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Message-ID: <ZcKFRqrUh5tTbsaJ@tpad>
Date: Tue, 6 Feb 2024 16:15:18 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
Zefan Li <lizefan.x@...edance.com>,
Johannes Weiner <hannes@...xchg.org>,
Jonathan Corbet <corbet@....net>,
"Paul E. McKenney" <paulmck@...nel.org>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
Joel Fernandes <joel@...lfernandes.org>,
Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang.zhang1211@...il.com>,
Davidlohr Bueso <dave@...olabs.net>, Shuah Khan <shuah@...nel.org>,
cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
linux-kselftest@...r.kernel.org, Mrunal Patel <mpatel@...hat.com>,
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Cestmir Kalina <ckalina@...hat.com>,
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Juri Lelli <juri.lelli@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Costa Shulyupin <cshulyup@...hat.com>
Subject: Re: [RFC PATCH 0/8] cgroup/cpuset: Support RCU_NOCB on isolated
partitions
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.
>
> 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.
>
>
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