linux-kernel - [PATCH 1/2] block: default to rq

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Date:	Tue, 09 Sep 2014 19:18:01 -0500
From:	Robert Elliott <relliott@...rdog.cce.hp.com>
To:	axboe@...nel.dk, elliott@...com, hch@....de,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] block: default to rq_affinity=2 for blk-mq

From: Robert Elliott <elliott@...com>

One change introduced by blk-mq is that it does all
the completion work in hard irq context rather than
soft irq context.

On a 6 core system, if all interrupts are routed to
one CPU, then you can easily run into this:
* 5 CPUs submitting IOs
* 1 CPU spending 100% of its time in hard irq context
processing IO completions, not able to submit anything
itself

Example with CPU5 receiving all interrupts:
   CPU usage:   CPU0   CPU1   CPU2   CPU3   CPU4   CPU5
        %usr:   0.00   3.03   1.01   2.02   2.00   0.00
        %sys:  14.58  75.76  14.14   4.04  78.00   0.00
        %irq:   0.00   0.00   0.00   1.01   0.00 100.00
       %soft:   0.00   0.00   0.00   0.00   0.00   0.00
%iowait idle:  85.42  21.21  84.85  92.93  20.00   0.00
       %idle:   0.00   0.00   0.00   0.00   0.00   0.00

When the submitting CPUs are forced to process their own
completion interrupts, this steals time from new
submissions and self-throttles them.

Without that, there is no direct feedback to the
submitters to slow down.  The only feedback is:
* reaching max queue depth
* lots of timeouts, resulting in aborts, resets, soft
  lockups and self-detected stalls on CPU5, bogus
  clocksource tsc unstable reports, network
  drop-offs, etc.

The SCSI LLD can set affinity_hint for each of its
interrupts to request that a program like irqbalance
route the interrupts back to the submitting CPU.
The latest version of irqbalance ignores those hints,
though, instead offering an option to run a policy
script that could honor them. Otherwise, it balances
them based on its own algorithms. So, we cannot rely
on this.

Hardware might perform interrupt coalescing to help,
but it cannot help 1 CPU keep up with the work
generated by many other CPUs.

rq_affinity=2 helps by pushing most of the block layer
and SCSI midlayer completion work back to the submitting
CPU (via an IPI).

Change the default rq_affinity=2 under blk-mq
so there's at least some feedback to slow down the
submitters.

Signed-off-by: Robert Elliott <elliott@...com>
---
 include/linux/blkdev.h |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 518b465..9f41a02 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -522,7 +522,8 @@ struct request_queue {
 				 (1 << QUEUE_FLAG_ADD_RANDOM))

 #define QUEUE_FLAG_MQ_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
-				 (1 << QUEUE_FLAG_SAME_COMP))
+				 (1 << QUEUE_FLAG_SAME_COMP)	|	\
+				 (1 << QUEUE_FLAG_SAME_FORCE))

 static inline void queue_lockdep_assert_held(struct request_queue *q)
 {

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