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Date:   Fri, 17 Jan 2020 09:58:41 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     kan.liang@linux.intel.com
Cc:     mingo@redhat.com, acme@kernel.org, linux-kernel@vger.kernel.org,
        eranian@google.com, ak@linux.intel.com
Subject: Re: [RESEND PATCH V3] perf/x86: Consider pinned events for group
 validation
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On Thu, Jan 16, 2020 at 11:00:25AM -0800, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
> 
> perf stat -M metrics relies on weak groups to reject unschedulable
> groups and run them as non-groups.
> This uses the group validation code in the kernel. Unfortunately
> that code doesn't take pinned events, such as the NMI watchdog, into
> account. So some groups can pass validation, but then later still
> never schedule.
> 
> For example,
> 
>  $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%)
> 
>        1.176613558 seconds time elapsed

More unreadable mess.