[PATCH 5/6] blk-mq: Use all available hardware queues

[Bart Van Assche <bvanassche@acm.org>]

Suppose that a system has two CPU sockets, three cores per socket,
that it does not support hyperthreading and that four hardware
queues are provided by a block driver. With the current algorithm
this will lead to the following assignment of CPU cores to hardware
queues:

  HWQ 0: 0 1
  HWQ 1: 2 3
  HWQ 2: 4 5
  HWQ 3: (none)

This patch changes the queue assignment into:

  HWQ 0: 0 1
  HWQ 1: 2
  HWQ 2: 3 4
  HWQ 3: 5

In other words, this patch has the following three effects:
- All four hardware queues are used instead of only three.
- CPU cores are spread more evenly over hardware queues. For the
  above example the range of the number of CPU cores associated
  with a single HWQ is reduced from [0..2] to [1..2].
- If the number of HWQ's is a multiple of the number of CPU sockets
  it is now guaranteed that all CPU cores associated with a single
  HWQ reside on the same CPU socket.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Cc: Jens Axboe <axboe@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@canonical.com>
Cc: Alexander Gordeev <agordeev@redhat.com>
---
 block/blk-mq-cpumap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index 1065d7c..8e56455 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -17,7 +17,7 @@
 static int cpu_to_queue_index(unsigned int nr_cpus, unsigned int nr_queues,
 			      const int cpu)
 {
-	return cpu / ((nr_cpus + nr_queues - 1) / nr_queues);
+	return cpu * nr_queues / nr_cpus;
 }
 
 static int get_first_sibling(unsigned int cpu)
-- 
2.1.2