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

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Message-ID: <4DA2F02D.2000903@fusionio.com>
Date:	Mon, 11 Apr 2011 14:12:29 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	NeilBrown <neilb@...e.de>
CC:	Mike Snitzer <snitzer@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>
Subject: Re: [PATCH 05/10] block: remove per-queue plugging

On 2011-04-11 13:55, NeilBrown wrote:
> On Mon, 11 Apr 2011 20:59:28 +1000 NeilBrown <neilb@...e.de> wrote:
> 
>> On Mon, 11 Apr 2011 11:19:58 +0200 Jens Axboe <jaxboe@...ionio.com> 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:

Ah, so it was pretty close. That does explain it. I've already queued up
the patch, I'll ammend the commit message.

-- 
Jens Axboe

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