Re: [PATCHv2 3/4] arm-cci: Add routines to enable/disable all counters

[Suzuki K. Poulose]

On 04/11/15 18:28, Mark Rutland wrote:
> On Tue, Oct 20, 2015 at 02:05:25PM +0100, Suzuki K. Poulose wrote:
> > Adds helper routines to manipulate the counter controls for
> > all the counters on the CCI PMU.
> >
> > pmu_disable_counters_ctrl: Iterates over the counters,
> > checking the status of each counter and disabling any enabled
> > counters. For each such changed counter, the mask is updated so that
> > one can restore the state later using pmu_enable_counters_ctrl.

> >   /*
> > + * Restore the status of the counters.
> > + * For each counter set in the mask, enable the counter back.
> > + */
> > +static void pmu_restore_counters_ctrl(struct cci_pmu *cci_pmu, unsigned long *mask)
> > +{
> > +	int i;
> > +
> > +	for_each_set_bit(i, mask, cci_pmu->num_cntrs)
> > +		pmu_enable_counter(cci_pmu, i);
> > +}
> > +
> > +/*
> > + * For all counters on the CCI-PMU, disable any 'enabled' counters,
> > + * saving the changed counters in the mask, so that we can restore
> > + * it later using pmu_restore_counters_ctrl.
> > + */
> > +static void pmu_disable_counters_ctrl(struct cci_pmu *cci_pmu, unsigned long *mask)
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < cci_pmu->num_cntrs; i++) {
> > +		clear_bit(i, mask);
> > +		if (pmu_get_counter_ctrl(cci_pmu, i)) {
> > +			set_bit(i, mask);
> > +			pmu_disable_counter(cci_pmu, i);
> > +		}
> > +	}
> > +}
>
> I don't understand what's going on with the mask here. Why do we clear
> ieach bit when the only user (introduced in the next patch) explicitly
> clears the mask anyway?

To be more precise, it should have been :

	if (pmu_get_counter_ctrl(cci_pmu, i)) {
		set_bit(i, mask);
		pmu_disable_counter(cci_pmu, i);
	} else
		clear_bit(i, mask);

> Can we not get rid of the mask entirely? The combination of used_mask
> and each event's hwc->state tells us which counters are actually in use.

The problem is that neither hwc->state nor the cci_pmu->hw_events->events is
protected by pmu_lock, while enable/disable counter is. So we cannot really
rely on ((struct perf_event *)(cci_pmu->hw_events->events[counter]))->hw->state.

What we do above is, create a mask of the counters which are enabled at the
moment and disable all of them. We then program the counter and then re-enable
those which were enabled (as marked in the mask).

Suzuki


> Thanks,
> Mark.

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