linux-kernel - Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock

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Date:   Tue, 5 Dec 2017 15:49:48 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>
Cc:     Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dave Chinner <david@...morbit.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before
 acquiring lock

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@...hat.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