linux-kernel - Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and fix blkcg percpu alloc deadlock

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Message-ID: <20120105012842.GQ31746@google.com>
Date:	Wed, 4 Jan 2012 17:28:42 -0800
From:	Tejun Heo <tj@...nel.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, avi@...hat.com,
	nate@...nel.net, cl@...ux-foundation.org, oleg@...hat.com,
	axboe@...nel.dk, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool
 and fix blkcg percpu alloc deadlock

On Tue, Dec 27, 2011 at 04:41:02PM -0800, Tejun Heo wrote:
> Hello,
> 
> On Wed, Dec 28, 2011 at 09:14:02AM +0900, KAMEZAWA Hiroyuki wrote:
> > > This is essentially more specialized form of the mempool approach.  It
> > > doesn't seem any simpler to me while being less generic.  I don't see
> > > what the upside would be.
> > 
> > Hm, but this never causes -ENOMEM error, at all.
> 
> Ooh, I missed the part it falls back to the global counter if percpu
> counters aren't allocated yet.  Yeah, this is an interesting approach.
> I'll think more about it.

I've been staring at the blkcg stats code and commit logs and am
wondering whether we can just scrap percpu counters there.  It seems
the reason why it was introduced in the first place is to avoid
stats->lock, which BTW is extremely heavy handed for gathering stats,
overhead in fast paths and I think there can be easier ways to avoid
stats->lock.

Thanks.

-- 
tejun
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