linux-kernel - Re: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning

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Date:   Fri, 20 Nov 2020 15:44:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        Phil Auld <pauld@...hat.com>
Subject: Re: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning

On Tue, Nov 17, 2020 at 09:35:56PM -0800, Davidlohr Bueso wrote:
> On Tue, 17 Nov 2020, Waiman Long wrote:
> 
> > The column "CS Load" represents the number of pause instructions issued
> > in the locking critical section. A CS load of 1 is extremely short and
> > is not likey in real situations. A load of 20 (moderate) and 100 (long)
> > are more realistic.
> > 
> > It can be seen that the previous patches in this series have reduced
> > performance in general except in highly contended cases with moderate
> > or long critical sections that performance improves a bit. This change
> > is mostly caused by the "Prevent potential lock starvation" patch that
> > reduce reader optimistic spinning and hence reduce reader fragmentation.
> > 
> > The patch that further limit reader optimistic spinning doesn't seem to
> > have too much impact on overall performance as shown in the benchmark
> > data.
> > 
> > The patch that disables reader optimistic spinning shows reduced
> > performance at lightly loaded cases, but comparable or slightly better
> > performance on with heavier contention.
> 
> I'm not overly worried about the lightly loaded cases here as the users
> (mostly thinking mmap_sem) most likely won't care for real workloads,
> not, ie: will-it-scale type things.
> 
> So at SUSE we also ran into this very same problem with reader optimistic
> spinning and considering the fragmentation went with disabling it, much
> like this patch - but without the reader optimistic lock stealing bits
> you have. So far nothing has really shown to fall out in our performance
> automation. And per your data a single reader spinner does not seem to be
> worth the added complexity of keeping reader spinning vs ripping it out.

I'm fine with ripping it... It was finnicky to begin with.