linux-kernel - Re: [mm/page_alloc] f26b3fa046: netperf.Throughput

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Message-Id: <20220512110634.712057e4663ecc5f697bf185@linux-foundation.org>
Date:   Thu, 12 May 2022 11:06:34 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Aaron Lu <aaron.lu@...el.com>,
        "ying.huang@...el.com" <ying.huang@...el.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        kernel test robot <oliver.sang@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>,
        fengwei.yin@...el.com, linux-mm@...ck.org
Subject: Re: [mm/page_alloc] f26b3fa046: netperf.Throughput_Mbps -18.0%
 regression

On Thu, 12 May 2022 10:42:09 -0700 Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, May 12, 2022 at 5:46 AM Aaron Lu <aaron.lu@...el.com> 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.

(hopefully adds linux-mm to cc)