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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
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