linux-kernel - Re: [patch 02/13] workqueue: Provide work_on_cpu

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Message-ID: <20170414095655.qb3r2mkqvsgaqnut@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Apr 2017 11:56:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "David S. Miller" <davem@...emloft.net>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Len Brown <lenb@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [patch 02/13] workqueue: Provide work_on_cpu_safe()

On Fri, Apr 14, 2017 at 11:51:07AM +0200, Thomas Gleixner wrote:
> On Fri, 14 Apr 2017, Peter Zijlstra wrote:
> 
> > On Wed, Apr 12, 2017 at 10:07:28PM +0200, Thomas Gleixner wrote:
> > > +long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg)
> > > +{
> > > +	long ret = -ENODEV;
> > > +
> > > +	get_online_cpus();
> > > +	if (cpu_online(cpu))
> > > +		ret = work_on_cpu(cpu, fn, arg);
> > > +	put_online_cpus();
> > > +	return ret;
> > > +}
> > 
> > But doesn't workqueue have this lovelt 'feature' where it will unbind
> > per-cpu work and run it on random CPUs when hotplug happens?
> > 
> > That is, I think you need a flush_work() before put_online_cpus() if you
> > want to guarantee anything.
> 
> work_on_cpu() is sychnronous, it flushes already.

So much for being awake ...