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

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Date:	Mon, 16 Jan 2012 10:28:53 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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 Wed, Jan 04, 2012 at 05:28:42PM -0800, Tejun Heo wrote:
> 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.

Tejun, 

If you get rid of stats->lock, and also don't use per cpu data strucutres
how would we avoid races in case of concurrent update (IO submission?).

Thanks
Vivek
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