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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Thu, May 15, 2025 at 11:10:42AM -0700, Namhyung Kim 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@cgroup_mutex true
>   lock delay is too long: 100000us (> 10ms)
> 
>    Usage: perf lock contention [<options>]
> 
>       -J, --inject-delay <TIME@FUNC>
>                             Inject delays to specific locks
> 
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Namhyung Kim <namhyung@kernel.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) {
> +		pr_err("lock delay is too long: %s (> 10ms)\n", spec);
> +		return false;
> +	}
> +

Please consider to replace those 1000 * 1000 her and in other places
with NSEC_PER_MSEC in a followup patch for the next merge window.

- Arnaldo

>  	tmp = realloc(delays, (nr_delays + 1) * sizeof(*delays));
>  	if (tmp == NULL) {
>  		pr_err("Memory allocation failure\n");
> -- 
> 2.49.0.1101.gccaa498523-goog