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

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Date:   Tue, 17 Nov 2020 21:35:56 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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, 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.

Thanks,
Davidlohr