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Message-ID: <293c16d2-f556-4d42-ab06-2e0acf1cc601@acm.org>
Date:   Tue, 24 Oct 2023 09:53:49 -0700
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Subject: Re: [PATCH V2] blk-mq: don't schedule block kworker on isolated CPUs
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To:     Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>
Cc:     linux-block@vger.kernel.org, Tejun Heo <tj@kernel.org>,
        linux-kernel@vger.kernel.org, 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>
References: <20231013124758.1492796-1-ming.lei@redhat.com>
From:   Bart Van Assche <bvanassche@acm.org>
In-Reply-To: <20231013124758.1492796-1-ming.lei@redhat.com>
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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.