Re: [RESEND PATCH] perf/x86/intel: Fix inaccurate period in context switch for auto-reload

Peter Zijlstra
2020-01-17T08:58:30Z

On Thu, Jan 16, 2020 at 10:31:54AM -0800, kan.liang@linux.intel.com wrote:
> From: Kan Liang <kan.liang@linux.intel.com>
>
> Perf doesn't take the left period into account when auto-reload is
> enabled with fixed period sampling mode in context switch.
> Here is the ftrace when recording PEBS event with fixed period.
>
> #perf record -e cycles:p -c 2000000 -- ./triad_loop
>
> //Task is scheduled out
> triad_loop-17222 [000] d... 861765.878032: write_msr:
> MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0 //Disable global counter
> triad_loop-17222 [000] d... 861765.878033: write_msr:
> MSR_IA32_PEBS_ENABLE(3f1), value 0 //Disable PEBS
> triad_loop-17222 [000] d... 861765.878033: write_msr:
> MSR_P6_EVNTSEL0(186), value 40003003c //Disable the counter
> triad_loop-17222 [000] d... 861765.878033: rdpmc: 0, value
> fffffff82840 //Read value of the counter
> triad_loop-17222 [000] d... 861765.878034: write_msr:
> MSR_CORE_PERF_GLOBAL_CTRL(38f), value 1000f000000ff //Re-enable global
> counter

This is unreadable garbage, please don't wrap trace output.