linux-kernel - Re: [PATCH 4.19 156/191] powerpc: Warn about use of smt_snooze

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Date:   Thu, 5 Nov 2020 23:44:11 +0000
From:   Joel Stanley <joel@....id.au>
To:     Pavel Machek <pavel@....cz>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 4.19 156/191] powerpc: Warn about use of smt_snooze_delay

On Thu, 5 Nov 2020 at 19:24, Pavel Machek <pavel@....cz> wrote:
>
> Hi!
>
> > From: Joel Stanley <joel@....id.au>
> >
> > commit a02f6d42357acf6e5de6ffc728e6e77faf3ad217 upstream.
> >
> > It's not done anything for a long time. Save the percpu variable, and
> > emit a warning to remind users to not expect it to do anything.
> >
> > This uses pr_warn_once instead of pr_warn_ratelimit as testing
> > 'ppc64_cpu --smt=off' on a 24 core / 4 SMT system showed the warning
> > to be noisy, as the online/offline loop is slow.
>
> I don't believe this is good idea for stable. It is in 5.9-rc2, and
> likely mainline users will get userspace fixed, but that warning is
> less useful for -stable users.

The warning is about the existing behaviour of the kernel. It does let
the user know that they won't see any difference in behaviour when
tweaking the smt_snooze_delay variable, which was a real issue that
Anton hit.

I agree that the future commit that removes smt_snooze_delay from the
kernel should not be backported.

Cheers,

Joel

>
> (And besides, it does not fix any serious bug).
>
> Best regards,
>                                                                 Pavel
>
> --
> http://www.livejournal.com/~pavelmachek