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

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Message-ID: <CAHk-=whPbMBGWiTdC3wqXMGMaCCHQ4WQh5ObB5iwa9gk-nCtzA@mail.gmail.com>
Date:   Wed, 5 Jun 2019 13:16:46 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jan Glauber <jglauber@...ium.com>
Cc:     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>,
        Jan Glauber <jglauber@...vell.com>
Subject: Re: [PATCH] lockref: Limit number of cmpxchg loop retries

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

                       Linus