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From:   Jan Glauber <jglauber@marvell.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
CC:     Jan Glauber <jglauber@cavium.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Jayachandran Chandrasekharan Nair <jnair@marvell.com>
Subject: Re: [PATCH] lockref: Limit number of cmpxchg loop retries
Thread-Topic: [PATCH] lockref: Limit number of cmpxchg loop retries
Thread-Index: AQHVHD5S/Fy6pcse0E6tWtS4S5X27w==
Date:   Thu, 6 Jun 2019 08:03:27 +0000
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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@cavium.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.
>=20
> 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=3D4 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.
>=20
>                        Linus