linux-kernel - Re: [PATCH V6] powercap/drivers/idle_injection: Add an idle injection framework

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Message-ID: <20180612125247.GO12217@hirez.programming.kicks-ass.net>
Date:   Tue, 12 Jun 2018 14:52:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     viresh.kumar@...aro.org, rjw@...ysocki.net,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Eduardo Valentin <edubezval@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Leo Yan <leo.yan@...aro.org>,
        Kevin Wangtao <kevin.wangtao@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rui Zhang <rui.zhang@...el.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Subject: Re: [PATCH V6] powercap/drivers/idle_injection: Add an idle
 injection framework

On Tue, Jun 12, 2018 at 02:44:29PM +0200, Daniel Lezcano wrote:
> On 12/06/2018 14:30, Peter Zijlstra wrote:
> > On Tue, Jun 12, 2018 at 02:00:11PM +0200, Daniel Lezcano wrote:
> >> +static void __idle_injection_wakeup(struct idle_injection_device *ii_dev)
> >> +{
> >> +	struct idle_injection_thread *iit;
> >> +	struct cpumask tmp;
> >> +	unsigned int cpu;
> >> +
> >> +	cpumask_and(&tmp, ii_dev->cpumask, cpu_online_mask);
> > 
> > You should not be having a cpumask on the stack. Those things can be
> > ginormous.
> 
> Ok, the kernel code uses of cpumask_t on the stack when dealing with
> cpumask_and. I assume it is also not recommended.

Yes, that should all get fixed. It's mostly legacy code I suppose. It's
been at least 10 years I think since we merged the whole
CPUMASK_OFFSTACK stuff.

> What would be the best practice ? Allocate a per cpumask at init time as
> a temporary mask to work with ?

In this case, you can do:

+       for_each_cpu_and(cpu, &ii_dev->cpumask, cpu_online_mask) {
+               iit = per_cpu_ptr(&idle_injection_thread, cpu);
+               iit->should_run = 1;
+               wake_up_process(iit->tsk);
+       }