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

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Date:   Fri, 20 Nov 2020 13:38:28 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Waiman Long' <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning

On Fri, 20 Nov 2020, David Laight wrote:
>I got massive performance improvements from changing a driver
>we have to use mutex instead of the old semaphores (the driver
>was written a long time ago).
>
>While these weren't 'rw' the same issue will apply.
>
>The problem was that the semaphore/mutex was typically only held over
>a few instructions (eg to add an item to a list).
>But with semaphore if you got contention the process always slept.
>OTOH mutex spin 'for a while' before sleeping so the code rarely slept.

The caveat here is if you are using trylock/unlock from irq, which
is the only reason why regular semaphores are still around today. If
not, indeed a mutex is better.

Thanks,
Davidlohr