linux-kernel - Re: [PATCH 04/14] blk-mq: Do not limit number of queues to 'nr_cpu

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Date:   Tue, 20 Sep 2016 13:44:36 +0200
From:   Alexander Gordeev <agordeev@...hat.com>
To:     Omar Sandoval <osandov@...ndov.com>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH 04/14] blk-mq: Do not limit number of queues to
 'nr_cpu_ids' in allocations

On Mon, Sep 19, 2016 at 10:48:49AM -0700, Omar Sandoval wrote:
> On Sun, Sep 18, 2016 at 09:37:14AM +0200, Alexander Gordeev wrote:
> > Currently maximum number of used hardware queues is limited to
> > number of CPUs in the system. However, using 'nr_cpu_ids' as
> > the limit for (de-)allocations of data structures instead of
> > existing data structures' counters (a) worsens readability and
> > (b) leads to unused memory when number of hardware queues is
> > less than number of CPUs.
> > 
> > CC: linux-block@...r.kernel.org
> > Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> > ---
> >  block/blk-mq.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 276ec7b..2c77b68 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2054,8 +2054,8 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
> >  	if (!q->queue_ctx)
> >  		goto err_exit;
> >  
> > -	q->queue_hw_ctx = kzalloc_node(nr_cpu_ids * sizeof(*(q->queue_hw_ctx)),
> > -						GFP_KERNEL, set->numa_node);
> > +	q->queue_hw_ctx = kzalloc_node(set->nr_hw_queues *
> > +			sizeof(*(q->queue_hw_ctx)), GFP_KERNEL, set->numa_node);
> >  	if (!q->queue_hw_ctx)
> >  		goto err_percpu;
> >  
> > @@ -2319,7 +2319,7 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
> >  	if (set->nr_hw_queues > nr_cpu_ids)
> >  		set->nr_hw_queues = nr_cpu_ids;
> >  
> > -	set->tags = kzalloc_node(nr_cpu_ids * sizeof(struct blk_mq_tags *),
> > +	set->tags = kzalloc_node(set->nr_hw_queues * sizeof(*set->tags),
> >  				 GFP_KERNEL, set->numa_node);
> >  	if (!set->tags)
> >  		return -ENOMEM;
> > @@ -2360,7 +2360,7 @@ void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
> >  {
> >  	int i;
> >  
> > -	for (i = 0; i < nr_cpu_ids; i++) {
> > +	for (i = 0; i < set->nr_hw_queues; i++) {
> >  		if (set->tags[i])
> >  			blk_mq_free_rq_map(set, set->tags[i], i);
> >  	}
> 
> I don't think this is safe since we might increase the number of
> hardware queues (blk_mq_update_nr_hw_queues()).

It is safe, because blk_mq_update_nr_hw_queues() limits number of
hardware queues to nr_cpu_ids. But still using nr_cpu_ids is wrong,
because (a) set->nr_hw_queues could be less than nr_cpu_ids and
(b) it is set->nr_hw_queues counter that tracks size of the array.

> -- 
> Omar