Re: [PATCH] blk-mq: balance mapping between CPUs and queues

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

On Thu, Jul 25, 2019 at 05:18:08PM +0800, Ming Lei wrote:
> On Thu, Jul 25, 2019 at 04:35:30PM +0800, Bob Liu wrote:
> > On 7/25/19 4:26 PM, Ming Lei wrote:
> > > Spread queues among present CPUs first, then building the mapping
> > > on other non-present CPUs.
> > > 
> > > So we can minimize count of dead queues which are mapped by un-present
> > > CPUs only. Then bad IO performance can be avoided by this unbalanced
> > > mapping between CPUs and queues.
> > > 
> > > The similar policy has been applied on Managed IRQ affinity.
> > > 
> > > Reported-by: Yi Zhang <yi.zhang@redhat.com>
> > > Cc: Yi Zhang <yi.zhang@redhat.com>
> > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > ---
> > >  block/blk-mq-cpumap.c | 34 +++++++++++++++++++++++-----------
> > >  1 file changed, 23 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> > > index f945621a0e8f..e217f3404dc7 100644
> > > --- a/block/blk-mq-cpumap.c
> > > +++ b/block/blk-mq-cpumap.c
> > > @@ -15,10 +15,9 @@
> > >  #include "blk.h"
> > >  #include "blk-mq.h"
> > >  
> > > -static int cpu_to_queue_index(struct blk_mq_queue_map *qmap,
> > > -			      unsigned int nr_queues, const int cpu)
> > > +static int queue_index(struct blk_mq_queue_map *qmap, const int q)
> > >  {
> > > -	return qmap->queue_offset + (cpu % nr_queues);
> > > +	return qmap->queue_offset + q;
> > >  }
> > >  
> > >  static int get_first_sibling(unsigned int cpu)
> > > @@ -36,23 +35,36 @@ int blk_mq_map_queues(struct blk_mq_queue_map *qmap)
> > >  {
> > >  	unsigned int *map = qmap->mq_map;
> > >  	unsigned int nr_queues = qmap->nr_queues;
> > > -	unsigned int cpu, first_sibling;
> > > +	unsigned int cpu, first_sibling, q = 0;
> > > +
> > > +	for_each_possible_cpu(cpu)
> > > +		map[cpu] = -1;
> > > +
> > > +	/*
> > > +	 * Spread queues among present CPUs first for minimizing
> > > +	 * count of dead queues which are mapped by all un-present CPUs
> > > +	 */
> > > +	for_each_present_cpu(cpu) {
> > > +		if (q >= nr_queues)
> > > +			break;
> > > +		map[cpu] = queue_index(qmap, q++);
> > > +	}
> > >  
> > >  	for_each_possible_cpu(cpu) {
> > > +		if (map[cpu] != -1)
> > > +			continue;
> > >  		/*
> > >  		 * First do sequential mapping between CPUs and queues.
> > >  		 * In case we still have CPUs to map, and we have some number of
> > >  		 * threads per cores then map sibling threads to the same queue
> > >  		 * for performance optimizations.
> > >  		 */
> > > -		if (cpu < nr_queues) {
> > > -			map[cpu] = cpu_to_queue_index(qmap, nr_queues, cpu);
> > 
> > Why not keep this similarly? 
> 
> Because the sequential mapping has been done already among present CPUs.
> 
> > 
> > > +		first_sibling = get_first_sibling(cpu);
> > > +		if (first_sibling == cpu) {
> > > +			map[cpu] = queue_index(qmap, q);
> > > +			q = (q + 1) % nr_queues;
> > >  		} else {
> > > -			first_sibling = get_first_sibling(cpu);
> > > -			if (first_sibling == cpu)
> > > -				map[cpu] = cpu_to_queue_index(qmap, nr_queues, cpu);
> > > -			else
> > > -				map[cpu] = map[first_sibling];
> > > +			map[cpu] = map[first_sibling];
> > 
> > Then no need to share queue if nr_queues is enough for all possible cpu.
> 
> I am not sure I follow your idea. There isn't 'enough' stuff wrt.
> nr_queues, which is just usually <= nr_queues.

The 2nd 'nr_queues' should have been 'nr_cpu_ids'.


thanks,
Ming