Re: [PATCH] blk-mq: implement queue quiesce via percpu_ref for BLK_MQ_F_BLOCKING

[Ming Lei <ming.lei@redhat.com>]

On Fri, Aug 21, 2020 at 01:14:41PM -0700, Sagi Grimberg wrote:
> 
> > In case of BLK_MQ_F_BLOCKING, blk-mq uses SRCU to mark read critical
> > section during dispatching request, then request queue quiesce is based on
> > SRCU. What we want to get is low cost added in fast path.
> > 
> > With percpu-ref, it is cleaner and simpler & enough for implementing queue
> > quiesce. The main requirement is to make sure all read sections to observe
> > QUEUE_FLAG_QUIESCED once blk_mq_quiesce_queue() returns.
> > 
> > Also it becomes much easier to add interface of async queue quiesce.
> > 
> > Meantime memory footprint can be reduced with per-request-queue percpu-ref.
> > 
> >  From implementation viewpoint, in fast path, not see percpu_ref is
> > slower than SRCU, and srcu tree(default option in most distributions)
> > could be slower since memory barrier is required in both lock & unlock,
> > and rcu_read_lock()/rcu_read_unlock() should be much cheap than
> > smp_mb().
> > 
> > 1) percpu_ref just hold the rcu_read_lock, then run a check &
> >     increase/decrease on the percpu variable:
> > 
> >     rcu_read_lock()
> >     if (__ref_is_percpu(ref, &percpu_count))
> > 	this_cpu_inc(*percpu_count);
> >     rcu_read_unlock()
> > 
> > 2) srcu tree:
> >          idx = READ_ONCE(ssp->srcu_idx) & 0x1;
> >          this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
> >          smp_mb(); /* B */  /* Avoid leaking the critical section. */
> > 
> > Also from my test on null_blk(blocking), not observe percpu-ref performs
> > worse than srcu, see the following test:
> > 
> > 1) test steps:
> > 
> > rmmod null_blk > /dev/null 2>&1
> > modprobe null_blk nr_devices=1 submit_queues=1 blocking=1
> > fio --bs=4k --size=512G  --rw=randread --norandommap --direct=1 --ioengine=libaio \
> > 	--iodepth=64 --runtime=60 --group_reporting=1  --name=nullb0 \
> > 	--filename=/dev/nullb0 --numjobs=32
> > 
> > test machine: HP DL380, 16 cpu cores, 2 threads per core, dual
> > sockets/numa, Intel(R) Xeon(R) Silver 4110 CPU @ 2.10GHz
> > 
> > 2) test result:
> > - srcu quiesce: 6063K IOPS
> > - percpu-ref quiesce: 6113K IOPS
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Josh Triplett <josh@joshtriplett.org>
> > Cc: Sagi Grimberg <sagi@grimberg.me>
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
> > Cc: Chao Leng <lengchao@huawei.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > 
> > ---
> > V1:
> > 	- remove SRCU related comment
> > 	- remove RFC
> > 	- not dispatch when the dispatch percpu ref becomes not live
> > 	- add test result on commit log
> > 
> >   block/blk-mq-sysfs.c   |   2 -
> >   block/blk-mq.c         | 104 ++++++++++++++++++++---------------------
> >   block/blk-sysfs.c      |   6 ++-
> >   include/linux/blk-mq.h |   7 ---
> >   include/linux/blkdev.h |   4 ++
> >   5 files changed, 61 insertions(+), 62 deletions(-)
> > 
> > diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
> > index 062229395a50..799db7937105 100644
> > --- a/block/blk-mq-sysfs.c
> > +++ b/block/blk-mq-sysfs.c
> > @@ -38,8 +38,6 @@ static void blk_mq_hw_sysfs_release(struct kobject *kobj)
> >   	cancel_delayed_work_sync(&hctx->run_work);
> > -	if (hctx->flags & BLK_MQ_F_BLOCKING)
> > -		cleanup_srcu_struct(hctx->srcu);
> >   	blk_free_flush_queue(hctx->fq);
> >   	sbitmap_free(&hctx->ctx_map);
> >   	free_cpumask_var(hctx->cpumask);
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 6294fa5c7ed9..e198bd565109 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -220,19 +220,13 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
> >    */
> >   void blk_mq_quiesce_queue(struct request_queue *q)
> >   {
> > -	struct blk_mq_hw_ctx *hctx;
> > -	unsigned int i;
> > -	bool rcu = false;
> > -
> >   	blk_mq_quiesce_queue_nowait(q);
> > -	queue_for_each_hw_ctx(q, hctx, i) {
> > -		if (hctx->flags & BLK_MQ_F_BLOCKING)
> > -			synchronize_srcu(hctx->srcu);
> > -		else
> > -			rcu = true;
> > -	}
> > -	if (rcu)
> > +	if (q->tag_set->flags & BLK_MQ_F_BLOCKING) {
> > +		percpu_ref_kill(&q->dispatch_counter);
> > +		wait_event(q->mq_quiesce_wq,
> > +				percpu_ref_is_zero(&q->dispatch_counter));
> 
> Looking at the q_usage_counter percpu, it's protected with the
> mq_freeze_lock and mq_freeze_depth, the fact that it's not protected
> here makes this non-nesting, which scares me... We had issues before
> in this area...

In theory, percpu_ref_kill() could be called nested, since percpu_ref_switch_lock
covers it. However, the warning in percpu_ref_kill_and_confirm() may be
triggered.

So looks we just need one mutex to cover quiesce and unquiesce. If queue
is being quiesced, return immediately from blk_mq_quiesce_queue().

> 
> > +	} else
> >   		synchronize_rcu();
> >   }
> >   EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
> > @@ -248,6 +242,9 @@ void blk_mq_unquiesce_queue(struct request_queue *q)
> >   {
> >   	blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
> > +	if (q->tag_set->flags & BLK_MQ_F_BLOCKING)
> > +		percpu_ref_resurrect(&q->dispatch_counter);
> 
> Same comment here...
> 
> > +
> >   	/* dispatch requests which are inserted during quiescing */
> >   	blk_mq_run_hw_queues(q, true);
> >   }
> > @@ -699,24 +696,21 @@ void blk_mq_complete_request(struct request *rq)
> >   }
> >   EXPORT_SYMBOL(blk_mq_complete_request);
> > -static void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx)
> > -	__releases(hctx->srcu)
> > +static void hctx_unlock(struct blk_mq_hw_ctx *hctx)
> >   {
> >   	if (!(hctx->flags & BLK_MQ_F_BLOCKING))
> >   		rcu_read_unlock();
> >   	else
> > -		srcu_read_unlock(hctx->srcu, srcu_idx);
> > +		percpu_ref_put(&hctx->queue->dispatch_counter);
> >   }
> > -static void hctx_lock(struct blk_mq_hw_ctx *hctx, int *srcu_idx)
> > -	__acquires(hctx->srcu)
> > +static inline bool hctx_lock(struct blk_mq_hw_ctx *hctx)
> >   {
> >   	if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
> > -		/* shut up gcc false positive */
> > -		*srcu_idx = 0;
> >   		rcu_read_lock();
> > +		return true;
> >   	} else
> > -		*srcu_idx = srcu_read_lock(hctx->srcu);
> > +		return percpu_ref_tryget_live(&hctx->queue->dispatch_counter);
> >   }
> >   /**
> > @@ -1495,8 +1489,6 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
> >    */
> >   static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> >   {
> > -	int srcu_idx;
> > -
> >   	/*
> >   	 * We should be running this queue from one of the CPUs that
> >   	 * are mapped to it.
> > @@ -1530,9 +1522,10 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
> >   	might_sleep_if(hctx->flags & BLK_MQ_F_BLOCKING);
> > -	hctx_lock(hctx, &srcu_idx);
> > -	blk_mq_sched_dispatch_requests(hctx);
> > -	hctx_unlock(hctx, srcu_idx);
> > +	if (hctx_lock(hctx)) {
> > +		blk_mq_sched_dispatch_requests(hctx);
> > +		hctx_unlock(hctx);
> > +	}
> 
> Maybe invert?
> 	if (!hctx_lock(hctx))
> 		return;
> 	blk_mq_sched_dispatch_requests(hctx);
> 	hctx_unlock(hctx);
> 
> I think we need a comment to why this is OK, both in the change log
> and in the code.

Fine, it is because hctx_lock() failure can be thought as queue being
quiesced.

> 
> >   }
> >   static inline int blk_mq_first_mapped_cpu(struct blk_mq_hw_ctx *hctx)
> > @@ -1644,7 +1637,6 @@ EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
> >    */
> >   void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
> >   {
> > -	int srcu_idx;
> >   	bool need_run;
> >   	/*
> > @@ -1655,10 +1647,12 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
> >   	 * And queue will be rerun in blk_mq_unquiesce_queue() if it is
> >   	 * quiesced.
> >   	 */
> > -	hctx_lock(hctx, &srcu_idx);
> > +	if (!hctx_lock(hctx))
> > +		return;
> > +
> >   	need_run = !blk_queue_quiesced(hctx->queue) &&
> >   		blk_mq_hctx_has_pending(hctx);
> > -	hctx_unlock(hctx, srcu_idx);
> > +	hctx_unlock(hctx);
> >   	if (need_run)
> >   		__blk_mq_delay_run_hw_queue(hctx, async, 0);
> > @@ -1997,7 +1991,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> >   	bool run_queue = true;
> >   	/*
> > -	 * RCU or SRCU read lock is needed before checking quiesced flag.
> > +	 * hctx_lock() is needed before checking quiesced flag.
> >   	 *
> >   	 * When queue is stopped or quiesced, ignore 'bypass_insert' from
> >   	 * blk_mq_request_issue_directly(), and return BLK_STS_OK to caller,
> > @@ -2045,11 +2039,13 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> >   		struct request *rq, blk_qc_t *cookie)
> >   {
> >   	blk_status_t ret;
> > -	int srcu_idx;
> >   	might_sleep_if(hctx->flags & BLK_MQ_F_BLOCKING);
> > -	hctx_lock(hctx, &srcu_idx);
> > +	if (!hctx_lock(hctx)) {
> > +		blk_mq_sched_insert_request(rq, false, false, false);
> > +		return;
> > +	}
> 
> I think we want the same flow for both modes. Maybe a preparation patch
> that lifts the checks in __blk_mq_try_issue_directly to do this, and
> then have the same flow with the hctx_lock failure (with a comment
> explaining this).

Looks a good idea.

> 
> >   	ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false, true);
> >   	if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE)
> > @@ -2057,19 +2053,21 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> >   	else if (ret != BLK_STS_OK)
> >   		blk_mq_end_request(rq, ret);
> > -	hctx_unlock(hctx, srcu_idx);
> > +	hctx_unlock(hctx);
> >   }
> >   blk_status_t blk_mq_request_issue_directly(struct request *rq, bool last)
> >   {
> >   	blk_status_t ret;
> > -	int srcu_idx;
> >   	blk_qc_t unused_cookie;
> >   	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> > -	hctx_lock(hctx, &srcu_idx);
> > +	if (!hctx_lock(hctx)) {
> > +		blk_mq_sched_insert_request(rq, false, false, false);
> > +		return BLK_STS_OK;
> > +	}
> 
> Same comment here.
> 
> >   	ret = __blk_mq_try_issue_directly(hctx, rq, &unused_cookie, true, last);
> > -	hctx_unlock(hctx, srcu_idx);
> > +	hctx_unlock(hctx);
> >   	return ret;
> >   }
> > @@ -2600,20 +2598,6 @@ static void blk_mq_exit_hw_queues(struct request_queue *q,
> >   	}
> >   }
> > -static int blk_mq_hw_ctx_size(struct blk_mq_tag_set *tag_set)
> > -{
> > -	int hw_ctx_size = sizeof(struct blk_mq_hw_ctx);
> > -
> > -	BUILD_BUG_ON(ALIGN(offsetof(struct blk_mq_hw_ctx, srcu),
> > -			   __alignof__(struct blk_mq_hw_ctx)) !=
> > -		     sizeof(struct blk_mq_hw_ctx));
> > -
> > -	if (tag_set->flags & BLK_MQ_F_BLOCKING)
> > -		hw_ctx_size += sizeof(struct srcu_struct);
> > -
> > -	return hw_ctx_size;
> > -}
> > -
> >   static int blk_mq_init_hctx(struct request_queue *q,
> >   		struct blk_mq_tag_set *set,
> >   		struct blk_mq_hw_ctx *hctx, unsigned hctx_idx)
> > @@ -2651,7 +2635,7 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set,
> >   	struct blk_mq_hw_ctx *hctx;
> >   	gfp_t gfp = GFP_NOIO | __GFP_NOWARN | __GFP_NORETRY;
> > -	hctx = kzalloc_node(blk_mq_hw_ctx_size(set), gfp, node);
> > +	hctx = kzalloc_node(sizeof(struct blk_mq_hw_ctx), gfp, node);
> >   	if (!hctx)
> >   		goto fail_alloc_hctx;
> > @@ -2693,8 +2677,6 @@ blk_mq_alloc_hctx(struct request_queue *q, struct blk_mq_tag_set *set,
> >   	if (!hctx->fq)
> >   		goto free_bitmap;
> > -	if (hctx->flags & BLK_MQ_F_BLOCKING)
> > -		init_srcu_struct(hctx->srcu);
> >   	blk_mq_hctx_kobj_init(hctx);
> >   	return hctx;
> > @@ -3171,6 +3153,13 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
> >   	mutex_unlock(&q->sysfs_lock);
> >   }
> > +static void blk_mq_dispatch_counter_release(struct percpu_ref *ref)
> > +{
> > +	struct request_queue *q = container_of(ref, struct request_queue,
> > +				dispatch_counter);
> > +	wake_up_all(&q->mq_quiesce_wq);
> > +}
> > +
> >   struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> >   						  struct request_queue *q,
> >   						  bool elevator_init)
> > @@ -3187,6 +3176,14 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> >   	if (blk_mq_alloc_ctxs(q))
> >   		goto err_poll;
> > +	if (set->flags & BLK_MQ_F_BLOCKING) {
> > +		init_waitqueue_head(&q->mq_quiesce_wq);
> > +		if (percpu_ref_init(&q->dispatch_counter,
> > +					blk_mq_dispatch_counter_release,
> > +					PERCPU_REF_ALLOW_REINIT, GFP_KERNEL))
> > +			goto err_hctxs;
> > +	}
> > +
> >   	/* init q->mq_kobj and sw queues' kobjects */
> >   	blk_mq_sysfs_init(q);
> > @@ -3195,7 +3192,7 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> >   	blk_mq_realloc_hw_ctxs(set, q);
> >   	if (!q->nr_hw_queues)
> > -		goto err_hctxs;
> > +		goto err_dispatch_counter;
> >   	INIT_WORK(&q->timeout_work, blk_mq_timeout_work);
> >   	blk_queue_rq_timeout(q, set->timeout ? set->timeout : 30 * HZ);
> > @@ -3229,6 +3226,9 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> >   	return q;
> > +err_dispatch_counter:
> > +	if (set->flags & BLK_MQ_F_BLOCKING)
> > +		percpu_ref_exit(&q->dispatch_counter);
> >   err_hctxs:
> >   	kfree(q->queue_hw_ctx);
> >   	q->nr_hw_queues = 0;
> > diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> > index 7dda709f3ccb..56b6c045e30c 100644
> > --- a/block/blk-sysfs.c
> > +++ b/block/blk-sysfs.c
> > @@ -941,9 +941,13 @@ static void blk_release_queue(struct kobject *kobj)
> >   	blk_queue_free_zone_bitmaps(q);
> > -	if (queue_is_mq(q))
> > +	if (queue_is_mq(q)) {
> >   		blk_mq_release(q);
> > +		if (q->tag_set->flags & BLK_MQ_F_BLOCKING)
> > +			percpu_ref_exit(&q->dispatch_counter);
> > +	}
> > +
> >   	blk_trace_shutdown(q);
> >   	mutex_lock(&q->debugfs_mutex);
> >   	debugfs_remove_recursive(q->debugfs_dir);
> > diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> > index 9d2d5ad367a4..ea3461298de5 100644
> > --- a/include/linux/blk-mq.h
> > +++ b/include/linux/blk-mq.h
> > @@ -169,13 +169,6 @@ struct blk_mq_hw_ctx {
> >   	 * q->unused_hctx_list.
> >   	 */
> >   	struct list_head	hctx_list;
> > -
> > -	/**
> > -	 * @srcu: Sleepable RCU. Use as lock when type of the hardware queue is
> > -	 * blocking (BLK_MQ_F_BLOCKING). Must be the last member - see also
> > -	 * blk_mq_hw_ctx_size().
> > -	 */
> > -	struct srcu_struct	srcu[];
> >   };
> >   /**
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index bb5636cc17b9..5fa8bc1bb7a8 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -572,6 +572,10 @@ struct request_queue {
> >   	struct list_head	tag_set_list;
> >   	struct bio_set		bio_split;
> > +	/* only used for BLK_MQ_F_BLOCKING */
> > +	struct percpu_ref	dispatch_counter;
> 
> Can this be moved to be next to the q_usage_counter? they
> will be taken together for blocking drivers...

I don't think it is a good idea, at least hctx->srcu is put at the end
of hctx, do you want to move it at beginning of hctx?

.q_usage_counter should have been put in the 1st cacheline of
request queue. If it is moved to the 1st cacheline of request queue,
we shouldn't put 'dispatch_counter' there, because it may hurt other
non-blocking drivers.

> 
> Also maybe a better name is needed here since it's just
> for blocking hctxs.
> 
> > +	wait_queue_head_t	mq_quiesce_wq;
> > +
> >   	struct dentry		*debugfs_dir;
> >   #ifdef CONFIG_BLK_DEBUG_FS
> > 
> 
> What I think is needed here is at a minimum test quiesce/unquiesce loops
> during I/O. code auditing is not enough, there may be driver assumptions
> broken with this change (although I hope there shouldn't be).

We have elevator switch / updating nr_request stress test, and both relies
on quiesce/unquiesce, and I did run such test with this patch.


thanks, 
Ming