Re: [PATCH 07/10] perf tools: Separate exclude_hv fallback

[Liang, Kan]

On 2024-09-05 4:24 p.m., Namhyung Kim wrote:
> The exclude_hv was added in the evsel__fallback() in the commit
> 4ec8d984895fef43a ("perf record: Fix priv level with branch sampling
> for paranoid=2") to address branch stack samples on Intel PMUs.
> As some other PMUs might not support that, let's separate the bit from
> exclude_kernel to make sure it can add the bit only if required.
> 
> Technically it should change the modifier string at the end of the
> event name.  ":u" is for exclude_kernel + exclude_hv, so it should be
> ":uh" if it has exclude_kernel only.  That means the default events for
> regular users will looks like "cycles:Puh" (for perf record) or
> "instructions:uh" (for perf stat).  But I'm not sure if it's worth the
> trouble so I didn't touch the name in this patch.
> 
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/util/evsel.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 1a4f52767942e5ad..c5df45bb74dfc1b5 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -3389,10 +3389,20 @@ bool evsel__fallback(struct evsel *evsel, struct target *target, int err,
>  		free(evsel->name);
>  		evsel->name = new_name;
>  		scnprintf(msg, msgsize, "kernel.perf_event_paranoid=%d, trying "
> -			  "to fall back to excluding kernel and hypervisor "
> -			  " samples", paranoid);
> +			  "to fall back to excluding kernel samples", paranoid);
>  		evsel->core.attr.exclude_kernel = 1;
> -		evsel->core.attr.exclude_hv     = 1;
> +
> +		return true;
> +	} else if (err == EACCES && !evsel->core.attr.exclude_hv &&
> +		   (paranoid = perf_event_paranoid()) > 1) {
> +		/* If event has exclude user then don't exclude hv. */
> +		if (evsel->core.attr.exclude_user)
> +			return false;
> +
> +		/* Intel branch stack requires exclude_hv */

I don't think it's an requirement for Intel branch stack. The HV is
ignored for all X86.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/events/core.c#n542

I think it's a generic request for branch on all arch.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/events/core.c#n366

Thanks,
Kan

> +		scnprintf(msg, msgsize, "kernel.perf_event_paranoid=%d, trying "
> +			  "to fall back to excluding hypervisor samples", paranoid);
> +		evsel->core.attr.exclude_hv = 1;
>  
>  		return true;
>  	} else if (err == EOPNOTSUPP && !evsel->core.attr.exclude_guest &&