linux-kernel - Re: [PATCH v3 15/15] blk-mq: use hk cpus only when isolcpus=io

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Message-ID: <ZrI5TcaAU82avPZn@fedora>
Date: Tue, 6 Aug 2024 22:55:09 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Daniel Wagner <dwagner@...e.de>
Cc: Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
	Sagi Grimberg <sagi@...mberg.me>,
	Thomas Gleixner <tglx@...utronix.de>,
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Subject: Re: [PATCH v3 15/15] blk-mq: use hk cpus only when isolcpus=io_queue
 is enabled

On Tue, Aug 06, 2024 at 02:06:47PM +0200, Daniel Wagner wrote:
> When isolcpus=io_queue is enabled all hardware queues should run on the
> housekeeping CPUs only. Thus ignore the affinity mask provided by the
> driver. Also we can't use blk_mq_map_queues because it will map all CPUs
> to first hctx unless, the CPU is the same as the hctx has the affinity
> set to, e.g. 8 CPUs with isolcpus=io_queue,2-3,6-7 config

What is the expected behavior if someone still tries to submit IO on isolated
CPUs?

BTW, I don't see any change in blk_mq_get_ctx()/blk_mq_map_queue() in this
patchset, that means one random hctx(or even NULL) may be used for submitting
IO from isolated CPUs, then there can be io hang risk during cpu hotplug, or
kernel panic when submitting bio.

Thanks,
Ming