linux-kernel - Re: [PATCH V3] blk-mq: don't schedule block kworker on isolated CPUs

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Date:   Mon, 6 Nov 2023 09:14:57 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
        Andrew Theurer <atheurer@...hat.com>,
        Joe Mario <jmario@...hat.com>,
        Sebastian Jug <sejug@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Bart Van Assche <bvanassche@....org>, ming.lei@...hat.com
Subject: Re: [PATCH V3] blk-mq: don't schedule block kworker on isolated CPUs

On Wed, Oct 25, 2023 at 10:57:37AM +0800, Ming Lei wrote:
> Kernel parameter of `isolcpus=` or 'nohz_full=' are used for isolating CPUs
> for specific task, and user often won't want block IO to disturb these CPUs,
> also long IO latency may be caused if blk-mq kworker is scheduled on these
> isolated CPUs.
> 
> Kernel workqueue only respects this limit for WQ_UNBOUND, for bound wq,
> the responsibility should be on wq user.
> 
> So don't not run block kworker on isolated CPUs by ruling out isolated CPUs
> from hctx->cpumask. Meantime in cpuhp handler, use queue map to check if
> all CPUs in this hw queue are offline, this way can avoid any cost in fast
> IO code path.
> 
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Andrew Theurer <atheurer@...hat.com>
> Cc: Joe Mario <jmario@...hat.com>
> Cc: Sebastian Jug <sejug@...hat.com>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: Bart Van Assche <bvanassche@....org>
> Signed-off-by: Ming Lei <ming.lei@...hat.com>
> ---
> 
> V3:
> 	- avoid to check invalid cpu as reported by Bart
> 	- take current cpu(to be offline, not done yet) into account
> 	- simplify blk_mq_hctx_has_online_cpu()

Hello Jens and Guys,

Ping...

thanks,
Ming