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

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Date:   Tue, 10 May 2022 21:27:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "ying.huang@...el.com" <ying.huang@...el.com>,
        Waiman Long <longman@...hat.com>,
        Will Deacon <will@...nel.org>, Aaron Lu <aaron.lu@...el.com>,
        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>,
        Andrew Morton <akpm@...ux-foundation.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
Subject: Re: [mm/page_alloc] f26b3fa046: netperf.Throughput_Mbps -18.0%
 regression

On Tue, May 10, 2022 at 11:05:01AM -0700, Linus Torvalds wrote:

> I think from a pure lock standpoint, it's the right thing to do (no
> unnecessary bouncing, with the lock releaser doing just one write, and
> the head waiter spinning on it is doing the right thing).
> 
> But I think this is an example of where you end up having that
> spinning on the lock possibly then being a disturbance on the other
> fields around the lock.
> 
> I wonder if Waiman / PeterZ / Will have any comments on that. Maybe
> that "spin on the lock itself" is just fundamentally the only correct
> thing, but since my initial reaction was "no, we're spinning on the
> mcs node", maybe that would be _possible_?
> 
> We do have a lot of those spinlocks embedded in other data structures
> cases. And if "somebody else is waiting for the lock" contends badly
> with "the lock holder is doing a lot of writes close to the lock",
> then that's not great.

The immediate problem is that we don't always have a node. Notably we
only do the whole MCS queueing thing when there's more than 1 contender.

Always doing the MCS thing had a hefty performance penalty vs the
simpler spinlock implementations for the uncontended and light contended
lock cases (by far the most common scenario) due to the extra cache-miss
of getting an MCS node.