linux-kernel - Re: [PATCH] lockref: Limit number of cmpxchg loop retries

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Date:   Thu, 6 Jun 2019 08:03:27 +0000
From:   Jan Glauber <jglauber@...vell.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Jan Glauber <jglauber@...ium.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Jayachandran Chandrasekharan Nair <jnair@...vell.com>
Subject: Re: [PATCH] lockref: Limit number of cmpxchg loop retries

On Wed, Jun 05, 2019 at 01:16:46PM -0700, Linus Torvalds wrote:
> On Wed, Jun 5, 2019 at 6:49 AM Jan Glauber <jglauber@...ium.com> wrote:
> >
> > Add an upper bound to the loop to force the fallback to spinlocks
> > after some time. A retry value of 100 should not impact any hardware
> > that does not have this issue.
> >
> > With the retry limit the performance of an open-close testcase
> > improved between 60-70% on ThunderX2.
> 
> Btw, did you do any kind of performance analysis across different
> retry limit values?

I tried 15/50/100/200/500, results were largely identical up to 100.
For SMT=4 a higher retry value might be better, but unless we can add a
sysctl value 100 looked like a good compromise to me.

--Jan

> I'm perfectly happy to just pick a random number and '100' looks fine
> to me, so this is mainly out of curiosity.
> 
>                        Linus