linux-kernel - Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU mask

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Message-ID: <20170406093426.qf3zqrkuj7zvyhlm@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2017 11:34:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU
 mask

On Wed, Apr 05, 2017 at 09:39:43AM +0200, Ingo Molnar wrote:

> > While converting the existing users I tried to stick with the rules
> > above however… well mostly CPUFREQ tries to temporary switch the CPU
> > mask to do something on a certain CPU and then switches the mask back it
> > its original value. So in theory `cpus_ptr' could or should be used.
> > However if this is invoked in a migration disabled region (which is not
> > the case because it would require something like preempt_disable() and
> > set_cpus_allowed_ptr() might sleep so it can't be) then the "restore"
> > part would restore the wrong mask. So it only looks strange and I go for
> > the pointer…
> 
> So maybe we could add the following facility:
> 
> 	ptr = sched_migrate_to_cpu_save(cpu);
> 
> 	...
> 
> 	sched_migrate_to_cpu_restore(ptr);
> 
> ... and use it in the cpufreq code. Then -rt could simply define migrate_disable() 
> to be:

Argh, no....

We have plenty of proper per-cpu kthreads / workqueue and other APIs to
do this.

Not to mention that implementing the above in a non-broken way has user
visible side-effects, which ugly.