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Subject: Re: [PATCH v2] block: Limit number of items taken from the I/O
 scheduler in one go
To:     Salman Qazi <sqazi@google.com>, Ming Lei <ming.lei@redhat.com>,
        Bart Van Assche <bvanassche@acm.org>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Jesse Barnes <jsbarnes@google.com>,
        Gwendal Grignou <gwendal@google.com>,
        Hannes Reinecke <hare@suse.com>, Christoph Hellwig <hch@lst.de>
References: <20200423210523.52833-1-sqazi@google.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <83078447-831c-2921-db5e-9cab4c4c12df@kernel.dk>
Date:   Thu, 23 Apr 2020 15:30:01 -0600
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On 4/23/20 3:05 PM, 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.

Any changes since v1? It's customary to put that in here too, below
the --- lines.

-- 
Jens Axboe