linux-kernel - Re: [PATCH v3] sched/fair: fix unthrottle_cfs_rq for leaf_cfs

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Message-ID: <CAKfTPtDknAbQdx6E6LhOn3AaBcN5v9TSnR=FP_rEXvrJq7kPww@mail.gmail.com>
Date:   Thu, 24 Jun 2021 14:31:12 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Po-Hsu Lin <po-hsu.lin@...onical.com>
Cc:     "Guilherme G. Piccoli" <gpiccoli@...onical.com>,
        Tao Zhou <t1zhou@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Sasha Levin <sashal@...nel.org>,
        SeongJae Park <sjpark@...zon.com>,
        Ben Segall <bsegall@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Tao Zhou <zohooouoto@...o.com.cn>,
        Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Tao Zhou <ouwen210@...mail.com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jay Vosburgh <jay.vosburgh@...onical.com>,
        Gavin Guo <gavin.guo@...onical.com>, halves@...onical.com,
        nivedita.singhvi@...onical.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "# v4 . 16+" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] sched/fair: fix unthrottle_cfs_rq for leaf_cfs_rq list

On Thu, 24 Jun 2021 at 12:29, Po-Hsu Lin <po-hsu.lin@...onical.com> wrote:
>
> Hello Vincent,
>
> sorry to resurrect this thread again,
> I was trying to backport this patch and corresponding fixes to our
> Ubuntu 4.15 kernel [1] to fix an issue report by LTP cfs_bandwidth01
> test[2], my colleague Guilherme told me there once a discussion about
> backporting this on this thread.
>
> You mentioned here this should not be backported to earlier stable
> kernel, I am curious if there is any specific reason of it? Too risky
> maybe?

Yes, IIRC there are some dependencies with other patchsets that make
the backport complex and not straight forward


> Thanks!
> PHLin
>
> [1] https://lists.ubuntu.com/archives/kernel-team/2021-June/121571.html
> [2] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/sched/cfs-scheduler/cfs_bandwidth01.c
>
>
> On Thu, Nov 19, 2020 at 9:25 PM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > On Thu, 19 Nov 2020 at 12:36, Guilherme G. Piccoli
> > <gpiccoli@...onical.com> wrote:
> > >
> > >
> > >
> > > On 19/11/2020 05:36, Vincent Guittot wrote:
> > > > On Thu, 19 Nov 2020 at 01:36, Tao Zhou <t1zhou@....com> wrote:
> > > >>
> > > >> On Thu, Nov 19, 2020 at 07:50:15AM +0800, Tao Zhou wrote:
> > > >>> On Wed, Nov 18, 2020 at 07:56:38PM -0300, Guilherme G. Piccoli wrote:
> > > >>>> Hi Vincent (and all CCed), I'm sorry to ping about such "old" patch, but
> > > >>>> we experienced a similar condition to what this patch addresses; it's an
> > > >>>> older kernel (4.15.x) but when suggesting the users to move to an
> > > >>>> updated 5.4.x kernel, we noticed that this patch is not there, although
> > > >>>> similar ones are (like [0] and [1]).
> > > >>>>
> > > >>>> So, I'd like to ask if there's any particular reason to not backport
> > > >>>> this fix to stable kernels, specially the longterm 5.4. The main reason
> > > >>>> behind the question is that the code is very complex for non-experienced
> > > >>>> scheduler developers, and I'm afraid in suggesting such backport to 5.4
> > > >>>> and introduce complex-to-debug issues.
> > > >>>>
> > > >>>> Let me know your thoughts Vincent (and all CCed), thanks in advance.
> > > >>>> Cheers,
> > > >>>>
> > > >>>>
> > > >>>> Guilherme
> > > >>>>
> > > >>>>
> > > >>>> P.S. For those that deleted this thread from the email client, here's a
> > > >>>> link:
> > > >>>> https://lore.kernel.org/lkml/20200513135528.4742-1-vincent.guittot@linaro.org/
> > > >>>>
> > > >>>>
> > > >>>> [0]
> > > >>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fe61468b2cb
> > > >>>>
> > > >>>> [1]
> > > >>>> https://lore.kernel.org/lkml/20200506141821.GA9773@lorien.usersys.redhat.com/
> > > >>>> <- great thread BTW!
> > > >>>
> > > >>> 'sched/fair: Fix unthrottle_cfs_rq() for leaf_cfs_rq list" failed to apply to
> > > >>> 5.4-stable tree'
> > > >>>
> > > >>> You could check above. But I do not have the link about this. Can't search it
> > > >>> on LKML web: https://lore.kernel.org/lkml/
> > > >>>
> > > >>> BTW: 'ouwen210@...mail.com' and 'zohooouoto@...o.com.cn' all is myself.
> > > >>>
> > > >>> Sorry for the confusing..
> > > >>>
> > > >>> Thanks.
> > > >>
> > > >> Sorry again. I forget something. It is in the stable.
> > > >>
> > > >> Here it is:
> > > >>
> > > >>   https://lore.kernel.org/stable/159041776924279@kroah.com/
> > > >
> > > > I think it has never been applied to stable.
> > > > As you mentioned, the backport has been sent :
> > > > https://lore.kernel.org/stable/20200525172709.GB7427@vingu-book/
> > > >
> > > > I received another emailed in September and pointed out to the
> > > > backport : https://www.spinics.net/lists/stable/msg410445.html
> > > >
> > > >
> > > >>
> > >
> > > Thanks a lot Tao and Vincent! Nice to know that you already worked the
> > > backport, gives much more confidence when the author does that heheh
> > >
> > > So, this should go to stable 5.4.y, but not 4.19.y IIUC?
> >
> > Yeah. they should be backported up to v5.1 but not earlier
> >
> > Regards,
> > Vincent
> >
> > > Cheers,
> > >
> > >
> > > Guilherme