Re: [PATCH 05/10] block: remove per-queue plugging

From: NeilBrown
Date: Mon Apr 11 2011 - 07:55:49 EST

On Mon, 11 Apr 2011 20:59:28 +1000 NeilBrown <neilb@xxxxxxx> wrote:

> On Mon, 11 Apr 2011 11:19:58 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote:
> > On 2011-04-11 06:50, NeilBrown wrote:
> > > The only explanation I can come up with is that very occasionally schedule on
> > > 2 separate cpus calls blk_flush_plug for the same task. I don't understand
> > > the scheduler nearly well enough to know if or how that can happen.
> > > However with this patch in place I can write to a RAID1 constantly for half
> > > an hour, and without it, the write rarely lasts for 3 minutes.
> >
> > Or perhaps if the request_fn blocks, that would be problematic. So the
> > patch is likely a good idea even for that case.
> >
> > I'll merge it, changing it to list_splice_init() as I think that would
> > be more clear.
> OK - though I'm not 100% the patch fixes the problem - just that it hides the
> symptom for me.
> I might try instrumenting the code a bit more and see if I can find exactly
> where it is re-entering flush_plug_list - as that seems to be what is
> happening.

OK, I found how it re-enters.

The request_fn doesn't exactly block, but when scsi_request_fn calls
spin_unlock_irq, this calls preempt_enable which can call schedule, which is
a recursive call.

The patch I provided will stop that from recursing again as the blk_plug.list
will be empty.

So it is almost what you suggested, however the request_fn doesn't block, it
just enabled preempt.

So the comment I would put at the top of that patch would be something like:

From: NeilBrown <neilb@xxxxxxx>

As request_fn called by __blk_run_queue is allowed to 'schedule()' (after
dropping the queue lock of course), it is possible to get a recursive call:

schedule -> blk_flush_plug -> __blk_finish_plug -> flush_plug_list
-> __blk_run_queue -> request_fn -> schedule

We must make sure that the second schedule does not call into blk_flush_plug
again. So instead of leaving the list of requests on blk_plug->list, move
them to a separate list leaving blk_plug->list empty.

Signed-off-by: NeilBrown <neilb@xxxxxxx>

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