linux-kernel - Re: [PATCH 05/10] nohz: New tick dependency mask

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Date:	Mon, 3 Aug 2015 15:09:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 05/10] nohz: New tick dependency mask

On Mon, Aug 03, 2015 at 02:57:17PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 23, 2015 at 06:42:10PM +0200, Frederic Weisbecker wrote:
> > +void tick_nohz_set_tick_dependency(enum tick_dependency_bit bit)
> > +{
> > +	unsigned long prev;
> > +
> > +	prev = __tick_nohz_set_tick_dependency(bit, &tick_dependency);
> > +	if (!prev)
> > +		tick_nohz_full_kick_all();
> > +}
> 
> > +void tick_nohz_set_tick_dependency_cpu(enum tick_dependency_bit bit, int cpu)
> > +{
> > +	unsigned long prev;
> > +	struct tick_sched *ts;
> > +
> > +	ts = per_cpu_ptr(&tick_cpu_sched, cpu);
> > +
> > +	prev = __tick_nohz_set_tick_dependency(bit, &ts->tick_dependency);
> > +	if (!prev)
> > +		tick_nohz_full_kick_cpu(cpu);
> > +}
> 
> > +/*
> > + * Local dependency must have its own flavour due to NMI-safe requirement
> > + * on perf.
> > + */
> 
> That doesn't make any sense:
> 
>   tick_nohz_set_tick_dependency_this_cpu();
> 
> (shees, you're nowhere near lazy enough, that's insane to type) is
> almost identical to:
> 
>   tick_nohz_set_tick_dependency_cpu(.cpu = smp_processor_id());
> 
> The only difference is a _very_ slight reduction in cost for computing
> the per-cpu offset.

But the local one must be NMI-safe. Now I can do:

    if (cpu == smp_processor_id())
        tick_nohz_full_kick() // NMI-safe
    else
        tick_nohz_full_kick_cpu(cpu); // not NMI-safe.

> 
> > +void tick_nohz_set_tick_dependency_this_cpu(enum tick_dependency_bit bit)
> > +{
> > +	unsigned long prev;
> > +	struct tick_sched *ts;
> > +
> > +	ts = this_cpu_ptr(&tick_cpu_sched);
> > +
> > +	prev = __tick_nohz_set_tick_dependency(bit, &ts->tick_dependency);
> > +	if (!prev)
> > +		tick_nohz_full_kick();
> > +}
> 
> 
> And on that naming; could we please shorten them, this is really
> ridiculous, it has 'tick' in it twice.
> 
> What's wrong with:
> 
> 	tick_nohz_set_dep()
> 	tick_nohz_set_dep_cpu()

Right.

Thanks.
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