Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v3] 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: <20200424150321.38617-1-sqazi@google.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <8998250d-4945-9efd-6936-07e55d505e25@kernel.dk>
Date:   Fri, 24 Apr 2020 09:17:23 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.7.0
MIME-Version: 1.0
In-Reply-To: <20200424150321.38617-1-sqazi@google.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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