linux-kernel - Re: [PATCH] perf tools: Refactor precise

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Message-ID: <aPrktlANBHFtV52B@google.com>
Date: Thu, 23 Oct 2025 19:30:14 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Zide Chen <zide.chen@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	thomas.falcon@...el.com, dapeng1.mi@...ux.intel.com,
	xudong.hao@...el.com
Subject: Re: [PATCH] perf tools: Refactor precise_ip fallback logic

Hello,

On Wed, Oct 22, 2025 at 03:08:02PM -0700, Zide Chen wrote:
> Commit c33aea446bf555ab ("perf tools: Fix precise_ip fallback logic")
> unconditionally called the precise_ip fallback and moved it after the
> missing-feature checks so that it could handle EINVAL as well.
> 
> However, this introduced an issue: after disabling missing features,
> the event could fail to open, which makes the subsequent precise_ip
> fallback useless since it will always fail.
> 
> For example, run the following command on Intel SPR:
> 
> $ perf record -e '{cpu/mem-loads-aux/S,cpu/mem-loads,ldlat=3/PS}' -- ls
> 
> Opening the event "cpu/mem-loads,ldlat=3/PS" returns EINVAL when
> precise_ip == 3. It then sets attr.inherit = false, which triggers a

I'm curious about this part.  Why the kernel set 'inherit = false'?  IOW
how did the leader event (mem-loads-aux) succeed with inherit = true
then?

> kernel check failure since it doesn't match the group leader's inherit
> attribute. As a result, it continues to fail even after precise_ip is
> reduced.
> 
> By moving the precise_ip fallback earlier, this issue is resolved, as
> well as the kernel test robot report mentioned in commit
> c33aea446bf555ab.
> 
> No negative side effects are expected, because the precise_ip level is
> restored by evsel__precise_ip_fallback() if the fallback does not help.

I'm not sure.. some events may need a different (i.e. lower) precise
level than the max.  I think checking the missing feature later will
use the max precise level always.

Thanks,
Namhyung

> 
> This also aligns with commit 2b70702917337a8d ("perf tools: Remove
> evsel__handle_error_quirks()").
> 
> Fixes: af954f76eea56453 ("perf tools: Check fallback error and order")
> Fixes: c33aea446bf555ab ("perf tools: Fix precise_ip fallback logic")
> Reviewed-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> Signed-off-by: Zide Chen <zide.chen@...el.com>
> ---
>  tools/perf/util/evsel.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index ca74514c8707..6ce32533a213 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -2714,12 +2714,12 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,
>  	if (err == -EMFILE && rlimit__increase_nofile(&set_rlimit))
>  		goto retry_open;
>  
> +	if (evsel__precise_ip_fallback(evsel))
> +		goto retry_open;
> +
>  	if (err == -EINVAL && evsel__detect_missing_features(evsel, cpu))
>  		goto fallback_missing_features;
>  
> -	if (evsel__precise_ip_fallback(evsel))
> -		goto retry_open;
> -
>  out_close:
>  	if (err)
>  		threads->err_thread = thread;
> -- 
> 2.51.0
>