linux-kernel - Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler

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Date:   Mon, 6 Aug 2018 23:50:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     kan.liang@...ux.intel.com, tglx@...utronix.de, mingo@...hat.com,
        acme@...nel.org, linux-kernel@...r.kernel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH 2/3] x86, perf: Add a separate Arch Perfmon v4 PMI handler

On Mon, Aug 06, 2018 at 02:33:23PM -0700, Andi Kleen wrote:
> On Mon, Aug 06, 2018 at 08:35:15PM +0200, Peter Zijlstra wrote:
> > > +static bool disable_counter_freezing;
> > > +module_param(disable_counter_freezing, bool, 0444);
> > > +MODULE_PARM_DESC(disable_counter_freezing, "Disable counter freezing feature."
> > > +		"The PMI handler will fall back to generic handler."
> > > +		"Default is false (enable counter freezing feature).");
> > 
> > Why?
> 
> See the description. Counter freezing took some time to stabilize,
> so it seemed better to have a knob to ask users to try in case
> there are more problems.

But it is not a module.. did you want early_param() or __setup()?

> > > +	/*
> > > +	 * Ack the PMU late after the APIC.  This avoids bogus
> > 
> > > +	 * freezing on Skylake CPUs.  The acking unfreezes the PMU
> > > +	 */
> > That doesn't make sense. PMU and APIC do not have order.> 
> 
> It makes a difference for the hardware.

I still have no clue what it wants to say.

> > > +	/*
> > > +	 * For arch perfmon 4 use counter freezing to avoid
> > > +	 * several MSR accesses in the PMI.
> > > +	 */
> > > +	if (x86_pmu.counter_freezing) {
> > > +		x86_pmu.handle_irq = intel_pmu_handle_irq_v4;
> > > +		pr_cont("counter freezing, ");
> > > +	}
> > 
> > Lets not print the counter freezing, we already print v4, right?
> 
> I find it useful to see that the kernel has the support, otherwise
> you would need to look at the version number, but it gets difficult
> with backports. This is another paranoia bit, in case there
> are problems.

That line will get ver long if we keep adding every dinky bit to it.

> > > @@ -561,6 +566,7 @@ struct x86_pmu {
> > >  	struct x86_pmu_quirk *quirks;
> > >  	int		perfctr_second_write;
> > >  	bool		late_ack;
> > > +	bool		counter_freezing;
> > 
> > Please make the both of them int or something.
> 
> That would make them bigger for no reason?

Then use u8 or something, I just don't much like _Bool in composite
types.