linux-kernel - Re: [PATCH V2] blk-mq: don't schedule block kworker on isolated CPUs

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Date:   Tue, 24 Oct 2023 09:53:49 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Ming Lei <ming.lei@...hat.com>, 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>
Subject: Re: [PATCH V2] blk-mq: don't schedule block kworker on isolated CPUs


On 10/13/23 05:47, Ming Lei wrote:
> @@ -3926,6 +3941,15 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>   		 */
>   		sbitmap_resize(&hctx->ctx_map, hctx->nr_ctx);
>   
> +		/*
> +		 * rule out isolated CPUs from hctx->cpumask for avoiding to
> +		 * run wq worker on isolated CPU
> +		 */
> +		for_each_cpu(cpu, hctx->cpumask) {
> +			if (cpu_is_isolated(cpu))
> +				cpumask_clear_cpu(cpu, hctx->cpumask);
> +		}

What will happen if this code makes hctx->cpumask empty? Code like
blk_mq_first_mapped_cpu() and blk_mq_hctx_next_cpu() assumes that
hctx->cpumask is not empty. There may be other code that assumes that
hctx->cpumask is not empty.

Thanks,

Bart.