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Date: Tue, 6 Feb 2024 13:56:23 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: Waiman Long <longman@redhat.com>
Cc: Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
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	Josh Triplett <josh@joshtriplett.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
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	Zqiang <qiang.zhang1211@gmail.com>,
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	Peter Zijlstra <peterz@infradead.org>,
	Costa Shulyupin <cshulyup@redhat.com>
Subject: Re: [RFC PATCH 0/8] cgroup/cpuset: Support RCU_NOCB on isolated
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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...

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.