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Date:   Mon, 11 Jan 2021 13:50:52 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively

On Mon, Jan 11, 2021 at 10:09:07AM -0800, Paul E. McKenney wrote:
> On Mon, Jan 11, 2021 at 06:16:39PM +0100, Peter Zijlstra wrote:
> > 
> > While thinking more about this, I'm thinking a big part of the problem
> > is that we're not dinstinguishing between geniuine per-cpu kthreads and
> > kthreads that just happen to be per-cpu.
> > 
> > Geniuine per-cpu kthreads are kthread_bind() and have PF_NO_SETAFFINITY,
> > but sadly a lot of non-per-cpu kthreads, that might happen to still be
> > per-cpu also have that -- again workqueue does that even to it's unbound
> > workers :-(
> > 
> > Now, anything created by smpboot, is created through
> > kthread_create_on_cpu() and that additionally sets to_kthread(p)->flags
> > KTHREAD_IS_PER_CPU.
> > 
> > And I'm thinking that might be sufficient, if we modify
> > is_per_cpu_kthread() to check that, then we only match smpboot threads
> > (which include the hotplug and stopper threads, but notably not the idle
> > thread)
> > 
> > Sadly it appears like io_uring() uses kthread_create_on_cpu() without
> > then having any hotplug crud on, so that needs additinoal frobbing.
> > 
> > Also, init_task is PF_KTHREAD but doesn't have a struct kthread on.. and
> > I suppose bound workqueues don't go through this either.
> > 
> > Let me rummage around a bit...
> > 
> > This seems to not insta-explode... opinions?
> 
> It passes quick tests on -rcu both with and without the rcutorture fixes,
> which is encouraging.  I will start a more vigorous test in about an hour.

And 672 ten-minute instances of RUDE01 passed with this patch applied
and with my rcutorture patch reverted.  So looking good, thank you!!!

							Thanx, Paul

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