From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754688AbbCaO2F (ORCPT ); Tue, 31 Mar 2015 10:28:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42731 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751289AbbCaO2C (ORCPT ); Tue, 31 Mar 2015 10:28:02 -0400 Date: Tue, 31 Mar 2015 10:27:26 -0400 From: Rik van Riel To: axboe@kernel.org Cc: fweisbec@redhat.com, mingo@kernel.org, linux-kernel@vger.kernel.org, lcapitulino@redhat.com, mtosatti@redhat.com Subject: [PATCH RFC] nohz,blk-mq: do not create blk-mq workqueues on nohz dedicated CPUs Message-ID: <20150331102726.076a6860@annuminas.surriel.com> Organization: Red Hat, Inc. MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org CPUs with nohz_full do not want disruption from timer interrupts, or other random system things. This includes block mq work. There is another issue with block mq vs. realtime tasks that run 100% of the time, which is not uncommon on systems that have CPUs dedicated to real time use with isolcpus= and nohz_full= Specifically, on systems like that, a block work item may never get to run, which could lead to filesystems getting stuck forever. We can avoid both issues by not scheduling blk-mq workqueues on cpus in nohz_full mode. Question for Jens: should we try to spread out the load for currently offline and nohz CPUs across the remaining CPUs in the system, to get the full benefit of blk-mq in these situations? If so, do you have any preference on how I should implement that? Cc: Frederic Weisbecker Cc: Ingo Molnar Cc: Jens Axboe Signed-off-by: Rik van Riel --- block/blk-mq.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/block/blk-mq.c b/block/blk-mq.c index 4f4bea21052e..1004d6817fa4 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -21,6 +21,7 @@ #include #include #include +#include #include @@ -1760,6 +1761,10 @@ static void blk_mq_init_cpu_queues(struct request_queue *q, if (!cpu_online(i)) continue; + /* Do not schedule work on nohz full dedicated CPUs. */ + if (tick_nohz_full_cpu(i)) + continue; + hctx = q->mq_ops->map_queue(q, i); cpumask_set_cpu(i, hctx->cpumask); hctx->nr_ctx++;