Re: [PATCH V2 04/23] perf/x86/intel: Support adaptive PEBSv4

[Liang, Kan]

On 3/21/2019 5:20 PM, Peter Zijlstra wrote:
> On Thu, Mar 21, 2019 at 01:56:44PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> > @@ -933,6 +1001,34 @@ pebs_update_state(bool needed_cb, struct cpu_hw_events *cpuc, struct pmu *pmu)
> >   		update = true;
> >   	}
> >   
> > +	/*
> > +	 * The PEBS record doesn't shrink on the del. Because to get
> > +	 * an accurate config needs to go through all the existing pebs events.
> > +	 * It's not necessary.
> > +	 * There is no harmful for a bigger PEBS record, except little
> > +	 * performance impacts.
> > +	 * Also, for most cases, the same pebs config is applied for all
> > +	 * pebs events.
> > +	 */
> > +	if (x86_pmu.intel_cap.pebs_baseline && add) {
> > +		u64 pebs_data_cfg;
> > +
> > +		/* Clear pebs_data_cfg and pebs_record_size for first PEBS. */
> > +		if (cpuc->n_pebs == 1) {
> > +			cpuc->pebs_data_cfg = 0;
> > +			cpuc->pebs_record_size = sizeof(struct pebs_basic);
> > +		}
>
> Argh, no. This is daft. The previous site was fine, it was just the
> pebs_record_size assignment I'm confused about.
>
> Note how by setting ->pebs_data_cfs to 0, you force the below branch to
> true and call adaptive_pebs_record_size_update() ? So _why_ do you have
> to set pebs_record_size()?

I think we have to reset both cpuc->pebs_data_cfg and 
cpuc->pebs_record_size. Because pebs_update_adaptive_cfg() can return 0.
If so, adaptive_pebs_record_size_update() will not be called. The 
cpuc->pebs_record_size still use the stale data, which may be wrong.

I think there is no difference to reset them in first add or last del.
If so, I will keep the code here unchanged.

I will prepare V3 to address other comments.

Thanks,
Kan

> > +
> > +		pebs_data_cfg = pebs_update_adaptive_cfg(event);
> > +
> > +		/* Update pebs_record_size if new event requires more data. */
> > +		if (pebs_data_cfg & ~cpuc->pebs_data_cfg) {
> > +			cpuc->pebs_data_cfg |= pebs_data_cfg;
> > +			adaptive_pebs_record_size_update();
> > +			update = true;
> > +		}
> > +	}
> > +
> >   	if (update)
> >   		pebs_update_threshold(cpuc);
> >   }