linux-kernel - Re: [PATCH 2/2] mm/percpu.c: don't bother to re-walk the pcpu_slot list if nobody free space since we last drop pcpu

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Message-ID: <20140327150038.GD18503@htj.dyndns.org>
Date:	Thu, 27 Mar 2014 11:00:38 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jianyu Zhan <nasa4836@...il.com>
Cc:	linux-mm@...ck.org, akpm@...ux-foundation.org,
	cl@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/percpu.c: don't bother to re-walk the pcpu_slot
 list if nobody free space since we last drop pcpu_lock.

On Thu, Mar 27, 2014 at 07:06:03PM +0800, Jianyu Zhan wrote:
> Presently, after we fail the first try to walk the pcpu_slot list
> to find a chunk for allocating, we just drop the pcpu_lock spinlock,
> and go allocating a new chunk. Then we re-gain the pcpu_lock and
> anchoring our hope on that during this period, some guys might have
> freed space for us(we still hold the pcpu_alloc_mutex during this
> period, so only freeing or reclaiming could happen), we do a fully
> rewalk of the pcpu_slot list.
> 
> However if nobody free space, this fully rewalk may seem too silly,
> and we would eventually fall back to the new chunk.
> 
> And since we hold pcpu_alloc_mutex, only freeing or reclaiming path
> could touch the pcpu_slot(which just need holding a pcpu_lock), we
> could maintain a pcpu_slot_stat bitmap to record that during the period
> we don't have the pcpu_lock, if anybody free space to any slot we
> interest in. If so, we just just go inside these slots for a try;
> if not, we just do allocation using the newly-allocated fully-free
> new chunk.

The patch probably needs to be refreshed on top of percpu/for-3.15.
Hmmm... I'm not sure whether the added complexity is worthwhile.  It's
a fairly cold path.  Can you show how helpful this optimization is?

Thanks.

-- 
tejun
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