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Date: Tue, 5 Dec 2017 15:49:48 +0100
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before
 acquiring lock
Message-ID: <20171205144948.ezgo3xpjeytkq6ua@dhcp22.suse.cz>
References: <1511965054-6328-1-git-send-email-longman@redhat.com>
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In-Reply-To: <1511965054-6328-1-git-send-email-longman@redhat.com>
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To: Andrew Morton <akpm@linux-foundation.org>, Waiman Long <longman@redhat.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Dave Chinner <david@fromorbit.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org

On Wed 29-11-17 09:17:34, Waiman Long wrote:
> The list_lru_del() function removes the given item from the LRU list.
> The operation looks simple, but it involves writing into the cachelines
> of the two neighboring list entries in order to get the deletion done.
> That can take a while if the cachelines aren't there yet, thus
> prolonging the lock hold time.
> 
> To reduce the lock hold time, the cachelines of the two neighboring
> list entries are now prefetched before acquiring the list_lru_node's
> lock.
> 
> Using a multi-threaded test program that created a large number
> of dentries and then killed them, the execution time was reduced
> from 38.5s to 36.6s after applying the patch on a 2-socket 36-core
> 72-thread x86-64 system.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>

The patch still seems to be in the mmotm tree while it breaks
compilation. At least m32r defconfig complains with
mm/list_lru.c: In function 'list_lru_del':
mm/list_lru.c:141:2: error: implicit declaration of function 'prefetchw' [-Werror=implicit-function-declaration]
  prefetchw(item->prev);

It also seems that there is no general agreement in the patch. Andrew,
do you plan to keep it?
-- 
Michal Hocko
SUSE Labs

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