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

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 30, 2017 at 08:54:04AM -0500, Waiman Long wrote:
> On 11/29/2017 07:42 PM, Dave Chinner wrote:
> > On Wed, Nov 29, 2017 at 01:53:19PM -0800, Andrew Morton wrote:
> >> On Wed, 29 Nov 2017 09:17:34 -0500 Waiman Long <longman@redhat.com> 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.
> >> Patch looks good.
> >>
> >> Can someone (Dave?) please explain why list_lru_del() supports deletion
> >> of an already list_empty(item)?
> >> This seems a rather dangerous thing to
> >> encourage.  Use cases I can think of are:
> >>
> >> a) item is already reliably deleted, so why the heck was the caller
> >>    calling list_lru_del() and 
> > Higher level operations can race. e.g. caller looks up an object,
> > finds it on the LRU, takes a reference. Then calls list_lru_del()
> > to remove it from the LRU. It blocks 'cause it can't get the list
> > lock as....
> >
> > ... Meanwhile, the list shrinker is running, sees the object on the
> > LRU list, sees it has a valid reference count, does lazy LRU cleanup
> > by runnning list_lru_isolate() on the object which removes it from
> > the LRU list. Eventually it drops the list lock, and ....
> >
> > ... the original thread gets the lock in list_lru_del() and sees the
> > object has already been removed from the LRU....
> >
> > IOWs, this sort of boilerplate code is potentially dangerous if
> > list_lru_del() can't handle items that have already been removed
> > from the list:
> >
> > 	if (!list_empty(&obj->lru))
> > 		list_lru_del(&obj->lru);
> >
> > Because this:
> >
> > 	if (!list_empty(&obj->lru))
> > 		<preempt>
> > 		<shrinker removes obj from LRU>
> > 		list_lru_del(&obj->lru);
> > 			<SPLAT>
> >
> > Would result in bad things happening....
> >
> > 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.

Which is what I said was wrong. You haven't addressed why you think
it's safe to add racy specualtive checks to this code in your quest
for speed.

Also, I'm curious about is how much of the gain is from the
prefetching, and how much of the gain is from avoiding the lock
altogether by the early bailout...

> 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.

Yup, but in many cases programmers get the static branch prediction
hints are wrong. In this case, you are supposing that nobody ever
calls list_lru_del() on objects that aren't on the lru. That's not
true - inodes that are being evicted may never have been on the LRU
at all, but we still call through list_lru_del() so it can determine
the LRU state correctly (e.g. cache cold rm -rf workloads)....

IOWs, I'm pretty sure even just adding static branch prediction
hints here is wrong....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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