linux-kernel - Re: [PATCH] perf lock contention: Reject more than 10ms delays for safety

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Message-ID: <CAP-5=fUTXv00r1B=2JQX4nPhZfG+WOQwGrAWmcWAh29fNZz-Kg@mail.gmail.com>
Date: Thu, 15 May 2025 11:18:38 -0700
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Kan Liang <kan.liang@...ux.intel.com>, 
	Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, 
	LKML <linux-kernel@...r.kernel.org>, linux-perf-users@...r.kernel.org, 
	Song Liu <song@...nel.org>, bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH] perf lock contention: Reject more than 10ms delays for safety

Nit, in the subject line for clarity perhaps rather than "perf lock
contention:" call it "perf lock contention/delay" or "perf lock
delay".

On Thu, May 15, 2025 at 11:10 AM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Delaying kernel operations can be dangerous and the kernel may kill
> (non-sleepable) BPF programs running for long in the future.
>
> Limit the max delay to 10ms and update the document about it.
>
>   $ sudo ./perf lock con -abl -J 100000us@...oup_mutex true
>   lock delay is too long: 100000us (> 10ms)
>
>    Usage: perf lock contention [<options>]
>
>       -J, --inject-delay <TIME@...C>
>                             Inject delays to specific locks
>
> Suggested-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> ---
>  tools/perf/Documentation/perf-lock.txt | 8 ++++++--
>  tools/perf/builtin-lock.c              | 5 +++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/Documentation/perf-lock.txt b/tools/perf/Documentation/perf-lock.txt
> index 2d9aecf630422aa6..c17b3e318169f9dc 100644
> --- a/tools/perf/Documentation/perf-lock.txt
> +++ b/tools/perf/Documentation/perf-lock.txt
> @@ -224,8 +224,12 @@ CONTENTION OPTIONS
>         only with -b/--use-bpf.
>
>         The 'time' is specified in nsec but it can have a unit suffix.  Available
> -       units are "ms" and "us".  Note that it will busy-wait after it gets the
> -       lock.  Please use it at your own risk.
> +       units are "ms", "us" and "ns".  Currently it accepts up to 10ms of delays
> +       for safety reasons.
> +
> +       Note that it will busy-wait after it gets the lock. Delaying locks can
> +       have significant consequences including potential kernel crashes.  Please
> +       use it at your own risk.
>
>
>  SEE ALSO
> diff --git a/tools/perf/builtin-lock.c b/tools/perf/builtin-lock.c
> index 41f6f3d2b779b986..3b3ade7a39cad01f 100644
> --- a/tools/perf/builtin-lock.c
> +++ b/tools/perf/builtin-lock.c
> @@ -2537,6 +2537,11 @@ static bool add_lock_delay(char *spec)
>                 return false;
>         }
>
> +       if (duration > 10 * 1000 * 1000) {

nit: It's unfortunate the variable name isn't carrying the time unit.
For example, this could be:
```
if (duration_ns > 10 * NSEC_PER_SEC) {
```
which should hopefully make it clearer what the time units are and
that they aren't messed up.

Thanks,
Ian

> +               pr_err("lock delay is too long: %s (> 10ms)\n", spec);
> +               return false;
> +       }
> +
>         tmp = realloc(delays, (nr_delays + 1) * sizeof(*delays));
>         if (tmp == NULL) {
>                 pr_err("Memory allocation failure\n");
> --
> 2.49.0.1101.gccaa498523-goog
>