Messages in this thread | | | Subject | Re: [PATCH 0/5] Support metric group constraint | From | "Liang, Kan" <> | Date | Thu, 20 Feb 2020 11:03:35 -0500 |
| |
On 2/20/2020 6:39 AM, Jiri Olsa wrote: > On Wed, Feb 19, 2020 at 11:08:35AM -0800, kan.liang@linux.intel.com wrote: >> From: Kan Liang <kan.liang@linux.intel.com> >> >> 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@hirez.programming.kicks-ass.net/ >> 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 >
| |