linux-kernel - Re: [PATCH v3] block: Limit number of items taken from the I/O scheduler in one go

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Date:   Fri, 24 Apr 2020 09:17:23 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Salman Qazi <sqazi@...gle.com>, Ming Lei <ming.lei@...hat.com>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jesse Barnes <jsbarnes@...gle.com>,
        Gwendal Grignou <gwendal@...gle.com>,
        Hannes Reinecke <hare@...e.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3] block: Limit number of items taken from the I/O
 scheduler in one go

On 4/24/20 9:03 AM, Salman Qazi wrote:
> Flushes bypass the I/O scheduler and get added to hctx->dispatch
> in blk_mq_sched_bypass_insert.  This can happen while a kworker is running
> hctx->run_work work item and is past the point in
> blk_mq_sched_dispatch_requests where hctx->dispatch is checked.
> 
> The blk_mq_do_dispatch_sched call is not guaranteed to end in bounded time,
> because the I/O scheduler can feed an arbitrary number of commands.
> 
> Since we have only one hctx->run_work, the commands waiting in
> hctx->dispatch will wait an arbitrary length of time for run_work to be
> rerun.
> 
> A similar phenomenon exists with dispatches from the software queue.
> 
> The solution is to poll hctx->dispatch in blk_mq_do_dispatch_sched and
> blk_mq_do_dispatch_ctx and return from the run_work handler and let it
> rerun.

Applied for 5.8, thanks.

-- 
Jens Axboe