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

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Date:   Fri, 17 Jan 2020 09:58:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, ak@...ux.intel.com
Subject: Re: [RESEND PATCH] perf/x86/intel: Fix inaccurate period in context
 switch for auto-reload

On Thu, Jan 16, 2020 at 10:31:54AM -0800, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.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.