Re: [PATCH] perf lock contention: Clear lock addr after use

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

Em Thu, Sep 28, 2023 at 04:50:18PM -0700, Namhyung Kim escreveu:
> It checks the current lock to calculated the delta of contention time.

> The address is saved in the tstamp map which is allocated at begining of
> contention and released at end of contention.
> 
> But it's possible for bpf_map_delete_elem() to fail.  In that case, the

How can it fail? 

You do:

        pelem = bpf_map_lookup_elem(&tstamp, &pid);
        if (!pelem || pelem->lock != ctx[0])
                return 0;

So it is there, why would the removal using the same key fail?

The patch should work as-is, I'm just curious about what would make
there removal of a map entry that was successfully looked up on the same
contention_end prog to fail when being removed...

- Arnaldo

> element in the tstamp map kept for the current lock and it makes the
> next contention for the same lock tracked incorrectly.  Specificially
> the next contention begin will see the existing element for the task and
> it'd just return.  Then the next contention end will see the element and
> calculate the time using the timestamp for the previous begin.
> 
> This can result in a large value for two small contentions happened from
> time to time.  Let's clear the lock address so that it can be updated
> next time even if the bpf_map_delete_elem() failed.
> 
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/bpf_skel/lock_contention.bpf.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index 4900a5dfb4a4..b11179452e19 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -389,6 +389,7 @@ int contention_end(u64 *ctx)
>  
>  	duration = bpf_ktime_get_ns() - pelem->timestamp;
>  	if ((__s64)duration < 0) {
> +		pelem->lock = 0;
>  		bpf_map_delete_elem(&tstamp, &pid);
>  		__sync_fetch_and_add(&time_fail, 1);
>  		return 0;
> @@ -422,6 +423,7 @@ int contention_end(u64 *ctx)
>  	data = bpf_map_lookup_elem(&lock_stat, &key);
>  	if (!data) {
>  		if (data_map_full) {
> +			pelem->lock = 0;
>  			bpf_map_delete_elem(&tstamp, &pid);
>  			__sync_fetch_and_add(&data_fail, 1);
>  			return 0;
> @@ -445,6 +447,7 @@ int contention_end(u64 *ctx)
>  				data_map_full = 1;
>  			__sync_fetch_and_add(&data_fail, 1);
>  		}
> +		pelem->lock = 0;
>  		bpf_map_delete_elem(&tstamp, &pid);
>  		return 0;
>  	}
> @@ -458,6 +461,7 @@ int contention_end(u64 *ctx)
>  	if (data->min_time > duration)
>  		data->min_time = duration;
>  
> +	pelem->lock = 0;
>  	bpf_map_delete_elem(&tstamp, &pid);
>  	return 0;
>  }
> -- 
> 2.42.0.582.g8ccd20d70d-goog
> 

-- 

- Arnaldo