linux-kernel - [PATCH 4.9 02/36] IB/device: Convert ib-comp-wq to be CPU-bound

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Date:   Mon, 14 May 2018 08:48:36 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Sagi Grimberg <sagi@...mberg.me>,
        Doug Ledford <dledford@...hat.com>,
        Raju Rangoju <rajur@...lsio.com>
Subject: [PATCH 4.9 02/36] IB/device: Convert ib-comp-wq to be CPU-bound

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sagi Grimberg <sagi@...mberg.me>

commit b7363e67b23e04c23c2a99437feefac7292a88bc upstream.

This workqueue is used by our storage target mode ULPs
via the new CQ API. Recent observations when working
with very high-end flash storage devices reveal that
UNBOUND workqueue threads can migrate between cpu cores
and even numa nodes (although some numa locality is accounted
for).

While this attribute can be useful in some workloads,
it does not fit in very nicely with the normal
run-to-completion model we usually use in our target-mode
ULPs and the block-mq irq<->cpu affinity facilities.

The whole block-mq concept is that the completion will
land on the same cpu where the submission was performed.
The fact that our submitter thread is migrating cpus
can break this locality.

We assume that as a target mode ULP, we will serve multiple
initiators/clients and we can spread the load enough without
having to use unbound kworkers.

Also, while we're at it, expose this workqueue via sysfs which
is harmless and can be useful for debug.

Signed-off-by: Sagi Grimberg <sagi@...mberg.me>
Reviewed-by: Bart Van Assche <bart.vanassche@...disk.com>--
Signed-off-by: Doug Ledford <dledford@...hat.com>
Cc: Raju  Rangoju <rajur@...lsio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/infiniband/core/device.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/infiniband/core/device.c
+++ b/drivers/infiniband/core/device.c
@@ -999,8 +999,7 @@ static int __init ib_core_init(void)
 		return -ENOMEM;

 	ib_comp_wq = alloc_workqueue("ib-comp-wq",
-			WQ_UNBOUND | WQ_HIGHPRI | WQ_MEM_RECLAIM,
-			WQ_UNBOUND_MAX_ACTIVE);
+			WQ_HIGHPRI | WQ_MEM_RECLAIM | WQ_SYSFS, 0);
 	if (!ib_comp_wq) {
 		ret = -ENOMEM;
 		goto err;