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Date: Thu, 21 Mar 2024 20:49:23 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Andrew Theurer <atheurer@redhat.com>, Joe Mario <jmario@redhat.com>,
	Sebastian Jug <sejug@redhat.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Bart Van Assche <bvanassche@acm.org>, Tejun Heo <tj@kernel.org>,
	ming.lei@redhat.com
Subject: Re: [PATCH V4] blk-mq: don't schedule block kworker on isolated CPUs
Message-ID: <Zfws04M0p3QUPmPJ@fedora>
References: <20240320023446.882006-1-ming.lei@redhat.com>
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In-Reply-To: <20240320023446.882006-1-ming.lei@redhat.com>

On Wed, Mar 20, 2024 at 10:34:46AM +0800, Ming Lei wrote:
> Kernel parameter of `isolcpus=` or 'nohz_full=' are used to isolate CPUs
> for specific task, and it isn't expected to let block IO disturb these CPUs.
> blk-mq kworker shouldn't be scheduled on isolated CPUs. Also if isolated
> CPUs is run for blk-mq kworker, long block IO latency can be caused.
> 
> Kernel workqueue only respects CPU isolation for WQ_UNBOUND, for bound
> WQ, the responsibility is on user because CPU is specified as WQ API
> parameter, such as mod_delayed_work_on(cpu), queue_delayed_work_on(cpu)
> and queue_work_on(cpu).
> 
> So not run blk-mq kworker on isolated CPUs by removing isolated CPUs
> from hctx->cpumask. Meantime use queue map to check if all CPUs in this
> hw queue are offline instead of hctx->cpumask, this way can avoid any
> cost in fast IO code path, and is safe since hctx->cpumask are only
> used in the two cases.
> 
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Andrew Theurer <atheurer@redhat.com>
> Cc: Joe Mario <jmario@redhat.com>
> Cc: Sebastian Jug <sejug@redhat.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Tejun Heo <tj@kernel.org>
> Tested-by: Joe Mario <jmario@redhat.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> V4:
> 	- improve comment & commit log as suggested by Tim

Hello Jens, Tejun and Guys,

This patch fixes one issue in OpenShift low latency environment, I appreciate
you may take a look at the patch and merge it if you are fine.


Thanks,
Ming