On Thu, Nov 30, 2017 at 12:47:36PM -0800, Andrew Morton wrote: > On Thu, 30 Nov 2017 08:54:04 -0500 Waiman Long <long...@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 <a...@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 <da...@fromorbit.com> > Cc: Johannes Weiner <han...@cmpxchg.org> > Cc: Vladimir Davydov <vdavydov....@gmail.com> > Cc: Waiman Long <long...@redhat.com> > Signed-off-by: Andrew Morton <a...@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.