Re: Questions about replacing isolcpus by cgroup-v2

Henning Schild
2021-11-15T12:39:25Z

Am Fri, 12 Nov 2021 17:37:07 +0100
schrieb Frederic Weisbecker <frederic-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>:

> On Fri, Nov 12, 2021 at 04:36:56PM +0100, Sebastian Andrzej Siewior
> wrote:
> > On 2021-11-04 17:29:08 [+0000], Moessbauer, Felix wrote:
> > > Dear subscribers,
> > Hi,
> >
> > I Cced cgroups@vger since thus question fits there better.
> > I Cced Frederic in case he has come clues regarding isolcpus and
> > cgroups.
> >
> > > we are currently evaluating how to rework realtime tuning to use
> > > cgroup-v2 cpusets instead of the isolcpus kernel parameter. Our
> > > use-case are realtime applications with rt and non-rt threads.
> > > Hereby, the non-rt thread might create additional non-rt threads:
> > >
> > > Example (RT CPU=1, 4 CPUs):
> > > - Non-RT Thread (A) with default affinity 0xD (1101b)
> > > - RT Thread (B) with Affinity 0x2 (0010b, via set_affinity)
> > >
> > > When using pure isolcpus and cgroup-v1, just setting isolcpus=1
> > > perfectly works: Thread A gets affinity 0xD, Thread B gets 0x2
> > > and additional threads get a default affinity of 0xD. By that,
> > > independent of the threads' priorities, we can ensure that
> > > nothing is scheduled on our RT cpu (except from kernel threads,
> > > etc...).
> > >
> > > During this journey, we discovered the following:
> > >
> > > Using cgroup-v2 cpusets and isolcpus together seems to be
> > > incompatible: When activating the cpuset controller on a cgroup
> > > (for the first time), all default CPU affinities are reset. By
> > > that, also the default affinity is set to 0xFFFF..., while with
> > > isolcpus we expect it to be (0xFFFF - isolcpus). This breaks the
> > > example from above, as now the non-RT thread can also be
> > > scheduled on the RT CPU.
>
> That sounds buggy from the cpuset-v2 side (adding the maintainers in
> Cc).

We thought the same but ended up saying that it might just be related
to the deprecation. But regarding that as a bug could help fix it in
stable kernels and distros. Because the patches will need some time and
will likely not get backported.

Henning

> Also please have a look into "[PATCH v8 0/6] cgroup/cpuset: Add new
> cpuset partition type & empty effecitve cpus":
>
> https://lore.kernel.org/lkml/20211018143619.205065-1-longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org/
>
> This stuff adds support for a new "isolated" partition type on
> cpuset/cgroup-v2 which should behave just like isolcpus.
>
> > >
> > > When only using cgroup-v2, we can isolate our RT process by
> > > placing it in a cgroup with CPUs=0,1 and remove CPU=1 from all
> > > other cgroups. However, we do not know of a strategy to set a
> > > default affinity: Given the example above, we have no way to
> > > ensure that newly created threads are born with an affinity of
> > > just 0x2 (without changing the application).
> > >
> > > Finally, isolcpus itself is deprecated since kernel 5.4.
> >
> > Where is this the deprecation of isolcpus announced/ written?
>
> We tried to deprecate it but too many people are still using it.
> Better pick an interface that allows you to change the isolated set
> at runtime like cpuset.sched_load_balance on cpuset/cgroup-v1 or the
> above patchset on v2.
>
> Thanks.