Re: [PATCH 0/5] Support metric group constraint

[Liang, Kan]

On 2/20/2020 6:39 AM, Jiri Olsa wrote:
> On Wed, Feb 19, 2020 at 11:08:35AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> >
> > Some metric groups, e.g. Page_Walks_Utilization, will never count when
> > NMI watchdog is enabled.
> >
> >   $echo 1 > /proc/sys/kernel/nmi_watchdog
> >   $perf stat -M Page_Walks_Utilization
> >
> >   Performance counter stats for 'system wide':
> >
> >   <not counted>      itlb_misses.walk_pending       (0.00%)
> >   <not counted>      dtlb_load_misses.walk_pending  (0.00%)
> >   <not counted>      dtlb_store_misses.walk_pending (0.00%)
> >   <not counted>      ept.walk_pending               (0.00%)
> >   <not counted>      cycles                         (0.00%)
> >
> >         2.343460588 seconds time elapsed
> >
> >   Some events weren't counted. Try disabling the NMI watchdog:
> >          echo 0 > /proc/sys/kernel/nmi_watchdog
> >          perf stat ...
> >          echo 1 > /proc/sys/kernel/nmi_watchdog
> >   The events in group usually have to be from the same PMU. Try
> >   reorganizing the group.
> >
> > A metric group is a weak group, which relies on group validation
> > code in the kernel to determine whether to be opened as a group or
> > a non-group. However, group validation code may return false-positives,
> > especially when NMI watchdog is enabled. (The metric group is allowed
> > as a group but will never be scheduled.)
> >
> > The attempt to fix the group validation code has been rejected.
> > https://lore.kernel.org/lkml/20200117091341.GX2827@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Because we cannot accurately predict whether the group can be scheduled
> > as a group, only by checking current status.
> >
> > This patch set provides another solution to mitigate the issue.
> > Add "MetricConstraint" in event list, which provides a hint for perf tool,
> > e.g. "MetricConstraint": "NO_NMI_WATCHDOG". Perf tool can change the
> > metric group to non-group (standalone metrics) if NMI watchdog is enabled.
>
> the problem is in the missing counter, that's taken by NMI watchdog, right?
>
> and it's problem for any metric that won't fit to the available
> counters.. shouldn't we rather do this workaround for any metric
> that wouldn't fit in available counters? 

I think current perf already did this.
All metric groups are weak group. Kernel (validate_group()) tells perf 
tool whether a metric group fit to available counters.
If yes, the metric group will be scheduled as a group.
If no, perf tool will not using a group and re-try. The code is as below.

 try_again:
 		if (create_perf_stat_counter(counter, &stat_config, &target) < 0) {

 			/* Weak group failed. Reset the group. */
 			if ((errno == EINVAL || errno == EBADF) &&
 			    counter->leader != counter &&
 			    counter->weak_group) {
 				counter = perf_evlist__reset_weak_group(evsel_list, counter);
 				goto try_again;
 			}


This patch set is to workaroud the false-positives from the kernel.
Kernel only validate the group itself. It assumes that all counters are 
available. So, when any counter is permanently occupied by others, e.g. 
watchdog, the validate_group() may return false-positives.

? not just for chosen
> ones..?

For now, I think we only need to workaround the Page_Walks_Utilization 
metric group. Because it has 5 events, and one of the events is cycles.
The cycles has to be scheduled on fixed counter 2. But it's already 
occupied by watchdog.
The false-positives of validate_group() will trigger the issue (metric 
group never be scheduled.).

For other metric groups, even they have cycles, the issue should not be 
triggered.
For example, if they have 4 or less events, the cycles can be scheduled 
to GP counter instead.
If they have 6 or more events, the weak group will be reject anyway.
Perf tool will open it as non-group (standalone metrics).

I think we only need to apply the constraint for the 
Page_Walks_Utilization metric group for now.


Thanks,
Kan

> thanks,
> jirka