Re: [tip:perf/core] perf/x86/intel: Disable userspace RDPMC usage for large PEBS

[Vince Weaver <vincent.weaver@maine.edu>]

On Fri, 9 Mar 2018, tip-bot for Kan Liang wrote:

> Commit-ID:  1af22eba248efe2de25658041a80a3d40fb3e92e
> Gitweb:     https://git.kernel.org/tip/1af22eba248efe2de25658041a80a3d40fb3e92e
> Author:     Kan Liang <kan.liang@linux.intel.com>
> AuthorDate: Mon, 12 Feb 2018 14:20:35 -0800
> Committer:  Ingo Molnar <mingo@kernel.org>
> 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 <kan.liang@linux.intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Stephane Eranian <eranian@google.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vince Weaver <vincent.weaver@maine.edu>
> Cc: acme@kernel.org
> Fixes: b8241d20699e ("perf/x86/intel: Implement batched PEBS interrupt handling (large PEBS interrupt threshold)")
> Link: http://lkml.kernel.org/r/1518474035-21006-6-git-send-email-kan.liang@linux.intel.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  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;
>