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.li...@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);
  }