linux-kernel - Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader optimistic spinning

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Message-ID: <db89a086-3719-cea5-e24e-339085728c29@redhat.com>
Date:   Tue, 4 Jun 2019 14:21:21 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader
 optimistic spinning

On 6/4/19 2:14 PM, Peter Zijlstra wrote:
>> I worked on this owner merging patch mainly to alleviate the need to use
>> cmpxchg for reader lock. cmpxchg_double() is certainly one possible
>> solution though it won't work on older CPUs. We can have a config option
>> to use cmpxchg_double as it may increase the size of other structures
>> that embedded rwsem and impose additional alignment constraint.
> cmpxchg8b was introduced with the Pentium (for PAE IIRC, it enabled
> atomic 64bit PTEs, but Linux never used it for that) and every Intel/AMD
> thereafter has had it. AFAIK there's no x86_64 chip without cmpxchg16b.

Thank for the clarification. I actually didn't check when cmpxch8b was
introduced. I know it is a bit slower than regular cmpxchg. So we may
still need to do some performance analysis to see how it compares with
my current approach.

Cheers,
Longman