Re: [mm/page_alloc] f26b3fa046: netperf.Throughput_Mbps -18.0% regression

[Linus Torvalds]

On Thu, May 12, 2022 at 5:46 AM Aaron Lu <aaron.lu@xxxxxxxxx> wrote:
>
> When nr_process=16, zone lock contention increased about 21% from 6% to
> 27%, performance dropped 17.8%, overall lock contention increased 14.3%:

So the contention issue seems real and nasty, and while the queued
locks may have helped a bit, I don't think they ended up making a
*huge* change: the queued locks help make sure the lock itself doesn't
bounce all over the place, but clearly if the lock holder writes close
to the lock, it will still bounce with at least *one* lock waiter.

And having looked at the qspinlock code, I have to agree with Waiman
and PeterZ that I don't think the locking code can reasonably eb
changed - I'm sure this particular case could be improved, but the
downsides for other cases would be quite large enough to make that a
bad idea.

So I think the issue is that

 (a) that zone lock is too hot.

 (b) given lock contention, the fields that get written to under the
lock are too close to the lock

Now, the optimal fix would of course be to just fix the lock so that
it isn't so hot. But assuming that's not possible, just looking at the
definition of that 'struct zone', I do have to say that the
ZONE_PADDING fields seem to have bit-rotted over the years.

The whole and only reason for them would be to avoid the cache
bouncing, but commit 6168d0da2b47 ("mm/lru: replace pgdat lru_lock
with lruvec lock") actively undid that for the 'lru_lock' case, and
way back when commit a368ab67aa55 ("mm: move zone lock to a different
cache line than order-0 free page lists") tried to make it true for
the 'lock' vs free_area[] cases, but did it without actually using the
ZONE_PADDING thing, but by moving things around, and not really
*guaranteeing* that 'lock' was in a different cacheline, but really
just making 'free_area[]' aligned, but still potentially in the same
cache-line as 'lock' (so now the lower-order 'free_area[]' entries are
not sharing a cache-line, but the higher-order 'free_area[]' ones
probably are).

So I get the feeling that those 'ZONE_PADDING' things are a bit random
and not really effective.

In a perfect world, somebody would fix the locking to just not have as
much contention. But assuming that isn't an option, maybe somebody
should just look at that 'struct zone' layout a bit more.

                     Linus