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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
> 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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