| Message ID | 20230517221022.325091-2-stefanha@redhat.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | block: add blk_io_plug_call() API | expand |
On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote: > Introduce a new API for thread-local blk_io_plug() that does not > traverse the block graph. The goal is to make blk_io_plug() multi-queue > friendly. > > Instead of having block drivers track whether or not we're in a plugged > section, provide an API that allows them to defer a function call until > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is > called multiple times with the same fn/opaque pair, then fn() is only > called once at the end of the function - resulting in batching. > > This patch introduces the API and changes blk_io_plug()/blk_io_unplug(). > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument > because the plug state is now thread-local. > > Later patches convert block drivers to blk_io_plug_call() and then we > can finally remove .bdrv_co_io_plug() once all block drivers have been > converted. > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> > --- > +++ b/block/plug.c > + > +/** > + * blk_io_plug_call: > + * @fn: a function pointer to be invoked > + * @opaque: a user-defined argument to @fn() > + * > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug() > + * section. > + * > + * Otherwise defer the call until the end of the outermost > + * blk_io_plug()/blk_io_unplug() section in this thread. If the same > + * @fn/@opaque pair has already been deferred, it will only be called once upon > + * blk_io_unplug() so that accumulated calls are batched into a single call. > + * > + * The caller must ensure that @opaque is not be freed before @fn() is invoked. s/be // > + */ > +void blk_io_plug_call(void (*fn)(void *), void *opaque) Reviewed-by: Eric Blake <eblake@redhat.com>
On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote: >Introduce a new API for thread-local blk_io_plug() that does not >traverse the block graph. The goal is to make blk_io_plug() multi-queue >friendly. > >Instead of having block drivers track whether or not we're in a plugged >section, provide an API that allows them to defer a function call until >we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is >called multiple times with the same fn/opaque pair, then fn() is only >called once at the end of the function - resulting in batching. > >This patch introduces the API and changes blk_io_plug()/blk_io_unplug(). >blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument >because the plug state is now thread-local. > >Later patches convert block drivers to blk_io_plug_call() and then we >can finally remove .bdrv_co_io_plug() once all block drivers have been >converted. > >Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> >--- > MAINTAINERS | 1 + > include/sysemu/block-backend-io.h | 13 +-- > block/block-backend.c | 22 ----- > block/plug.c | 159 ++++++++++++++++++++++++++++++ > hw/block/dataplane/xen-block.c | 8 +- > hw/block/virtio-blk.c | 4 +- > hw/scsi/virtio-scsi.c | 6 +- > block/meson.build | 1 + > 8 files changed, 173 insertions(+), 41 deletions(-) > create mode 100644 block/plug.c > >diff --git a/MAINTAINERS b/MAINTAINERS >index 50585117a0..574202295c 100644 >--- a/MAINTAINERS >+++ b/MAINTAINERS >@@ -2644,6 +2644,7 @@ F: util/aio-*.c > F: util/aio-*.h > F: util/fdmon-*.c > F: block/io.c >+F: block/plug.c > F: migration/block* > F: include/block/aio.h > F: include/block/aio-wait.h >diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h >index d62a7ee773..be4dcef59d 100644 >--- a/include/sysemu/block-backend-io.h >+++ b/include/sysemu/block-backend-io.h >@@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error); > int blk_get_max_iov(BlockBackend *blk); > int blk_get_max_hw_iov(BlockBackend *blk); > >-/* >- * blk_io_plug/unplug are thread-local operations. This means that multiple >- * IOThreads can simultaneously call plug/unplug, but the caller must ensure >- * that each unplug() is called in the same IOThread of the matching plug(). >- */ >-void coroutine_fn blk_co_io_plug(BlockBackend *blk); >-void co_wrapper blk_io_plug(BlockBackend *blk); >- >-void coroutine_fn blk_co_io_unplug(BlockBackend *blk); >-void co_wrapper blk_io_unplug(BlockBackend *blk); >+void blk_io_plug(void); >+void blk_io_unplug(void); >+void blk_io_plug_call(void (*fn)(void *), void *opaque); > > AioContext *blk_get_aio_context(BlockBackend *blk); > BlockAcctStats *blk_get_stats(BlockBackend *blk); >diff --git a/block/block-backend.c b/block/block-backend.c >index ca537cd0ad..1f1d226ba6 100644 >--- a/block/block-backend.c >+++ b/block/block-backend.c >@@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify) > notifier_list_add(&blk->insert_bs_notifiers, notify); > } > >-void coroutine_fn blk_co_io_plug(BlockBackend *blk) >-{ >- BlockDriverState *bs = blk_bs(blk); >- IO_CODE(); >- GRAPH_RDLOCK_GUARD(); >- >- if (bs) { >- bdrv_co_io_plug(bs); >- } >-} >- >-void coroutine_fn blk_co_io_unplug(BlockBackend *blk) >-{ >- BlockDriverState *bs = blk_bs(blk); >- IO_CODE(); >- GRAPH_RDLOCK_GUARD(); >- >- if (bs) { >- bdrv_co_io_unplug(bs); >- } >-} >- > BlockAcctStats *blk_get_stats(BlockBackend *blk) > { > IO_CODE(); >diff --git a/block/plug.c b/block/plug.c >new file mode 100644 >index 0000000000..6738a568ba >--- /dev/null >+++ b/block/plug.c >@@ -0,0 +1,159 @@ >+/* SPDX-License-Identifier: GPL-2.0-or-later */ >+/* >+ * Block I/O plugging >+ * >+ * Copyright Red Hat. >+ * >+ * This API defers a function call within a blk_io_plug()/blk_io_unplug() >+ * section, allowing multiple calls to batch up. This is a performance >+ * optimization that is used in the block layer to submit several I/O requests >+ * at once instead of individually: >+ * >+ * blk_io_plug(); <-- start of plugged region >+ * ... >+ * blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call >+ * blk_io_plug_call(my_func, my_obj); <-- another >+ * blk_io_plug_call(my_func, my_obj); <-- another >+ * ... >+ * blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once >+ * >+ * This code is actually generic and not tied to the block layer. If another >+ * subsystem needs this functionality, it could be renamed. >+ */ >+ >+#include "qemu/osdep.h" >+#include "qemu/coroutine-tls.h" >+#include "qemu/notify.h" >+#include "qemu/thread.h" >+#include "sysemu/block-backend.h" >+ >+/* A function call that has been deferred until unplug() */ >+typedef struct { >+ void (*fn)(void *); >+ void *opaque; >+} UnplugFn; >+ >+/* Per-thread state */ >+typedef struct { >+ unsigned count; /* how many times has plug() been called? */ >+ GArray *unplug_fns; /* functions to call at unplug time */ >+} Plug; >+ >+/* Use get_ptr_plug() to fetch this thread-local value */ >+QEMU_DEFINE_STATIC_CO_TLS(Plug, plug); >+ >+/* Called at thread cleanup time */ >+static void blk_io_plug_atexit(Notifier *n, void *value) >+{ >+ Plug *plug = get_ptr_plug(); >+ g_array_free(plug->unplug_fns, TRUE); >+} >+ >+/* This won't involve coroutines, so use __thread */ >+static __thread Notifier blk_io_plug_atexit_notifier; >+ >+/** >+ * blk_io_plug_call: >+ * @fn: a function pointer to be invoked >+ * @opaque: a user-defined argument to @fn() >+ * >+ * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug() >+ * section. Just to understand better, what if two BlockDrivers share the same iothread but one calls blk_io_plug()/blk_io_unplug(), while the other calls this function not in a blk_io_plug()/blk_io_unplug() section? If the call is in the middle of the other BlockDriver's section, it is deferred, right? Is this situation possible? Or should we prevent blk_io_plug_call() from being called out of a blk_io_plug()/blk_io_unplug() section? Thanks, Stefano >+ * >+ * Otherwise defer the call until the end of the outermost >+ * blk_io_plug()/blk_io_unplug() section in this thread. If the same >+ * @fn/@opaque pair has already been deferred, it will only be called once upon >+ * blk_io_unplug() so that accumulated calls are batched into a single call. >+ * >+ * The caller must ensure that @opaque is not be freed before @fn() is invoked. >+ */ >+void blk_io_plug_call(void (*fn)(void *), void *opaque) >+{ >+ Plug *plug = get_ptr_plug(); >+ >+ /* Call immediately if we're not plugged */ >+ if (plug->count == 0) { >+ fn(opaque); >+ return; >+ } >+ >+ GArray *array = plug->unplug_fns; >+ if (!array) { >+ array = g_array_new(FALSE, FALSE, sizeof(UnplugFn)); >+ plug->unplug_fns = array; >+ blk_io_plug_atexit_notifier.notify = blk_io_plug_atexit; >+ qemu_thread_atexit_add(&blk_io_plug_atexit_notifier); >+ } >+ >+ UnplugFn *fns = (UnplugFn *)array->data; >+ UnplugFn new_fn = { >+ .fn = fn, >+ .opaque = opaque, >+ }; >+ >+ /* >+ * There won't be many, so do a linear search. If this becomes a bottleneck >+ * then a binary search (glib 2.62+) or different data structure could be >+ * used. >+ */ >+ for (guint i = 0; i < array->len; i++) { >+ if (memcmp(&fns[i], &new_fn, sizeof(new_fn)) == 0) { >+ return; /* already exists */ >+ } >+ } >+ >+ g_array_append_val(array, new_fn); >+} >+ >+/** >+ * blk_io_plug: Defer blk_io_plug_call() functions until blk_io_unplug() >+ * >+ * blk_io_plug/unplug are thread-local operations. This means that multiple >+ * threads can simultaneously call plug/unplug, but the caller must ensure that >+ * each unplug() is called in the same thread of the matching plug(). >+ * >+ * Nesting is supported. blk_io_plug_call() functions are only called at the >+ * outermost blk_io_unplug(). >+ */ >+void blk_io_plug(void) >+{ >+ Plug *plug = get_ptr_plug(); >+ >+ assert(plug->count < UINT32_MAX); >+ >+ plug->count++; >+} >+ >+/** >+ * blk_io_unplug: Run any pending blk_io_plug_call() functions >+ * >+ * There must have been a matching blk_io_plug() call in the same thread prior >+ * to this blk_io_unplug() call. >+ */ >+void blk_io_unplug(void) >+{ >+ Plug *plug = get_ptr_plug(); >+ >+ assert(plug->count > 0); >+ >+ if (--plug->count > 0) { >+ return; >+ } >+ >+ GArray *array = plug->unplug_fns; >+ if (!array) { >+ return; >+ } >+ >+ UnplugFn *fns = (UnplugFn *)array->data; >+ >+ for (guint i = 0; i < array->len; i++) { >+ fns[i].fn(fns[i].opaque); >+ } >+ >+ /* >+ * This resets the array without freeing memory so that appending is cheap >+ * in the future. >+ */ >+ g_array_set_size(array, 0); >+} >diff --git a/hw/block/dataplane/xen-block.c b/hw/block/dataplane/xen-block.c >index d8bc39d359..e49c24f63d 100644 >--- a/hw/block/dataplane/xen-block.c >+++ b/hw/block/dataplane/xen-block.c >@@ -537,7 +537,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane) > * is below us. > */ > if (inflight_atstart > IO_PLUG_THRESHOLD) { >- blk_io_plug(dataplane->blk); >+ blk_io_plug(); > } > while (rc != rp) { > /* pull request from ring */ >@@ -577,12 +577,12 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane) > > if (inflight_atstart > IO_PLUG_THRESHOLD && > batched >= inflight_atstart) { >- blk_io_unplug(dataplane->blk); >+ blk_io_unplug(); > } > xen_block_do_aio(request); > if (inflight_atstart > IO_PLUG_THRESHOLD) { > if (batched >= inflight_atstart) { >- blk_io_plug(dataplane->blk); >+ blk_io_plug(); > batched = 0; > } else { > batched++; >@@ -590,7 +590,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane) > } > } > if (inflight_atstart > IO_PLUG_THRESHOLD) { >- blk_io_unplug(dataplane->blk); >+ blk_io_unplug(); > } > > return done_something; >diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c >index 8f65ea4659..b4286424c1 100644 >--- a/hw/block/virtio-blk.c >+++ b/hw/block/virtio-blk.c >@@ -1134,7 +1134,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq) > bool suppress_notifications = virtio_queue_get_notification(vq); > > aio_context_acquire(blk_get_aio_context(s->blk)); >- blk_io_plug(s->blk); >+ blk_io_plug(); > > do { > if (suppress_notifications) { >@@ -1158,7 +1158,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq) > virtio_blk_submit_multireq(s, &mrb); > } > >- blk_io_unplug(s->blk); >+ blk_io_unplug(); > aio_context_release(blk_get_aio_context(s->blk)); > } > >diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c >index 612c525d9d..534a44ee07 100644 >--- a/hw/scsi/virtio-scsi.c >+++ b/hw/scsi/virtio-scsi.c >@@ -799,7 +799,7 @@ static int virtio_scsi_handle_cmd_req_prepare(VirtIOSCSI *s, VirtIOSCSIReq *req) > return -ENOBUFS; > } > scsi_req_ref(req->sreq); >- blk_io_plug(d->conf.blk); >+ blk_io_plug(); > object_unref(OBJECT(d)); > return 0; > } >@@ -810,7 +810,7 @@ static void virtio_scsi_handle_cmd_req_submit(VirtIOSCSI *s, VirtIOSCSIReq *req) > if (scsi_req_enqueue(sreq)) { > scsi_req_continue(sreq); > } >- blk_io_unplug(sreq->dev->conf.blk); >+ blk_io_unplug(); > scsi_req_unref(sreq); > } > >@@ -836,7 +836,7 @@ static void virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq) > while (!QTAILQ_EMPTY(&reqs)) { > req = QTAILQ_FIRST(&reqs); > QTAILQ_REMOVE(&reqs, req, next); >- blk_io_unplug(req->sreq->dev->conf.blk); >+ blk_io_unplug(); > scsi_req_unref(req->sreq); > virtqueue_detach_element(req->vq, &req->elem, 0); > virtio_scsi_free_req(req); >diff --git a/block/meson.build b/block/meson.build >index 486dda8b85..fb4332bd66 100644 >--- a/block/meson.build >+++ b/block/meson.build >@@ -23,6 +23,7 @@ block_ss.add(files( > 'mirror.c', > 'nbd.c', > 'null.c', >+ 'plug.c', > 'qapi.c', > 'qcow2-bitmap.c', > 'qcow2-cache.c', >-- >2.40.1 >
On Fri, May 19, 2023 at 10:45:57AM +0200, Stefano Garzarella wrote: > On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote: > > Introduce a new API for thread-local blk_io_plug() that does not > > traverse the block graph. The goal is to make blk_io_plug() multi-queue > > friendly. > > > > Instead of having block drivers track whether or not we're in a plugged > > section, provide an API that allows them to defer a function call until > > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is > > called multiple times with the same fn/opaque pair, then fn() is only > > called once at the end of the function - resulting in batching. > > > > This patch introduces the API and changes blk_io_plug()/blk_io_unplug(). > > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument > > because the plug state is now thread-local. > > > > Later patches convert block drivers to blk_io_plug_call() and then we > > can finally remove .bdrv_co_io_plug() once all block drivers have been > > converted. > > > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> > > --- > > MAINTAINERS | 1 + > > include/sysemu/block-backend-io.h | 13 +-- > > block/block-backend.c | 22 ----- > > block/plug.c | 159 ++++++++++++++++++++++++++++++ > > hw/block/dataplane/xen-block.c | 8 +- > > hw/block/virtio-blk.c | 4 +- > > hw/scsi/virtio-scsi.c | 6 +- > > block/meson.build | 1 + > > 8 files changed, 173 insertions(+), 41 deletions(-) > > create mode 100644 block/plug.c > > > > diff --git a/MAINTAINERS b/MAINTAINERS > > index 50585117a0..574202295c 100644 > > --- a/MAINTAINERS > > +++ b/MAINTAINERS > > @@ -2644,6 +2644,7 @@ F: util/aio-*.c > > F: util/aio-*.h > > F: util/fdmon-*.c > > F: block/io.c > > +F: block/plug.c > > F: migration/block* > > F: include/block/aio.h > > F: include/block/aio-wait.h > > diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h > > index d62a7ee773..be4dcef59d 100644 > > --- a/include/sysemu/block-backend-io.h > > +++ b/include/sysemu/block-backend-io.h > > @@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error); > > int blk_get_max_iov(BlockBackend *blk); > > int blk_get_max_hw_iov(BlockBackend *blk); > > > > -/* > > - * blk_io_plug/unplug are thread-local operations. This means that multiple > > - * IOThreads can simultaneously call plug/unplug, but the caller must ensure > > - * that each unplug() is called in the same IOThread of the matching plug(). > > - */ > > -void coroutine_fn blk_co_io_plug(BlockBackend *blk); > > -void co_wrapper blk_io_plug(BlockBackend *blk); > > - > > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk); > > -void co_wrapper blk_io_unplug(BlockBackend *blk); > > +void blk_io_plug(void); > > +void blk_io_unplug(void); > > +void blk_io_plug_call(void (*fn)(void *), void *opaque); > > > > AioContext *blk_get_aio_context(BlockBackend *blk); > > BlockAcctStats *blk_get_stats(BlockBackend *blk); > > diff --git a/block/block-backend.c b/block/block-backend.c > > index ca537cd0ad..1f1d226ba6 100644 > > --- a/block/block-backend.c > > +++ b/block/block-backend.c > > @@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify) > > notifier_list_add(&blk->insert_bs_notifiers, notify); > > } > > > > -void coroutine_fn blk_co_io_plug(BlockBackend *blk) > > -{ > > - BlockDriverState *bs = blk_bs(blk); > > - IO_CODE(); > > - GRAPH_RDLOCK_GUARD(); > > - > > - if (bs) { > > - bdrv_co_io_plug(bs); > > - } > > -} > > - > > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk) > > -{ > > - BlockDriverState *bs = blk_bs(blk); > > - IO_CODE(); > > - GRAPH_RDLOCK_GUARD(); > > - > > - if (bs) { > > - bdrv_co_io_unplug(bs); > > - } > > -} > > - > > BlockAcctStats *blk_get_stats(BlockBackend *blk) > > { > > IO_CODE(); > > diff --git a/block/plug.c b/block/plug.c > > new file mode 100644 > > index 0000000000..6738a568ba > > --- /dev/null > > +++ b/block/plug.c > > @@ -0,0 +1,159 @@ > > +/* SPDX-License-Identifier: GPL-2.0-or-later */ > > +/* > > + * Block I/O plugging > > + * > > + * Copyright Red Hat. > > + * > > + * This API defers a function call within a blk_io_plug()/blk_io_unplug() > > + * section, allowing multiple calls to batch up. This is a performance > > + * optimization that is used in the block layer to submit several I/O requests > > + * at once instead of individually: > > + * > > + * blk_io_plug(); <-- start of plugged region > > + * ... > > + * blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call > > + * blk_io_plug_call(my_func, my_obj); <-- another > > + * blk_io_plug_call(my_func, my_obj); <-- another > > + * ... > > + * blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once > > + * > > + * This code is actually generic and not tied to the block layer. If another > > + * subsystem needs this functionality, it could be renamed. > > + */ > > + > > +#include "qemu/osdep.h" > > +#include "qemu/coroutine-tls.h" > > +#include "qemu/notify.h" > > +#include "qemu/thread.h" > > +#include "sysemu/block-backend.h" > > + > > +/* A function call that has been deferred until unplug() */ > > +typedef struct { > > + void (*fn)(void *); > > + void *opaque; > > +} UnplugFn; > > + > > +/* Per-thread state */ > > +typedef struct { > > + unsigned count; /* how many times has plug() been called? */ > > + GArray *unplug_fns; /* functions to call at unplug time */ > > +} Plug; > > + > > +/* Use get_ptr_plug() to fetch this thread-local value */ > > +QEMU_DEFINE_STATIC_CO_TLS(Plug, plug); > > + > > +/* Called at thread cleanup time */ > > +static void blk_io_plug_atexit(Notifier *n, void *value) > > +{ > > + Plug *plug = get_ptr_plug(); > > + g_array_free(plug->unplug_fns, TRUE); > > +} > > + > > +/* This won't involve coroutines, so use __thread */ > > +static __thread Notifier blk_io_plug_atexit_notifier; > > + > > +/** > > + * blk_io_plug_call: > > + * @fn: a function pointer to be invoked > > + * @opaque: a user-defined argument to @fn() > > + * > > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug() > > + * section. > > Just to understand better, what if two BlockDrivers share the same > iothread but one calls blk_io_plug()/blk_io_unplug(), while the other > calls this function not in a blk_io_plug()/blk_io_unplug() section? > > If the call is in the middle of the other BlockDriver's section, it is > deferred, right? Yes, the call is deferred until blk_io_unplug(). > Is this situation possible? One scenario I can think of is when aio_poll() is called between plug/unplug. In that case, some I/O associated with device B might happen while device A is with plug/unplug. > Or should we prevent blk_io_plug_call() from being called out of a > blk_io_plug()/blk_io_unplug() section? blk_io_plug_call() is called outside blk_io_plug()/blk_io_unplug() when device emulation doesn't use plug/unplug. For example, IDE doesn't use it but still calls down into the same Linux AIO or io_uring code that invokes blk_io_plug_call(). This is why blk_io_plug_call() calls fn() immediately when invoked outside plug/unplug. Stefan
On Thu, May 18, 2023 at 07:04:52PM -0500, Eric Blake wrote: > On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote: > > Introduce a new API for thread-local blk_io_plug() that does not > > traverse the block graph. The goal is to make blk_io_plug() multi-queue > > friendly. > > > > Instead of having block drivers track whether or not we're in a plugged > > section, provide an API that allows them to defer a function call until > > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is > > called multiple times with the same fn/opaque pair, then fn() is only > > called once at the end of the function - resulting in batching. > > > > This patch introduces the API and changes blk_io_plug()/blk_io_unplug(). > > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument > > because the plug state is now thread-local. > > > > Later patches convert block drivers to blk_io_plug_call() and then we > > can finally remove .bdrv_co_io_plug() once all block drivers have been > > converted. > > > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> > > --- > > > +++ b/block/plug.c > > + > > +/** > > + * blk_io_plug_call: > > + * @fn: a function pointer to be invoked > > + * @opaque: a user-defined argument to @fn() > > + * > > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug() > > + * section. > > + * > > + * Otherwise defer the call until the end of the outermost > > + * blk_io_plug()/blk_io_unplug() section in this thread. If the same > > + * @fn/@opaque pair has already been deferred, it will only be called once upon > > + * blk_io_unplug() so that accumulated calls are batched into a single call. > > + * > > + * The caller must ensure that @opaque is not be freed before @fn() is invoked. > > s/be // Will fix, thanks! Stefan
On Tue, May 23, 2023 at 11:47:08AM -0400, Stefan Hajnoczi wrote: >On Fri, May 19, 2023 at 10:45:57AM +0200, Stefano Garzarella wrote: >> On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote: >> > Introduce a new API for thread-local blk_io_plug() that does not >> > traverse the block graph. The goal is to make blk_io_plug() multi-queue >> > friendly. >> > >> > Instead of having block drivers track whether or not we're in a plugged >> > section, provide an API that allows them to defer a function call until >> > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is >> > called multiple times with the same fn/opaque pair, then fn() is only >> > called once at the end of the function - resulting in batching. >> > >> > This patch introduces the API and changes blk_io_plug()/blk_io_unplug(). >> > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument >> > because the plug state is now thread-local. >> > >> > Later patches convert block drivers to blk_io_plug_call() and then we >> > can finally remove .bdrv_co_io_plug() once all block drivers have been >> > converted. >> > >> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> >> > --- >> > MAINTAINERS | 1 + >> > include/sysemu/block-backend-io.h | 13 +-- >> > block/block-backend.c | 22 ----- >> > block/plug.c | 159 ++++++++++++++++++++++++++++++ >> > hw/block/dataplane/xen-block.c | 8 +- >> > hw/block/virtio-blk.c | 4 +- >> > hw/scsi/virtio-scsi.c | 6 +- >> > block/meson.build | 1 + >> > 8 files changed, 173 insertions(+), 41 deletions(-) >> > create mode 100644 block/plug.c >> > >> > diff --git a/MAINTAINERS b/MAINTAINERS >> > index 50585117a0..574202295c 100644 >> > --- a/MAINTAINERS >> > +++ b/MAINTAINERS >> > @@ -2644,6 +2644,7 @@ F: util/aio-*.c >> > F: util/aio-*.h >> > F: util/fdmon-*.c >> > F: block/io.c >> > +F: block/plug.c >> > F: migration/block* >> > F: include/block/aio.h >> > F: include/block/aio-wait.h >> > diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h >> > index d62a7ee773..be4dcef59d 100644 >> > --- a/include/sysemu/block-backend-io.h >> > +++ b/include/sysemu/block-backend-io.h >> > @@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error); >> > int blk_get_max_iov(BlockBackend *blk); >> > int blk_get_max_hw_iov(BlockBackend *blk); >> > >> > -/* >> > - * blk_io_plug/unplug are thread-local operations. This means that multiple >> > - * IOThreads can simultaneously call plug/unplug, but the caller must ensure >> > - * that each unplug() is called in the same IOThread of the matching plug(). >> > - */ >> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk); >> > -void co_wrapper blk_io_plug(BlockBackend *blk); >> > - >> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk); >> > -void co_wrapper blk_io_unplug(BlockBackend *blk); >> > +void blk_io_plug(void); >> > +void blk_io_unplug(void); >> > +void blk_io_plug_call(void (*fn)(void *), void *opaque); >> > >> > AioContext *blk_get_aio_context(BlockBackend *blk); >> > BlockAcctStats *blk_get_stats(BlockBackend *blk); >> > diff --git a/block/block-backend.c b/block/block-backend.c >> > index ca537cd0ad..1f1d226ba6 100644 >> > --- a/block/block-backend.c >> > +++ b/block/block-backend.c >> > @@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify) >> > notifier_list_add(&blk->insert_bs_notifiers, notify); >> > } >> > >> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk) >> > -{ >> > - BlockDriverState *bs = blk_bs(blk); >> > - IO_CODE(); >> > - GRAPH_RDLOCK_GUARD(); >> > - >> > - if (bs) { >> > - bdrv_co_io_plug(bs); >> > - } >> > -} >> > - >> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk) >> > -{ >> > - BlockDriverState *bs = blk_bs(blk); >> > - IO_CODE(); >> > - GRAPH_RDLOCK_GUARD(); >> > - >> > - if (bs) { >> > - bdrv_co_io_unplug(bs); >> > - } >> > -} >> > - >> > BlockAcctStats *blk_get_stats(BlockBackend *blk) >> > { >> > IO_CODE(); >> > diff --git a/block/plug.c b/block/plug.c >> > new file mode 100644 >> > index 0000000000..6738a568ba >> > --- /dev/null >> > +++ b/block/plug.c >> > @@ -0,0 +1,159 @@ >> > +/* SPDX-License-Identifier: GPL-2.0-or-later */ >> > +/* >> > + * Block I/O plugging >> > + * >> > + * Copyright Red Hat. >> > + * >> > + * This API defers a function call within a blk_io_plug()/blk_io_unplug() >> > + * section, allowing multiple calls to batch up. This is a performance >> > + * optimization that is used in the block layer to submit several I/O requests >> > + * at once instead of individually: >> > + * >> > + * blk_io_plug(); <-- start of plugged region >> > + * ... >> > + * blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call >> > + * blk_io_plug_call(my_func, my_obj); <-- another >> > + * blk_io_plug_call(my_func, my_obj); <-- another >> > + * ... >> > + * blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once >> > + * >> > + * This code is actually generic and not tied to the block layer. If another >> > + * subsystem needs this functionality, it could be renamed. >> > + */ >> > + >> > +#include "qemu/osdep.h" >> > +#include "qemu/coroutine-tls.h" >> > +#include "qemu/notify.h" >> > +#include "qemu/thread.h" >> > +#include "sysemu/block-backend.h" >> > + >> > +/* A function call that has been deferred until unplug() */ >> > +typedef struct { >> > + void (*fn)(void *); >> > + void *opaque; >> > +} UnplugFn; >> > + >> > +/* Per-thread state */ >> > +typedef struct { >> > + unsigned count; /* how many times has plug() been called? */ >> > + GArray *unplug_fns; /* functions to call at unplug time */ >> > +} Plug; >> > + >> > +/* Use get_ptr_plug() to fetch this thread-local value */ >> > +QEMU_DEFINE_STATIC_CO_TLS(Plug, plug); >> > + >> > +/* Called at thread cleanup time */ >> > +static void blk_io_plug_atexit(Notifier *n, void *value) >> > +{ >> > + Plug *plug = get_ptr_plug(); >> > + g_array_free(plug->unplug_fns, TRUE); >> > +} >> > + >> > +/* This won't involve coroutines, so use __thread */ >> > +static __thread Notifier blk_io_plug_atexit_notifier; >> > + >> > +/** >> > + * blk_io_plug_call: >> > + * @fn: a function pointer to be invoked >> > + * @opaque: a user-defined argument to @fn() >> > + * >> > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug() >> > + * section. >> >> Just to understand better, what if two BlockDrivers share the same >> iothread but one calls blk_io_plug()/blk_io_unplug(), while the other >> calls this function not in a blk_io_plug()/blk_io_unplug() section? >> >> If the call is in the middle of the other BlockDriver's section, it is >> deferred, right? > >Yes, the call is deferred until blk_io_unplug(). > >> Is this situation possible? > >One scenario I can think of is when aio_poll() is called between >plug/unplug. In that case, some I/O associated with device B might >happen while device A is with plug/unplug. > >> Or should we prevent blk_io_plug_call() from being called out of a >> blk_io_plug()/blk_io_unplug() section? > >blk_io_plug_call() is called outside blk_io_plug()/blk_io_unplug() when >device emulation doesn't use plug/unplug. For example, IDE doesn't use >it but still calls down into the same Linux AIO or io_uring code that >invokes blk_io_plug_call(). This is why blk_io_plug_call() calls fn() >immediately when invoked outside plug/unplug. Got it, so it seems that everything should work properly. Thanks, Stefano
diff --git a/MAINTAINERS b/MAINTAINERS index 50585117a0..574202295c 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -2644,6 +2644,7 @@ F: util/aio-*.c F: util/aio-*.h F: util/fdmon-*.c F: block/io.c +F: block/plug.c F: migration/block* F: include/block/aio.h F: include/block/aio-wait.h diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h index d62a7ee773..be4dcef59d 100644 --- a/include/sysemu/block-backend-io.h +++ b/include/sysemu/block-backend-io.h @@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error); int blk_get_max_iov(BlockBackend *blk); int blk_get_max_hw_iov(BlockBackend *blk); -/* - * blk_io_plug/unplug are thread-local operations. This means that multiple - * IOThreads can simultaneously call plug/unplug, but the caller must ensure - * that each unplug() is called in the same IOThread of the matching plug(). - */ -void coroutine_fn blk_co_io_plug(BlockBackend *blk); -void co_wrapper blk_io_plug(BlockBackend *blk); - -void coroutine_fn blk_co_io_unplug(BlockBackend *blk); -void co_wrapper blk_io_unplug(BlockBackend *blk); +void blk_io_plug(void); +void blk_io_unplug(void); +void blk_io_plug_call(void (*fn)(void *), void *opaque); AioContext *blk_get_aio_context(BlockBackend *blk); BlockAcctStats *blk_get_stats(BlockBackend *blk); diff --git a/block/block-backend.c b/block/block-backend.c index ca537cd0ad..1f1d226ba6 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify) notifier_list_add(&blk->insert_bs_notifiers, notify); } -void coroutine_fn blk_co_io_plug(BlockBackend *blk) -{ - BlockDriverState *bs = blk_bs(blk); - IO_CODE(); - GRAPH_RDLOCK_GUARD(); - - if (bs) { - bdrv_co_io_plug(bs); - } -} - -void coroutine_fn blk_co_io_unplug(BlockBackend *blk) -{ - BlockDriverState *bs = blk_bs(blk); - IO_CODE(); - GRAPH_RDLOCK_GUARD(); - - if (bs) { - bdrv_co_io_unplug(bs); - } -} - BlockAcctStats *blk_get_stats(BlockBackend *blk) { IO_CODE(); diff --git a/block/plug.c b/block/plug.c new file mode 100644 index 0000000000..6738a568ba --- /dev/null +++ b/block/plug.c @@ -0,0 +1,159 @@ +/* SPDX-License-Identifier: GPL-2.0-or-later */ +/* + * Block I/O plugging + * + * Copyright Red Hat. + * + * This API defers a function call within a blk_io_plug()/blk_io_unplug() + * section, allowing multiple calls to batch up. This is a performance + * optimization that is used in the block layer to submit several I/O requests + * at once instead of individually: + * + * blk_io_plug(); <-- start of plugged region + * ... + * blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call + * blk_io_plug_call(my_func, my_obj); <-- another + * blk_io_plug_call(my_func, my_obj); <-- another + * ... + * blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once + * + * This code is actually generic and not tied to the block layer. If another + * subsystem needs this functionality, it could be renamed. + */ + +#include "qemu/osdep.h" +#include "qemu/coroutine-tls.h" +#include "qemu/notify.h" +#include "qemu/thread.h" +#include "sysemu/block-backend.h" + +/* A function call that has been deferred until unplug() */ +typedef struct { + void (*fn)(void *); + void *opaque; +} UnplugFn; + +/* Per-thread state */ +typedef struct { + unsigned count; /* how many times has plug() been called? */ + GArray *unplug_fns; /* functions to call at unplug time */ +} Plug; + +/* Use get_ptr_plug() to fetch this thread-local value */ +QEMU_DEFINE_STATIC_CO_TLS(Plug, plug); + +/* Called at thread cleanup time */ +static void blk_io_plug_atexit(Notifier *n, void *value) +{ + Plug *plug = get_ptr_plug(); + g_array_free(plug->unplug_fns, TRUE); +} + +/* This won't involve coroutines, so use __thread */ +static __thread Notifier blk_io_plug_atexit_notifier; + +/** + * blk_io_plug_call: + * @fn: a function pointer to be invoked + * @opaque: a user-defined argument to @fn() + * + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug() + * section. + * + * Otherwise defer the call until the end of the outermost + * blk_io_plug()/blk_io_unplug() section in this thread. If the same + * @fn/@opaque pair has already been deferred, it will only be called once upon + * blk_io_unplug() so that accumulated calls are batched into a single call. + * + * The caller must ensure that @opaque is not be freed before @fn() is invoked. + */ +void blk_io_plug_call(void (*fn)(void *), void *opaque) +{ + Plug *plug = get_ptr_plug(); + + /* Call immediately if we're not plugged */ + if (plug->count == 0) { + fn(opaque); + return; + } + + GArray *array = plug->unplug_fns; + if (!array) { + array = g_array_new(FALSE, FALSE, sizeof(UnplugFn)); + plug->unplug_fns = array; + blk_io_plug_atexit_notifier.notify = blk_io_plug_atexit; + qemu_thread_atexit_add(&blk_io_plug_atexit_notifier); + } + + UnplugFn *fns = (UnplugFn *)array->data; + UnplugFn new_fn = { + .fn = fn, + .opaque = opaque, + }; + + /* + * There won't be many, so do a linear search. If this becomes a bottleneck + * then a binary search (glib 2.62+) or different data structure could be + * used. + */ + for (guint i = 0; i < array->len; i++) { + if (memcmp(&fns[i], &new_fn, sizeof(new_fn)) == 0) { + return; /* already exists */ + } + } + + g_array_append_val(array, new_fn); +} + +/** + * blk_io_plug: Defer blk_io_plug_call() functions until blk_io_unplug() + * + * blk_io_plug/unplug are thread-local operations. This means that multiple + * threads can simultaneously call plug/unplug, but the caller must ensure that + * each unplug() is called in the same thread of the matching plug(). + * + * Nesting is supported. blk_io_plug_call() functions are only called at the + * outermost blk_io_unplug(). + */ +void blk_io_plug(void) +{ + Plug *plug = get_ptr_plug(); + + assert(plug->count < UINT32_MAX); + + plug->count++; +} + +/** + * blk_io_unplug: Run any pending blk_io_plug_call() functions + * + * There must have been a matching blk_io_plug() call in the same thread prior + * to this blk_io_unplug() call. + */ +void blk_io_unplug(void) +{ + Plug *plug = get_ptr_plug(); + + assert(plug->count > 0); + + if (--plug->count > 0) { + return; + } + + GArray *array = plug->unplug_fns; + if (!array) { + return; + } + + UnplugFn *fns = (UnplugFn *)array->data; + + for (guint i = 0; i < array->len; i++) { + fns[i].fn(fns[i].opaque); + } + + /* + * This resets the array without freeing memory so that appending is cheap + * in the future. + */ + g_array_set_size(array, 0); +} diff --git a/hw/block/dataplane/xen-block.c b/hw/block/dataplane/xen-block.c index d8bc39d359..e49c24f63d 100644 --- a/hw/block/dataplane/xen-block.c +++ b/hw/block/dataplane/xen-block.c @@ -537,7 +537,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane) * is below us. */ if (inflight_atstart > IO_PLUG_THRESHOLD) { - blk_io_plug(dataplane->blk); + blk_io_plug(); } while (rc != rp) { /* pull request from ring */ @@ -577,12 +577,12 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane) if (inflight_atstart > IO_PLUG_THRESHOLD && batched >= inflight_atstart) { - blk_io_unplug(dataplane->blk); + blk_io_unplug(); } xen_block_do_aio(request); if (inflight_atstart > IO_PLUG_THRESHOLD) { if (batched >= inflight_atstart) { - blk_io_plug(dataplane->blk); + blk_io_plug(); batched = 0; } else { batched++; @@ -590,7 +590,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane) } } if (inflight_atstart > IO_PLUG_THRESHOLD) { - blk_io_unplug(dataplane->blk); + blk_io_unplug(); } return done_something; diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c index 8f65ea4659..b4286424c1 100644 --- a/hw/block/virtio-blk.c +++ b/hw/block/virtio-blk.c @@ -1134,7 +1134,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq) bool suppress_notifications = virtio_queue_get_notification(vq); aio_context_acquire(blk_get_aio_context(s->blk)); - blk_io_plug(s->blk); + blk_io_plug(); do { if (suppress_notifications) { @@ -1158,7 +1158,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq) virtio_blk_submit_multireq(s, &mrb); } - blk_io_unplug(s->blk); + blk_io_unplug(); aio_context_release(blk_get_aio_context(s->blk)); } diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c index 612c525d9d..534a44ee07 100644 --- a/hw/scsi/virtio-scsi.c +++ b/hw/scsi/virtio-scsi.c @@ -799,7 +799,7 @@ static int virtio_scsi_handle_cmd_req_prepare(VirtIOSCSI *s, VirtIOSCSIReq *req) return -ENOBUFS; } scsi_req_ref(req->sreq); - blk_io_plug(d->conf.blk); + blk_io_plug(); object_unref(OBJECT(d)); return 0; } @@ -810,7 +810,7 @@ static void virtio_scsi_handle_cmd_req_submit(VirtIOSCSI *s, VirtIOSCSIReq *req) if (scsi_req_enqueue(sreq)) { scsi_req_continue(sreq); } - blk_io_unplug(sreq->dev->conf.blk); + blk_io_unplug(); scsi_req_unref(sreq); } @@ -836,7 +836,7 @@ static void virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq) while (!QTAILQ_EMPTY(&reqs)) { req = QTAILQ_FIRST(&reqs); QTAILQ_REMOVE(&reqs, req, next); - blk_io_unplug(req->sreq->dev->conf.blk); + blk_io_unplug(); scsi_req_unref(req->sreq); virtqueue_detach_element(req->vq, &req->elem, 0); virtio_scsi_free_req(req); diff --git a/block/meson.build b/block/meson.build index 486dda8b85..fb4332bd66 100644 --- a/block/meson.build +++ b/block/meson.build @@ -23,6 +23,7 @@ block_ss.add(files( 'mirror.c', 'nbd.c', 'null.c', + 'plug.c', 'qapi.c', 'qcow2-bitmap.c', 'qcow2-cache.c',
Introduce a new API for thread-local blk_io_plug() that does not traverse the block graph. The goal is to make blk_io_plug() multi-queue friendly. Instead of having block drivers track whether or not we're in a plugged section, provide an API that allows them to defer a function call until we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is called multiple times with the same fn/opaque pair, then fn() is only called once at the end of the function - resulting in batching. This patch introduces the API and changes blk_io_plug()/blk_io_unplug(). blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument because the plug state is now thread-local. Later patches convert block drivers to blk_io_plug_call() and then we can finally remove .bdrv_co_io_plug() once all block drivers have been converted. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> --- MAINTAINERS | 1 + include/sysemu/block-backend-io.h | 13 +-- block/block-backend.c | 22 ----- block/plug.c | 159 ++++++++++++++++++++++++++++++ hw/block/dataplane/xen-block.c | 8 +- hw/block/virtio-blk.c | 4 +- hw/scsi/virtio-scsi.c | 6 +- block/meson.build | 1 + 8 files changed, 173 insertions(+), 41 deletions(-) create mode 100644 block/plug.c