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Date: Wed, 7 Feb 2024 11:59:21 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>,
	Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
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Subject: Re: [RFC PATCH 0/8] cgroup/cpuset: Support RCU_NOCB on isolated
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On Wed, Feb 07, 2024 at 03:47:46PM +0100, Frederic Weisbecker wrote:
> Le Tue, Feb 06, 2024 at 04:15:18PM -0300, Marcelo Tosatti a ?crit :
> > 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 ?crit :
> > > > 
> > > > 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=

I mean i've never seen any use of the individual flags being set.

You either want full isolation (nohz_full and all the flags together,
except for HK_TYPE_DOMAIN which is sometimes enabled/disabled), or not.

So why not group them all together ?

Do you know of any separate uses of these flags (except for
HK_TYPE_DOMAIN).

> 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.
> > > 
> > > 
> > 
> 
>