On Fri, 9 Mar 2018, tip-bot for Kan Liang wrote:
> Commit-ID: 1af22eba248efe2de25658041a80a3d40fb3e92e
> Gitweb:
> https://git.kernel.org/tip/1af22eba248efe2de25658041a80a3d40fb3e92e
> Author: Kan Liang <[email protected]>
> AuthorDate: Mon, 12 Feb 2018 14:20:35 -0800
> Committer: Ingo Molnar <[email protected]>
> CommitDate: Fri, 9 Mar 2018 08:22:23 +0100
>
> perf/x86/intel: Disable userspace RDPMC usage for large PEBS
>
So this whole commit log is about disabling RDPMC usage for "large PEBS"
but the actual change disables RDPMC if "PERF_X86_EVENT_FREERUNNING"
Either the commit log is really misleading, or else a poor name was chosen
for this feature.
Vince
> Userspace RDPMC cannot possibly work for large PEBS, which was introduced in:
>
> b8241d20699e ("perf/x86/intel: Implement batched PEBS interrupt handling
> (large PEBS interrupt threshold)")
>
> When the PEBS interrupt threshold is larger than one, there is no way
> to get exact auto-reload times and value for userspace RDPMC. Disable
> the userspace RDPMC usage when large PEBS is enabled.
>
> The only exception is when the PEBS interrupt threshold is 1, in which
> case user-space RDPMC works well even with auto-reload events.
>
> Signed-off-by: Kan Liang <[email protected]>
> Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
> Cc: Alexander Shishkin <[email protected]>
> Cc: Arnaldo Carvalho de Melo <[email protected]>
> Cc: Jiri Olsa <[email protected]>
> Cc: Linus Torvalds <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Stephane Eranian <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Vince Weaver <[email protected]>
> Cc: [email protected]
> Fixes: b8241d20699e ("perf/x86/intel: Implement batched PEBS interrupt
> handling (large PEBS interrupt threshold)")
> Link:
> http://lkml.kernel.org/r/[email protected]
> Signed-off-by: Ingo Molnar <[email protected]>
> ---
> arch/x86/events/core.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index 00a6251981d2..9c86e10f1196 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2117,7 +2117,8 @@ static int x86_pmu_event_init(struct perf_event *event)
> event->destroy(event);
> }
>
> - if (READ_ONCE(x86_pmu.attr_rdpmc))
> + if (READ_ONCE(x86_pmu.attr_rdpmc) &&
> + !(event->hw.flags & PERF_X86_EVENT_FREERUNNING))
> event->hw.flags |= PERF_X86_EVENT_RDPMC_ALLOWED;
>
> return err;
>
