| Message ID | 20200408080255.v4.3.I28278ef8ea27afc0ec7e597752a6d4e58c16176f@changeid (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | blk-mq: Fix two causes of IO stalls found in reboot testing | expand |
On Wed, Apr 08, 2020 at 08:04:01AM -0700, Douglas Anderson wrote: > If ever a thread running blk-mq code tries to get budget and fails it > immediately stops doing work and assumes that whenever budget is freed > up that queues will be kicked and whatever work the thread was trying > to do will be tried again. > > One path where budget is freed and queues are kicked in the normal > case can be seen in scsi_finish_command(). Specifically: > - scsi_finish_command() > - scsi_device_unbusy() > - # Decrement "device_busy", AKA release budget > - scsi_io_completion() > - scsi_end_request() > - blk_mq_run_hw_queues() > > The above is all well and good. The problem comes up when a thread > claims the budget but then releases it without actually dispatching > any work. Since we didn't schedule any work we'll never run the path > of finishing work / kicking the queues. > > This isn't often actually a problem which is why this issue has > existed for a while and nobody noticed. Specifically we only get into > this situation when we unexpectedly found that we weren't going to do > any work. Code that later receives new work kicks the queues. All > good, right? > > The problem shows up, however, if timing is just wrong and we hit a > race. To see this race let's think about the case where we only have > a budget of 1 (only one thread can hold budget). Now imagine that a > thread got budget and then decided not to dispatch work. It's about > to call put_budget() but then the thread gets context switched out for > a long, long time. While in this state, any and all kicks of the > queue (like the when we received new work) will be no-ops because > nobody can get budget. Finally the thread holding budget gets to run > again and returns. All the normal kicks will have been no-ops and we > have an I/O stall. > > As you can see from the above, you need just the right timing to see > the race. To start with, the only case it happens if we thought we > had work, actually managed to get the budget, but then actually didn't > have work. That's pretty rare to start with. Even then, there's > usually a very small amount of time between realizing that there's no > work and putting the budget. During this small amount of time new > work has to come in and the queue kick has to make it all the way to > trying to get the budget and fail. It's pretty unlikely. > > One case where this could have failed is illustrated by an example of > threads running blk_mq_do_dispatch_sched(): > > * Threads A and B both run has_work() at the same time with the same > "hctx". Imagine has_work() is exact. There's no lock, so it's OK > if Thread A and B both get back true. > * Thread B gets interrupted for a long time right after it decides > that there is work. Maybe its CPU gets an interrupt and the > interrupt handler is slow. > * Thread A runs, get budget, dispatches work. > * Thread A's work finishes and budget is released. > * Thread B finally runs again and gets budget. > * Since Thread A already took care of the work and no new work has > come in, Thread B will get NULL from dispatch_request(). I believe > this is specifically why dispatch_request() is allowed to return > NULL in the first place if has_work() must be exact. > * Thread B will now be holding the budget and is about to call > put_budget(), but hasn't called it yet. > * Thread B gets interrupted for a long time (again). Dang interrupts. > * Now Thread C (maybe with a different "hctx" but the same queue) > comes along and runs blk_mq_do_dispatch_sched(). > * Thread C won't do anything because it can't get budget. > * Finally Thread B will run again and put the budget without kicking > any queues. > > Even though the example above is with blk_mq_do_dispatch_sched() I > believe the race is possible any time someone is holding budget but > doesn't do work. > > Unfortunately, the unlikely has become more likely if you happen to be > using the BFQ I/O scheduler. BFQ, by design, sometimes returns "true" > for has_work() but then NULL for dispatch_request() and stays in this > state for a while (currently up to 9 ms). Suddenly you only need one > race to hit, not two races in a row. With my current setup this is > easy to reproduce in reboot tests and traces have actually shown that > we hit a race similar to the one described above. > > Note that we only need to fix blk_mq_do_dispatch_sched() and > blk_mq_do_dispatch_ctx() and not the other places that put budget. In > other cases we know that we have work to do on at least one "hctx" and > code already exists to kick that "hctx"'s queue. When that work > finally finishes all the queues will be kicked using the normal flow. > > One last note is that (at least in the SCSI case) budget is shared by > all "hctx"s that have the same queue. Thus we need to make sure to > kick the whole queue, not just re-run dispatching on a single "hctx". > > Signed-off-by: Douglas Anderson <dianders@chromium.org> > --- > > Changes in v4: > - Only kick in blk_mq_do_dispatch_ctx() / blk_mq_do_dispatch_sched(). > > Changes in v3: > - Always kick when putting the budget. > - Delay blk_mq_do_dispatch_sched() kick by 3 ms for inexact has_work(). > - Totally rewrote commit message. > > Changes in v2: > - Replace ("scsi: core: Fix stall...") w/ ("blk-mq: Rerun dispatch...") > > block/blk-mq-sched.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > index 74cedea56034..eca81bd4010c 100644 > --- a/block/blk-mq-sched.c > +++ b/block/blk-mq-sched.c > @@ -80,6 +80,8 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx) > blk_mq_run_hw_queue(hctx, true); > } > > +#define BLK_MQ_BUDGET_DELAY 3 /* ms units */ > + > /* > * Only SCSI implements .get_budget and .put_budget, and SCSI restarts > * its queue by itself in its completion handler, so we don't need to > @@ -103,6 +105,14 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) > rq = e->type->ops.dispatch_request(hctx); > if (!rq) { > blk_mq_put_dispatch_budget(hctx); > + /* > + * We're releasing without dispatching. Holding the > + * budget could have blocked any "hctx"s with the > + * same queue and if we didn't dispatch then there's > + * no guarantee anyone will kick the queue. Kick it > + * ourselves. > + */ > + blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY); > break; > } > > @@ -149,6 +159,14 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx) > rq = blk_mq_dequeue_from_ctx(hctx, ctx); > if (!rq) { > blk_mq_put_dispatch_budget(hctx); > + /* > + * We're releasing without dispatching. Holding the > + * budget could have blocked any "hctx"s with the > + * same queue and if we didn't dispatch then there's > + * no guarantee anyone will kick the queue. Kick it > + * ourselves. > + */ > + blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY); > break; > } > > -- > 2.26.0.292.g33ef6b2f38-goog > Reviewed-by: Ming Lei <ming.lei@redhat.com>
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index 74cedea56034..eca81bd4010c 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c @@ -80,6 +80,8 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx) blk_mq_run_hw_queue(hctx, true); } +#define BLK_MQ_BUDGET_DELAY 3 /* ms units */ + /* * Only SCSI implements .get_budget and .put_budget, and SCSI restarts * its queue by itself in its completion handler, so we don't need to @@ -103,6 +105,14 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) rq = e->type->ops.dispatch_request(hctx); if (!rq) { blk_mq_put_dispatch_budget(hctx); + /* + * We're releasing without dispatching. Holding the + * budget could have blocked any "hctx"s with the + * same queue and if we didn't dispatch then there's + * no guarantee anyone will kick the queue. Kick it + * ourselves. + */ + blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY); break; } @@ -149,6 +159,14 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx) rq = blk_mq_dequeue_from_ctx(hctx, ctx); if (!rq) { blk_mq_put_dispatch_budget(hctx); + /* + * We're releasing without dispatching. Holding the + * budget could have blocked any "hctx"s with the + * same queue and if we didn't dispatch then there's + * no guarantee anyone will kick the queue. Kick it + * ourselves. + */ + blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY); break; }
If ever a thread running blk-mq code tries to get budget and fails it immediately stops doing work and assumes that whenever budget is freed up that queues will be kicked and whatever work the thread was trying to do will be tried again. One path where budget is freed and queues are kicked in the normal case can be seen in scsi_finish_command(). Specifically: - scsi_finish_command() - scsi_device_unbusy() - # Decrement "device_busy", AKA release budget - scsi_io_completion() - scsi_end_request() - blk_mq_run_hw_queues() The above is all well and good. The problem comes up when a thread claims the budget but then releases it without actually dispatching any work. Since we didn't schedule any work we'll never run the path of finishing work / kicking the queues. This isn't often actually a problem which is why this issue has existed for a while and nobody noticed. Specifically we only get into this situation when we unexpectedly found that we weren't going to do any work. Code that later receives new work kicks the queues. All good, right? The problem shows up, however, if timing is just wrong and we hit a race. To see this race let's think about the case where we only have a budget of 1 (only one thread can hold budget). Now imagine that a thread got budget and then decided not to dispatch work. It's about to call put_budget() but then the thread gets context switched out for a long, long time. While in this state, any and all kicks of the queue (like the when we received new work) will be no-ops because nobody can get budget. Finally the thread holding budget gets to run again and returns. All the normal kicks will have been no-ops and we have an I/O stall. As you can see from the above, you need just the right timing to see the race. To start with, the only case it happens if we thought we had work, actually managed to get the budget, but then actually didn't have work. That's pretty rare to start with. Even then, there's usually a very small amount of time between realizing that there's no work and putting the budget. During this small amount of time new work has to come in and the queue kick has to make it all the way to trying to get the budget and fail. It's pretty unlikely. One case where this could have failed is illustrated by an example of threads running blk_mq_do_dispatch_sched(): * Threads A and B both run has_work() at the same time with the same "hctx". Imagine has_work() is exact. There's no lock, so it's OK if Thread A and B both get back true. * Thread B gets interrupted for a long time right after it decides that there is work. Maybe its CPU gets an interrupt and the interrupt handler is slow. * Thread A runs, get budget, dispatches work. * Thread A's work finishes and budget is released. * Thread B finally runs again and gets budget. * Since Thread A already took care of the work and no new work has come in, Thread B will get NULL from dispatch_request(). I believe this is specifically why dispatch_request() is allowed to return NULL in the first place if has_work() must be exact. * Thread B will now be holding the budget and is about to call put_budget(), but hasn't called it yet. * Thread B gets interrupted for a long time (again). Dang interrupts. * Now Thread C (maybe with a different "hctx" but the same queue) comes along and runs blk_mq_do_dispatch_sched(). * Thread C won't do anything because it can't get budget. * Finally Thread B will run again and put the budget without kicking any queues. Even though the example above is with blk_mq_do_dispatch_sched() I believe the race is possible any time someone is holding budget but doesn't do work. Unfortunately, the unlikely has become more likely if you happen to be using the BFQ I/O scheduler. BFQ, by design, sometimes returns "true" for has_work() but then NULL for dispatch_request() and stays in this state for a while (currently up to 9 ms). Suddenly you only need one race to hit, not two races in a row. With my current setup this is easy to reproduce in reboot tests and traces have actually shown that we hit a race similar to the one described above. Note that we only need to fix blk_mq_do_dispatch_sched() and blk_mq_do_dispatch_ctx() and not the other places that put budget. In other cases we know that we have work to do on at least one "hctx" and code already exists to kick that "hctx"'s queue. When that work finally finishes all the queues will be kicked using the normal flow. One last note is that (at least in the SCSI case) budget is shared by all "hctx"s that have the same queue. Thus we need to make sure to kick the whole queue, not just re-run dispatching on a single "hctx". Signed-off-by: Douglas Anderson <dianders@chromium.org> --- Changes in v4: - Only kick in blk_mq_do_dispatch_ctx() / blk_mq_do_dispatch_sched(). Changes in v3: - Always kick when putting the budget. - Delay blk_mq_do_dispatch_sched() kick by 3 ms for inexact has_work(). - Totally rewrote commit message. Changes in v2: - Replace ("scsi: core: Fix stall...") w/ ("blk-mq: Rerun dispatch...") block/blk-mq-sched.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)