From: David Laight <David.Laight@ACULAB.COM>
To: 'Waiman Long' <longman@redhat.com>, Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Phil Auld <pauld@redhat.com>
Subject: RE: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning
Date: Fri, 20 Nov 2020 13:11:12 +0000 [thread overview]
Message-ID: <5fe76531782f4a8492b341d5f381147b@AcuMS.aculab.com> (raw)
In-Reply-To: <ee34bc01-9fef-23ff-ada1-1ec2d39533c9@redhat.com>
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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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2020-11-20 13:11 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-18 3:04 [PATCH 0/5] locking/rwsem: Rework reader optimistic spinning Waiman Long
2020-11-18 3:04 ` [PATCH 1/5] locking/rwsem: Pass the current atomic count to rwsem_down_read_slowpath() Waiman Long
2020-11-18 3:04 ` [PATCH 2/5] locking/rwsem: Prevent potential lock starvation Waiman Long
2020-11-20 14:44 ` Peter Zijlstra
2020-11-20 17:27 ` Waiman Long
2020-11-18 3:04 ` [PATCH 3/5] locking/rwsem: Enable reader optimistic lock stealing Waiman Long
2020-11-20 14:36 ` Peter Zijlstra
2020-11-20 17:26 ` Waiman Long
2020-12-08 3:53 ` Davidlohr Bueso
2020-11-18 3:04 ` [PATCH 4/5] locking/rwsem: Wake up all waiting readers if RWSEM_WAKE_READ_OWNED Waiman Long
2020-11-18 4:53 ` Davidlohr Bueso
2020-11-19 18:37 ` Waiman Long
2020-11-18 3:04 ` [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning Waiman Long
2020-11-18 5:35 ` Davidlohr Bueso
2020-11-19 18:40 ` Waiman Long
2020-11-20 13:11 ` David Laight [this message]
2020-11-20 17:04 ` Waiman Long
2020-11-20 17:37 ` David Laight
2020-11-20 21:38 ` Davidlohr Bueso
2020-11-21 11:50 ` David Laight
2020-11-20 14:44 ` Peter Zijlstra
2020-11-20 22:39 ` Waiman Long
2020-11-20 14:42 ` Peter Zijlstra
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