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From:   Salman Qazi <sqazi@google.com>
Date:   Thu, 23 Apr 2020 18:41:54 -0700
Message-ID: <CAKUOC8Vb4AZnU_Sm=rxGk7QDK9=NvQT+G3Kp1cV8uVcebxsVWQ@mail.gmail.com>
Subject: Re: [PATCH v2] block: Limit number of items taken from the I/O
 scheduler in one go
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Ming Lei <ming.lei@redhat.com>,
        Bart Van Assche <bvanassche@acm.org>,
        linux-block <linux-block@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jesse Barnes <jsbarnes@google.com>,
        Gwendal Grignou <gwendal@google.com>,
        Hannes Reinecke <hare@suse.com>, Christoph Hellwig <hch@lst.de>
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I remailed it with the changes since v1 added.  But just to answer directly:

Changes since v1:

* Removed max_sched_batch.
* Extended the fix to the software queue.
* Use a return value from blk_mq_do_dispatch_sched to indicate if
  the dispatch should be rerun.
* Some comments added.

On Thu, Apr 23, 2020 at 2:30 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> 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
>