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

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Message-ID: <20111223144647.GA16818@redhat.com>
Date:	Fri, 23 Dec 2011 09:46:47 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.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 Thu, Dec 22, 2011 at 05:58:34PM -0800, Andrew Morton wrote:
> On Thu, 22 Dec 2011 20:40:43 -0500 Vivek Goyal <vgoyal@...hat.com> wrote:
> 
> > That's why the need of per cpu data structures to make stat collection
> > lockless.
> 
> btw, (and this is a common refrain): was there any reason for avoiding
> using percpu_counters here?

IIUC, per cpu counters also call alloc_percpu() during initialization.

int __percpu_counter_init(struct percpu_counter *fbc, s64 amount,
                          struct lock_class_key *key)
{
        raw_spin_lock_init(&fbc->lock);
        lockdep_set_class(&fbc->lock, key);
        fbc->count = amount;
        fbc->counters = alloc_percpu(s32);
....
....
}

So they are no good either for allocation and initialization in IO path.

Thanks
Vivek
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