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SubjectRe: [PATCH V2 04/23] perf/x86/intel: Support adaptive PEBSv4
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On 3/21/2019 5:20 PM, Peter Zijlstra wrote:
> On Thu, Mar 21, 2019 at 01:56:44PM -0700, kan.liang@linux.intel.com 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);
>> }

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