linux-kernel - Re: Deadlock possibly caused by too_many

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Message-ID: <4CC146B1.8060906@kernel.dk>
Date:	Fri, 22 Oct 2010 10:09:21 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Wu Fengguang <fengguang.wu@...el.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Torsten Kaiser <just.for.lkml@...glemail.com>,
	Neil Brown <neilb@...e.de>, Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Li, Shaohua" <shaohua.li@...el.com>
Subject: Re: Deadlock possibly caused by too_many_isolated.

On 2010-10-22 10:07, Wu Fengguang wrote:
>>> We surely need 1 set aside for each level of that stack that will
>>> potentially consume one. 1 should be enough for the generic pool, and
>>> then clones will use a separate pool. So md and friends should really
>>> have a pool per device, so that stacking will always work properly.
>>
>> Agreed for the deadlock problem.
>>
>>> There should be no throughput concerns, it should purely be a safe guard
>>> measure to prevent us deadlocking when doing IO for reclaim.
>>
>> It's easy to verify whether the minimal size will have negative
>> impacts on IO throughput. In Torsten's case, increase BIO_POOL_SIZE
>> by one and check how it performs.
> 
> Sorry it seems simply increasing BIO_POOL_SIZE is not enough to fix
> possible deadlocks. We need adding new mempool(s). Because when there
> BIO_POOL_SIZE=2 and there are two concurrent reclaimers each take 1
> reservation, they will deadlock each other when trying to take the
> next bio at the raid1 level.

Yes, plus it's not a practical solution since you don't know how deep
the stack is. As I wrote in the initial email, each consumer needs it's
own private mempool (and just 1 entry should suffice).

-- 
Jens Axboe

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