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

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Date:	Thu, 22 Dec 2011 14:20:58 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	avi@...hat.com, nate@...nel.net, cl@...ux-foundation.org,
	oleg@...hat.com, axboe@...nel.dk, vgoyal@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and
 fix blkcg percpu alloc deadlock

On Thu, 22 Dec 2011 14:09:11 -0800
Tejun Heo <tj@...nel.org> wrote:

> Hello,
> 
> On Thu, Dec 22, 2011 at 01:59:25PM -0800, Andrew Morton wrote:
> > How about we just delete those statistics and then this patchset?
> > 
> > Or how about we change those statistics to not do percpu allocations,
> > then delete this patchset?
> 
> I'm not against above both

Don't just consider my suggestions - please try to come up with your own
alternatives too!  If all else fails then this patch is a last resort.

> but apparently those percpu stats reduced
> CPU overhead significantly.

Deleting them would save even more CPU.

Or make them runtime or compile-time configurable, so only the
developers see the impact.

Some specifics on which counters are causing the problems would help here.

> > Or how about we fix the percpu memory allocation code so that it
> > propagates the gfp flags, then delete this patchset?
> 
> Oh, no, this is gonna make things *way* more complex.  I tried.

But there's a difference between fixing a problem and working around it.

>  If
> we're gonna have many more NOIO percpu users, which I don't think we
> would or should, that might make sense but, for fringe cases,
> extending mempool to cover percpu is a much better sized solution.

I've long felt that we goofed with the gfp_flags thing and that it
should be a field in the task_struct.  Now *that* would be a large
patch!


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