Subject: RE: [PATCH mlx5-next] RDMA/mlx5: Don't use cached IRQ affinity mask

From: "Steve Wise" <[email-masked]>
Date: Wed, 18 Jul 2018 09:25:54 -0500

> -----Original Message-----
> From: Max Gurtovoy <[email-masked]>
> Sent: Wednesday, July 18, 2018 9:14 AM
> To: Sagi Grimberg <[email-masked]>; Steve Wise
> <[email-masked]>; 'Leon Romanovsky' <[email-masked]>
> Cc: 'Doug Ledford' <[email-masked]>; 'Jason Gunthorpe'
> <[email-masked]>; 'RDMA mailing list' <[email-masked]>;
> 'Saeed Mahameed' <[email-masked]>; 'linux-netdev'
> <[email-masked]>
> Subject: Re: [PATCH mlx5-next] RDMA/mlx5: Don't use cached IRQ affinity
> mask
> 
> 
> 
> On 7/18/2018 2:38 PM, Sagi Grimberg wrote:
> >
> >>> IMO we must fulfil the user wish to connect to N queues and not reduce
> >>> it because of affinity overlaps. So in order to push Leon's patch we
> >>> must also fix the blk_mq_rdma_map_queues to do a best effort
> mapping
> >>> according the affinity and map the rest in naive way (in that way we
> >>> will *always* map all the queues).
> >>
> >> That is what I would expect also.   For example, in my node, where
> >> there are
> >> 16 cpus, and 2 numa nodes, I observe much better nvmf IOPS
> performance by
> >> setting up my 16 driver completion event queues such that each is
> >> bound to a
> >> node-local cpu.  So I end up with each nodel-local cpu having 2 queues
> >> bound
> >> to it.   W/O adding support in iw_cxgb4 for ib_get_vector_affinity(),
> >> this
> >> works fine.   I assumed adding ib_get_vector_affinity() would allow
> >> this to
> >> all "just work" by default, but I'm running into this connection failure
> >> issue.
> >>
> >> I don't understand exactly what the blk_mq layer is trying to do, but I
> >> assume it has ingress event queues and processing that it trying to align
> >> with the drivers ingress cq event handling, so everybody stays on the
> >> same
> >> cpu (or at least node).   But something else is going on.  Is there
> >> documentation on how this works somewhere?
> >
> > Does this (untested) patch help?
> 
> I'm not sure (I'll test it tomorrow) because the issue is the unmapped
> queues and not the cpus.
> for example, if the affinity of q=6 and q=12 returned the same cpumask
> than q=6 will not be mapped and will fail to connect.
> 
> > --
> > diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> > index 3eb169f15842..dbe962cb537d 100644
> > --- a/block/blk-mq-cpumap.c
> > +++ b/block/blk-mq-cpumap.c
> > @@ -30,29 +30,34 @@ static int get_first_sibling(unsigned int cpu)
> >          return cpu;
> >   }
> >
> > -int blk_mq_map_queues(struct blk_mq_tag_set *set)
> > +void blk_mq_map_queue(struct blk_mq_tag_set *set, unsigned int cpu)
> >   {

There is already a static inline function named blk_mq_map_queue() in block/blk-mq.h.  Did you mean to replace that?  Or is this just a function name conflict?

> >          unsigned int *map = set->mq_map;
> >          unsigned int nr_queues = set->nr_hw_queues;
> > -       unsigned int cpu, first_sibling;
> > +       unsigned int first_sibling;
> >
> > -       for_each_possible_cpu(cpu) {
> > -               /*
> > -                * 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
> > -                * performace optimizations.
> > -                */
> > -               if (cpu < nr_queues) {
> > +       /*
> > +        * 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
> > +        * performace optimizations.
> > +        */
> > +       if (cpu < nr_queues) {
> > +               map[cpu] = cpu_to_queue_index(nr_queues, cpu);
> > +       } else {
> > +               first_sibling = get_first_sibling(cpu);
> > +               if (first_sibling == cpu)
> >                          map[cpu] = cpu_to_queue_index(nr_queues, cpu);
> > -               } else {
> > -                       first_sibling = get_first_sibling(cpu);
> > -                       if (first_sibling == cpu)
> > -                               map[cpu] = cpu_to_queue_index(nr_queues,
> > cpu);
> > -                       else
> > -                               map[cpu] = map[first_sibling];
> > -               }
> > +               else
> > +                       map[cpu] = map[first_sibling];
> >          }
> > +}
> > +EXPORT_SYMBOL_GPL(blk_mq_map_queue);
> > +
> > +int blk_mq_map_queues(struct blk_mq_tag_set *set)
> > +{
> > +       for_each_possible_cpu(cpu)
> > +                blk_mq_map_queue(set, cpu);
> >
> >          return 0;
> >   }
> > diff --git a/block/blk-mq-rdma.c b/block/blk-mq-rdma.c
> > index 996167f1de18..5e91789bea5b 100644
> > --- a/block/blk-mq-rdma.c
> > +++ b/block/blk-mq-rdma.c
> > @@ -35,6 +35,10 @@ int blk_mq_rdma_map_queues(struct
> blk_mq_tag_set *set,
> >          const struct cpumask *mask;
> >          unsigned int queue, cpu;
> >
> > +       /* reset all to  */
> > +       for_each_possible_cpu(cpu)
> > +               set->mq_map[cpu] = UINT_MAX;
> > +
> >          for (queue = 0; queue < set->nr_hw_queues; queue++) {
> >                  mask = ib_get_vector_affinity(dev, first_vec + queue);
> >                  if (!mask)
> > @@ -44,6 +48,11 @@ int blk_mq_rdma_map_queues(struct
> blk_mq_tag_set *set,
> >                          set->mq_map[cpu] = queue;
> >          }
> >
> > +       for_each_possible_cpu(cpu) {
> > +               if (set->mq_map[cpu] == UINT_MAX)
> > +                       blk_mq_map_queue(set, cpu);
> > +       }
> > +
> >          return 0;
> >
> >   fallback:
> > diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> > index e3147eb74222..7a9848a82475 100644
> > --- a/include/linux/blk-mq.h
> > +++ b/include/linux/blk-mq.h
> > @@ -283,6 +283,7 @@ int blk_mq_freeze_queue_wait_timeout(struct
> > request_queue *q,
> >                                       unsigned long timeout);
> >
> >   int blk_mq_map_queues(struct blk_mq_tag_set *set);
> > +void blk_mq_map_queue(struct blk_mq_tag_set *set, unsigned int cpu);
> >   void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int
> > nr_hw_queues);
> >
> >   void blk_mq_quiesce_queue_nowait(struct request_queue *q);
> > --
> >
> > It really is still a best effort thing...