Re: [RFC PATCH 0/8] cgroup/cpuset: Support RCU_NOCB on isolated partitions

Frederic Weisbecker
2024-02-07T14:47:49Z

Le Tue, Feb 06, 2024 at 04:15:18PM -0300, Marcelo Tosatti a écrit :
> On Tue, Feb 06, 2024 at 01:56:23PM +0100, Frederic Weisbecker wrote:
> > Le Wed, Jan 17, 2024 at 12:15:07PM -0500, Waiman Long a écrit :
> > >
> > > On 1/17/24 12:07, Tejun Heo wrote:
> > > > Hello,
> > > >
> > > > On Wed, Jan 17, 2024 at 11:35:03AM -0500, Waiman Long wrote:
> > > > > The first 2 patches are adopted from Federic with minor twists to fix
> > > > > merge conflicts and compilation issue. The rests are for implementing
> > > > > the new cpuset.cpus.isolation_full interface which is essentially a flag
> > > > > to globally enable or disable full CPU isolation on isolated partitions.
> > > > I think the interface is a bit premature. The cpuset partition feature is
> > > > already pretty restrictive and makes it really clear that it's to isolate
> > > > the CPUs. I think it'd be better to just enable all the isolation features
> > > > by default. If there are valid use cases which can't be served without
> > > > disabling some isolation features, we can worry about adding the interface
> > > > at that point.
> > >
> > > My current thought is to make isolated partitions act like isolcpus=domain,
> > > additional CPU isolation capabilities are optional and can be turned on
> > > using isolation_full. However, I am fine with making all these turned on by
> > > default if it is the consensus.
> >
> > Right it was the consensus last time I tried. Along with the fact that mutating
> > this isolation_full set has to be done on offline CPUs to simplify the whole
> > picture.
> >
> > So lemme try to summarize what needs to be done:
> >
> > 1) An all-isolation feature file (that is, all the HK_TYPE_* things) on/off for
> > now. And if it ever proves needed, provide a way later for more finegrained
> > tuning.
> >
> > 2) This file must only apply to offline CPUs because it avoids migrations and
> > stuff.
> >
> > 3) I need to make RCU NOCB tunable only on offline CPUs, which isn't that much
> > changes.
> >
> > 4) HK_TYPE_TIMER:
> > * Wrt. timers in general, not much needs to be done, the CPUs are
> > offline. But:
> > * arch/x86/kvm/x86.c does something weird
> > * drivers/char/random.c might need some care
> > * watchdog needs to be (de-)activated
> >
> > 5) HK_TYPE_DOMAIN:
> > * This one I fear is not mutable, this is isolcpus...
>
> Except for HK_TYPE_DOMAIN, i have never seen anyone use any of this
> flags.

HK_TYPE_DOMAIN is used by isolcpus=domain,....
HK_TYPE_MANAGED_IRQ is used by isolcpus=managed_irq,...

All the others (except HK_TYPE_SCHED) are used by nohz_full=

Thanks.

>
> >
> > 6) HK_TYPE_MANAGED_IRQ:
> > * I prefer not to think about it :-)
> >
> > 7) HK_TYPE_TICK:
> > * Maybe some tiny ticks internals to revisit, I'll check that.
> > * There is a remote tick to take into consideration, but again the
> > CPUs are offline so it shouldn't be too complicated.
> >
> > 8) HK_TYPE_WQ:
> > * Fortunately we already have all the mutable interface in place.
> > But we must make it live nicely with the sysfs workqueue affinity
> > files.
> >
> > 9) HK_FLAG_SCHED:
> > * Oops, this one is ignored by nohz_full/isolcpus, isn't it?
> > Should be removed?
> >
> > 10) HK_TYPE_RCU:
> > * That's point 3) and also some kthreads to affine, which leads us
> > to the following in HK_TYPE_KTHREAD:
> >
> > 11) HK_FLAG_KTHREAD:
> > * I'm guessing it's fine as long as isolation_full is also an
> > isolated partition. Then unbound kthreads shouldn't run there.
> >
> > 12) HK_TYPE_MISC:
> > * Should be fine as ILB isn't running on offline CPUs.
> >
> > Thanks.
> >
> >
>