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

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Message-ID: <5fe76531782f4a8492b341d5f381147b@AcuMS.aculab.com>
Date:   Fri, 20 Nov 2020 13:11:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Waiman Long' <longman@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>
CC:     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

From: Waiman Long
> Sent: 19 November 2020 18:40
...
> My own testing also show not too much performance difference when
> removing reader spinning except in the lightly loaded cases.

I'm confused.

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.

So I really expect that readers need to spin (for a while) if
a rwsem (etc) is locked for writing.

Clearly you really need a CBU (Crystal Ball Unit) to work out
whether to spin or not.

	David

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