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

[Minchan Kim]

On Thu, Nov 30, 2017 at 08:43:41AM -0500, Waiman Long wrote:
> On 11/29/2017 07:53 PM, Minchan Kim wrote:
> > Hello,
> >
> > On Wed, Nov 29, 2017 at 09:17:34AM -0500, 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@xxxxxxxxxx>
> >> ---
> >>  mm/list_lru.c | 10 +++++++++-
> >>  1 file changed, 9 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/mm/list_lru.c b/mm/list_lru.c
> >> index f141f0c..65aae44 100644
> >> --- a/mm/list_lru.c
> >> +++ b/mm/list_lru.c
> >> @@ -132,8 +132,16 @@ bool list_lru_del(struct list_lru *lru, struct list_head *item)
> >>  	struct list_lru_node *nlru = &lru->node[nid];
> >>  	struct list_lru_one *l;
> >>  
> >> +	/*
> >> +	 * Prefetch the neighboring list entries to reduce lock hold time.
> >> +	 */
> >> +	if (unlikely(list_empty(item)))
> >> +		return false;
> >> +	prefetchw(item->prev);
> >> +	prefetchw(item->next);
> >> +
> > A question:
> >
> > A few month ago, I had a chance to measure prefetch effect with my testing
> > workload. For the clarification, it's not list_lru_del but list traverse
> > stuff so it might be similar.
> >
> > With my experiment at that time, it was really hard to find best place to
> > add prefetchw. Sometimes, it was too eariler or late so the effect was
> > not good, even worse on some cases.
> >
> > Also, the performance was different with each machine although my testing
> > machines was just two. ;-)
> >
> > So my question is what's a rule of thumb to add prefetch command?
> > Like your code, putting prefetch right before touching?
> >
> > I'm really wonder what's the rule to make every arch/machines happy
> > with prefetch.
> 
> I add the prefetchw() before spin_lock() because the latency of the
> lockinig operation can be highly variable. There will have high latency
> when the lock is contended. With the prefetch, lock hold time will be
> reduced. In turn, it helps to reduce the amount of lock contention as
> well. If there is no lock contention, the prefetch won't help.

I knew it by your description. My point is prefetch optimization could
show different results by various architectures and workloads so
I wanted to know what kinds of rule we have to prove it's always win
or no harmful for *everycase* in geneal.

This is a performance patch and it's very micro-optimized topic so
I think we need more data to prove it. Maybe perf is best friend and need a
experiment with no lock contention case, at least.

Thanks.