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Date: Tue, 6 Feb 2024 16:15:18 -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>,
	Jonathan Corbet <corbet@lwn.net>,
	"Paul E. McKenney" <paulmck@kernel.org>,
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	Joel Fernandes <joel@joelfernandes.org>,
	Josh Triplett <josh@joshtriplett.org>,
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	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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	Zqiang <qiang.zhang1211@gmail.com>,
	Davidlohr Bueso <dave@stgolabs.net>, Shuah Khan <shuah@kernel.org>,
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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
 partitions
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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.

> 
> 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.
> 
>