Re: [PATCH v3] sched/fair: fix unthrottle_cfs_rq for leaf_cfs_rq list

[Po-Hsu Lin]

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?
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@xxxxxxxxxx> wrote:
>
> On Thu, 19 Nov 2020 at 12:36, Guilherme G. Piccoli
> <gpiccoli@xxxxxxxxxxxxx> wrote:
> >
> >
> >
> > On 19/11/2020 05:36, Vincent Guittot wrote:
> > > On Thu, 19 Nov 2020 at 01:36, Tao Zhou <t1zhou@xxxxxxx> 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@xxxxxxxxxx/
> > >>>>
> > >>>>
> > >>>> [0]
> > >>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fe61468b2cb
> > >>>>
> > >>>> [1]
> > >>>> https://lore.kernel.org/lkml/20200506141821.GA9773@xxxxxxxxxxxxxxxxxxxxxxxxx/
> > >>>> <- 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@xxxxxxxxxxx' and 'zohooouoto@xxxxxxxxxxx' 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@xxxxxxxxx/
> > >
> > > 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