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

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Message-ID: <20111222232433.GQ17084@google.com>
Date:	Thu, 22 Dec 2011 15:24:33 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.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

Hello,

On Thu, Dec 22, 2011 at 03:16:49PM -0800, Andrew Morton wrote:
> > The ones allocated in the last patch.  blk_group_cpu_stats.
> 
> What last patch.
> 
> I can find no occurence of "blk_group_cpu_stats" on linux-kernel or in
> the kernel tree.

Sorry it's blkio_group_stats_cpu.  It's in the seventh path in this
series.

> > The stats are per cgroup - request_queue pair.  We don't want to
> > allocate for all of them for each combination as there are
> > configurations with stupid number of request_queues and silly many
> > cgroups and #cgroups * #request_queue * #cpus can be huge.  So, we
> > want on-demand allocation.  While the stats are important, they are
> > not critical and allocations can be opportunistic.  If the allocation
> > fails this time, we can try it for the next time.
> 
> Without code to look at I am at a loss.

block/blk-cgroup.c blk-throttle.c cfq-iosched.c.  Have fun.

> request_queues are allocated in blk_alloc_queue_node(), which uses
> GFP_KERNEL (and also mysteriously takes a gfp_t arg).

Yeah, sure, we *can* allocate everything for every combination when
either request_queue or cgroup comes up.  That's the thing I tried to
explain in the above quoted paragraph.

Thanks.

-- 
tejun
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