| Message ID | 20201106110141.5887-1-damien.lemoal@wdc.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | null_blk: Fix scheduling in atomic with zoned mode | expand |
On Fri, Nov 06, 2020 at 08:01:41PM +0900, Damien Le Moal wrote: > Commit aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") changed > zone locking to using the potentially sleeping wait_on_bit_io() > function. This is acceptable when memory backing is enabled as the > device queue is in that case marked as blocking, but this triggers a > scheduling while in atomic context with memory backing disabled. > > Fix this by relying solely on the device zone spinlock for zone > information protection without temporarily releasing this lock around > null_process_cmd() execution in null_zone_write(). This is OK to do > since when memory backing is disabled, command processing does not > block and the memory backing lock nullb->lock is unused. This solution > avoids the overhead of having to mark a zoned null_blk device queue as > blocking when memory backing is unused. > > This patch also adds comments to the zone locking code to explain the > unusual locking scheme. > > Reported-by: kernel test robot <lkp@intel.com> > Fixes: aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") > Cc: stable@vger.kernel.org > Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com> > --- > drivers/block/null_blk.h | 2 +- > drivers/block/null_blk_zoned.c | 47 ++++++++++++++++++++++------------ > 2 files changed, 32 insertions(+), 17 deletions(-) > > diff --git a/drivers/block/null_blk.h b/drivers/block/null_blk.h > index cfd00ad40355..c24d9b5ad81a 100644 > --- a/drivers/block/null_blk.h > +++ b/drivers/block/null_blk.h > @@ -47,7 +47,7 @@ struct nullb_device { > unsigned int nr_zones_closed; > struct blk_zone *zones; > sector_t zone_size_sects; > - spinlock_t zone_dev_lock; > + spinlock_t zone_lock; > unsigned long *zone_locks; > > unsigned long size; /* device size in MB */ > diff --git a/drivers/block/null_blk_zoned.c b/drivers/block/null_blk_zoned.c > index 8775acbb4f8f..beb34b4f76b0 100644 > --- a/drivers/block/null_blk_zoned.c > +++ b/drivers/block/null_blk_zoned.c > @@ -46,11 +46,20 @@ int null_init_zoned_dev(struct nullb_device *dev, struct request_queue *q) > if (!dev->zones) > return -ENOMEM; > > - spin_lock_init(&dev->zone_dev_lock); > - dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL); > - if (!dev->zone_locks) { > - kvfree(dev->zones); > - return -ENOMEM; > + /* > + * With memory backing, the zone_lock spinlock needs to be temporarily > + * released to avoid scheduling in atomic context. To guarantee zone > + * information protection, use a bitmap to lock zones with > + * wait_on_bit_lock_io(). Sleeping on the lock is OK as memory backing > + * implies that the queue is marked with BLK_MQ_F_BLOCKING. > + */ > + spin_lock_init(&dev->zone_lock); > + if (dev->memory_backed) { > + dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL); > + if (!dev->zone_locks) { > + kvfree(dev->zones); > + return -ENOMEM; > + } > } > > if (dev->zone_nr_conv >= dev->nr_zones) { > @@ -137,12 +146,17 @@ void null_free_zoned_dev(struct nullb_device *dev) > > static inline void null_lock_zone(struct nullb_device *dev, unsigned int zno) > { > - wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE); > + if (dev->memory_backed) > + wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE); > + spin_lock_irq(&dev->zone_lock); > } > > static inline void null_unlock_zone(struct nullb_device *dev, unsigned int zno) > { > - clear_and_wake_up_bit(zno, dev->zone_locks); > + spin_unlock_irq(&dev->zone_lock); > + > + if (dev->memory_backed) > + clear_and_wake_up_bit(zno, dev->zone_locks); > } > > int null_report_zones(struct gendisk *disk, sector_t sector, > @@ -322,7 +336,6 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector, > return null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors); > > null_lock_zone(dev, zno); > - spin_lock(&dev->zone_dev_lock); > > switch (zone->cond) { > case BLK_ZONE_COND_FULL: > @@ -375,9 +388,17 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector, > if (zone->cond != BLK_ZONE_COND_EXP_OPEN) > zone->cond = BLK_ZONE_COND_IMP_OPEN; > > - spin_unlock(&dev->zone_dev_lock); > + /* > + * Memory backing allocation may sleep: release the zone_lock spinlock > + * to avoid scheduling in atomic context. Zone operation atomicity is > + * still guaranteed through the zone_locks bitmap. > + */ > + if (dev->memory_backed) > + spin_unlock_irq(&dev->zone_lock); > ret = null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors); > - spin_lock(&dev->zone_dev_lock); > + if (dev->memory_backed) > + spin_lock_irq(&dev->zone_lock); Do we actually need to take zone_lock at all for the memory_backed case? Should the !memory_backed just use a per-zone spinlock?
On 2020/11/06 22:55, Christoph Hellwig wrote: > On Fri, Nov 06, 2020 at 08:01:41PM +0900, Damien Le Moal wrote: >> Commit aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") changed >> zone locking to using the potentially sleeping wait_on_bit_io() >> function. This is acceptable when memory backing is enabled as the >> device queue is in that case marked as blocking, but this triggers a >> scheduling while in atomic context with memory backing disabled. >> >> Fix this by relying solely on the device zone spinlock for zone >> information protection without temporarily releasing this lock around >> null_process_cmd() execution in null_zone_write(). This is OK to do >> since when memory backing is disabled, command processing does not >> block and the memory backing lock nullb->lock is unused. This solution >> avoids the overhead of having to mark a zoned null_blk device queue as >> blocking when memory backing is unused. >> >> This patch also adds comments to the zone locking code to explain the >> unusual locking scheme. >> >> Reported-by: kernel test robot <lkp@intel.com> >> Fixes: aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") >> Cc: stable@vger.kernel.org >> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com> >> --- >> drivers/block/null_blk.h | 2 +- >> drivers/block/null_blk_zoned.c | 47 ++++++++++++++++++++++------------ >> 2 files changed, 32 insertions(+), 17 deletions(-) >> >> diff --git a/drivers/block/null_blk.h b/drivers/block/null_blk.h >> index cfd00ad40355..c24d9b5ad81a 100644 >> --- a/drivers/block/null_blk.h >> +++ b/drivers/block/null_blk.h >> @@ -47,7 +47,7 @@ struct nullb_device { >> unsigned int nr_zones_closed; >> struct blk_zone *zones; >> sector_t zone_size_sects; >> - spinlock_t zone_dev_lock; >> + spinlock_t zone_lock; >> unsigned long *zone_locks; >> >> unsigned long size; /* device size in MB */ >> diff --git a/drivers/block/null_blk_zoned.c b/drivers/block/null_blk_zoned.c >> index 8775acbb4f8f..beb34b4f76b0 100644 >> --- a/drivers/block/null_blk_zoned.c >> +++ b/drivers/block/null_blk_zoned.c >> @@ -46,11 +46,20 @@ int null_init_zoned_dev(struct nullb_device *dev, struct request_queue *q) >> if (!dev->zones) >> return -ENOMEM; >> >> - spin_lock_init(&dev->zone_dev_lock); >> - dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL); >> - if (!dev->zone_locks) { >> - kvfree(dev->zones); >> - return -ENOMEM; >> + /* >> + * With memory backing, the zone_lock spinlock needs to be temporarily >> + * released to avoid scheduling in atomic context. To guarantee zone >> + * information protection, use a bitmap to lock zones with >> + * wait_on_bit_lock_io(). Sleeping on the lock is OK as memory backing >> + * implies that the queue is marked with BLK_MQ_F_BLOCKING. >> + */ >> + spin_lock_init(&dev->zone_lock); >> + if (dev->memory_backed) { >> + dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL); >> + if (!dev->zone_locks) { >> + kvfree(dev->zones); >> + return -ENOMEM; >> + } >> } >> >> if (dev->zone_nr_conv >= dev->nr_zones) { >> @@ -137,12 +146,17 @@ void null_free_zoned_dev(struct nullb_device *dev) >> >> static inline void null_lock_zone(struct nullb_device *dev, unsigned int zno) >> { >> - wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE); >> + if (dev->memory_backed) >> + wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE); >> + spin_lock_irq(&dev->zone_lock); >> } >> >> static inline void null_unlock_zone(struct nullb_device *dev, unsigned int zno) >> { >> - clear_and_wake_up_bit(zno, dev->zone_locks); >> + spin_unlock_irq(&dev->zone_lock); >> + >> + if (dev->memory_backed) >> + clear_and_wake_up_bit(zno, dev->zone_locks); >> } >> >> int null_report_zones(struct gendisk *disk, sector_t sector, >> @@ -322,7 +336,6 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector, >> return null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors); >> >> null_lock_zone(dev, zno); >> - spin_lock(&dev->zone_dev_lock); >> >> switch (zone->cond) { >> case BLK_ZONE_COND_FULL: >> @@ -375,9 +388,17 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector, >> if (zone->cond != BLK_ZONE_COND_EXP_OPEN) >> zone->cond = BLK_ZONE_COND_IMP_OPEN; >> >> - spin_unlock(&dev->zone_dev_lock); >> + /* >> + * Memory backing allocation may sleep: release the zone_lock spinlock >> + * to avoid scheduling in atomic context. Zone operation atomicity is >> + * still guaranteed through the zone_locks bitmap. >> + */ >> + if (dev->memory_backed) >> + spin_unlock_irq(&dev->zone_lock); >> ret = null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors); >> - spin_lock(&dev->zone_dev_lock); >> + if (dev->memory_backed) >> + spin_lock_irq(&dev->zone_lock); > > Do we actually need to take zone_lock at all for the memory_backed > case? Should the !memory_backed just use a per-zone spinlock? For your first question, yes we do: the per zone wait_on_bit_io() lock serialize zone operations per zone but we need a device level zone lock for protecting the counters (imp open, exp open and closed) for zone resources management. For the !memory backed case, I wondered the same but decided to just reuse the device zone spinlock as the single lock for all zones. Not sure if a per zone spinlock would improve performance by that much given that the lock time frame becomes really tiny without the memory copy. I will try that to see how performance changes. That said, I would prefer to add such change as a different patch for next cycle and keep this patch as a bug fix though.
Looks good for now:
Reviewed-by: Christoph Hellwig <hch@lst.de>
although I think we should address the global lock for the
!memory_backed case rather sooner than later.
On 2020/11/06 23:19, Christoph Hellwig wrote: > Looks good for now: > > Reviewed-by: Christoph Hellwig <hch@lst.de> > > although I think we should address the global lock for the > !memory_backed case rather sooner than later. OK. I will send something next week.
On 11/6/20 4:01 AM, Damien Le Moal wrote: > Commit aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") changed > zone locking to using the potentially sleeping wait_on_bit_io() > function. This is acceptable when memory backing is enabled as the > device queue is in that case marked as blocking, but this triggers a > scheduling while in atomic context with memory backing disabled. > > Fix this by relying solely on the device zone spinlock for zone > information protection without temporarily releasing this lock around > null_process_cmd() execution in null_zone_write(). This is OK to do > since when memory backing is disabled, command processing does not > block and the memory backing lock nullb->lock is unused. This solution > avoids the overhead of having to mark a zoned null_blk device queue as > blocking when memory backing is unused. > > This patch also adds comments to the zone locking code to explain the > unusual locking scheme. Applied for 5.10, though I do agree that the locking for non-memory backed should go. The whole point of null_blk is to be able to benchmark the underlying layers, and if we end up having null_blk be the limiting factor, then it's pointless.
diff --git a/drivers/block/null_blk.h b/drivers/block/null_blk.h index cfd00ad40355..c24d9b5ad81a 100644 --- a/drivers/block/null_blk.h +++ b/drivers/block/null_blk.h @@ -47,7 +47,7 @@ struct nullb_device { unsigned int nr_zones_closed; struct blk_zone *zones; sector_t zone_size_sects; - spinlock_t zone_dev_lock; + spinlock_t zone_lock; unsigned long *zone_locks; unsigned long size; /* device size in MB */ diff --git a/drivers/block/null_blk_zoned.c b/drivers/block/null_blk_zoned.c index 8775acbb4f8f..beb34b4f76b0 100644 --- a/drivers/block/null_blk_zoned.c +++ b/drivers/block/null_blk_zoned.c @@ -46,11 +46,20 @@ int null_init_zoned_dev(struct nullb_device *dev, struct request_queue *q) if (!dev->zones) return -ENOMEM; - spin_lock_init(&dev->zone_dev_lock); - dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL); - if (!dev->zone_locks) { - kvfree(dev->zones); - return -ENOMEM; + /* + * With memory backing, the zone_lock spinlock needs to be temporarily + * released to avoid scheduling in atomic context. To guarantee zone + * information protection, use a bitmap to lock zones with + * wait_on_bit_lock_io(). Sleeping on the lock is OK as memory backing + * implies that the queue is marked with BLK_MQ_F_BLOCKING. + */ + spin_lock_init(&dev->zone_lock); + if (dev->memory_backed) { + dev->zone_locks = bitmap_zalloc(dev->nr_zones, GFP_KERNEL); + if (!dev->zone_locks) { + kvfree(dev->zones); + return -ENOMEM; + } } if (dev->zone_nr_conv >= dev->nr_zones) { @@ -137,12 +146,17 @@ void null_free_zoned_dev(struct nullb_device *dev) static inline void null_lock_zone(struct nullb_device *dev, unsigned int zno) { - wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE); + if (dev->memory_backed) + wait_on_bit_lock_io(dev->zone_locks, zno, TASK_UNINTERRUPTIBLE); + spin_lock_irq(&dev->zone_lock); } static inline void null_unlock_zone(struct nullb_device *dev, unsigned int zno) { - clear_and_wake_up_bit(zno, dev->zone_locks); + spin_unlock_irq(&dev->zone_lock); + + if (dev->memory_backed) + clear_and_wake_up_bit(zno, dev->zone_locks); } int null_report_zones(struct gendisk *disk, sector_t sector, @@ -322,7 +336,6 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector, return null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors); null_lock_zone(dev, zno); - spin_lock(&dev->zone_dev_lock); switch (zone->cond) { case BLK_ZONE_COND_FULL: @@ -375,9 +388,17 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector, if (zone->cond != BLK_ZONE_COND_EXP_OPEN) zone->cond = BLK_ZONE_COND_IMP_OPEN; - spin_unlock(&dev->zone_dev_lock); + /* + * Memory backing allocation may sleep: release the zone_lock spinlock + * to avoid scheduling in atomic context. Zone operation atomicity is + * still guaranteed through the zone_locks bitmap. + */ + if (dev->memory_backed) + spin_unlock_irq(&dev->zone_lock); ret = null_process_cmd(cmd, REQ_OP_WRITE, sector, nr_sectors); - spin_lock(&dev->zone_dev_lock); + if (dev->memory_backed) + spin_lock_irq(&dev->zone_lock); + if (ret != BLK_STS_OK) goto unlock; @@ -392,7 +413,6 @@ static blk_status_t null_zone_write(struct nullb_cmd *cmd, sector_t sector, ret = BLK_STS_OK; unlock: - spin_unlock(&dev->zone_dev_lock); null_unlock_zone(dev, zno); return ret; @@ -516,9 +536,7 @@ static blk_status_t null_zone_mgmt(struct nullb_cmd *cmd, enum req_opf op, null_lock_zone(dev, i); zone = &dev->zones[i]; if (zone->cond != BLK_ZONE_COND_EMPTY) { - spin_lock(&dev->zone_dev_lock); null_reset_zone(dev, zone); - spin_unlock(&dev->zone_dev_lock); trace_nullb_zone_op(cmd, i, zone->cond); } null_unlock_zone(dev, i); @@ -530,7 +548,6 @@ static blk_status_t null_zone_mgmt(struct nullb_cmd *cmd, enum req_opf op, zone = &dev->zones[zone_no]; null_lock_zone(dev, zone_no); - spin_lock(&dev->zone_dev_lock); switch (op) { case REQ_OP_ZONE_RESET: @@ -550,8 +567,6 @@ static blk_status_t null_zone_mgmt(struct nullb_cmd *cmd, enum req_opf op, break; } - spin_unlock(&dev->zone_dev_lock); - if (ret == BLK_STS_OK) trace_nullb_zone_op(cmd, zone_no, zone->cond);
Commit aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") changed zone locking to using the potentially sleeping wait_on_bit_io() function. This is acceptable when memory backing is enabled as the device queue is in that case marked as blocking, but this triggers a scheduling while in atomic context with memory backing disabled. Fix this by relying solely on the device zone spinlock for zone information protection without temporarily releasing this lock around null_process_cmd() execution in null_zone_write(). This is OK to do since when memory backing is disabled, command processing does not block and the memory backing lock nullb->lock is unused. This solution avoids the overhead of having to mark a zoned null_blk device queue as blocking when memory backing is unused. This patch also adds comments to the zone locking code to explain the unusual locking scheme. Reported-by: kernel test robot <lkp@intel.com> Fixes: aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") Cc: stable@vger.kernel.org Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com> --- drivers/block/null_blk.h | 2 +- drivers/block/null_blk_zoned.c | 47 ++++++++++++++++++++++------------ 2 files changed, 32 insertions(+), 17 deletions(-)