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

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Date:   Tue, 5 Dec 2017 15:56:18 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Waiman Long <longman@...hat.com>,
        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 Tue, 5 Dec 2017 15:49:48 +0100 Michal Hocko <mhocko@...nel.org> wrote:

> 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);

erp, I forgot to cc Stephen.

> It also seems that there is no general agreement in the patch. Andrew,
> do you plan to keep it?

It's in wait-and-see mode.