Re: [PATCH] block: build default queue map via irq_create_affinity_masks

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

On Thu, Jul 01, 2021 at 11:06:57AM +0100, John Garry wrote:
> On 30/06/2021 04:51, Ming Lei wrote:
> > The default queue mapping builder of blk_mq_map_queues doesn't take NUMA
> > topo into account, so the built mapping is pretty bad, since CPUs
> > belonging to different NUMA node are assigned to same queue. It is
> > observed that IOPS drops by ~30% when running two jobs on same hctx
> > of null_blk from two CPUs belonging to two NUMA nodes compared with
> > from same NUMA node.
> > 
> > Address the issue by reusing irq_create_affinity_masks() for building
> > the default queue mapping, so that we can re-use the mapping created
> > for managed irq.
> > 
> > Lots of drivers may benefit from the change, such as nvme pci poll,
> > nvme tcp, ...
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >   block/blk-mq-cpumap.c | 60 +++++++++----------------------------------
> >   1 file changed, 12 insertions(+), 48 deletions(-)
> > 
> > diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
> > index 3db84d3197f1..946e373296a3 100644
> > --- a/block/blk-mq-cpumap.c
> > +++ b/block/blk-mq-cpumap.c
> > @@ -10,67 +10,31 @@
> >   #include <linux/mm.h>
> >   #include <linux/smp.h>
> >   #include <linux/cpu.h>
> > +#include <linux/interrupt.h>
> 
> Similar to what Christoph mentioned, seems strange to be including
> interrupt.h

The ideal way is to abstract & move the affinity building code into lib/,
but it needs to refactor kernel/irq/affinity.c a bit.

Also here each queue means one blk-mq hw queue, it is still not too
strange to associate it with interrupt and re-use the interrupt affinity
building code.

Let's see how Thomas thinks about this usage.

> 
> >   #include <linux/blk-mq.h>
> >   #include "blk.h"
> >   #include "blk-mq.h"
> > -static int queue_index(struct blk_mq_queue_map *qmap,
> > -		       unsigned int nr_queues, const int q)
> > -{
> > -	return qmap->queue_offset + (q % nr_queues);
> > -}
> > -
> > -static int get_first_sibling(unsigned int cpu)
> > -{
> > -	unsigned int ret;
> > -
> > -	ret = cpumask_first(topology_sibling_cpumask(cpu));
> > -	if (ret < nr_cpu_ids)
> > -		return ret;
> > -
> > -	return cpu;
> > -}
> > -
> >   int blk_mq_map_queues(struct blk_mq_queue_map *qmap)
> >   {
> > +	struct irq_affinity_desc *masks = NULL;
> > +	struct irq_affinity affd = {0};
> 
> should this be simply {}? I forget...

I think both should be fine, and two usages can be found in kernel code.

> 
> >   	unsigned int *map = qmap->mq_map;
> >   	unsigned int nr_queues = qmap->nr_queues;
> > -	unsigned int cpu, first_sibling, q = 0;
> > +	unsigned int q;
> > -	for_each_possible_cpu(cpu)
> > -		map[cpu] = -1;
> > +	masks = irq_create_affinity_masks(nr_queues, &affd);
> > +	if (!masks)
> > +		return -ENOMEM;
> 
> should we fall back on something else here? Seems that this function does
> not fail just because out of memory.

The default case is nr_set == 1, so the only failure is out of
memory, and irq_create_affinity_masks() basically creates cpumask for
each vector/queue and assigns possible CPUs among these vector/queue.


Thanks,
Ming