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

[Minchan Kim <minchan@kernel.org>]

On Thu, Nov 30, 2017 at 12:47:36PM -0800, Andrew Morton wrote:
> On Thu, 30 Nov 2017 08:54:04 -0500 Waiman Long <longman@redhat.com> wrote:
> 
> > > And, from that perspective, the racy shortcut in the proposed patch
> > > is wrong, too. Prefetch is fine, but in general shortcutting list
> > > empty checks outside the internal lock isn't.
> > 
> > For the record, I add one more list_empty() check at the beginning of
> > list_lru_del() in the patch for 2 purpose:
> > 1. it allows the code to bail out early.
> > 2. It make sure the cacheline of the list_head entry itself is loaded.
> > 
> > Other than that, I only add a likely() qualifier to the existing
> > list_empty() check within the lock critical region.
> 
> But it sounds like Dave thinks that unlocked check should be removed?
> 
> How does this adendum look?
> 
> From: Andrew Morton <akpm@linux-foundation.org>
> Subject: list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix
> 
> include prefetch.h, remove unlocked list_empty() test, per Dave
> 
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Waiman Long <longman@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  mm/list_lru.c |    5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff -puN mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix mm/list_lru.c
> --- a/mm/list_lru.c~list_lru-prefetch-neighboring-list-entries-before-acquiring-lock-fix
> +++ a/mm/list_lru.c
> @@ -8,6 +8,7 @@
>  #include <linux/module.h>
>  #include <linux/mm.h>
>  #include <linux/list_lru.h>
> +#include <linux/prefetch.h>
>  #include <linux/slab.h>
>  #include <linux/mutex.h>
>  #include <linux/memcontrol.h>
> @@ -135,13 +136,11 @@ bool list_lru_del(struct list_lru *lru,
>  	/*
>  	 * Prefetch the neighboring list entries to reduce lock hold time.
>  	 */
> -	if (unlikely(list_empty(item)))
> -		return false;
>  	prefetchw(item->prev);
>  	prefetchw(item->next);
>  
>  	spin_lock(&nlru->lock);
> -	if (likely(!list_empty(item))) {
> +	if (!list_empty(item)) {
>  		l = list_lru_from_kmem(nlru, item);
>  		list_del_init(item);
>  		l->nr_items--;

If we cannot guarantee it's likely !list_empty, prefetch with NULL pointer
would be harmful by the lesson we have learned.

        https://lwn.net/Articles/444336/

So, with considering list_lru_del is generic library, it cannot see
whether a workload makes heavy lock contentions or not.
Maybe, right place for prefetching would be in caller, not in library
itself.

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