Re: [PATCH v2] tracing/hwlat: Do the latency calculations in nanoseconds

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] tracing/hwlat: get_sample() leaves NMI callback enabled on
  goto out path
--

commit b2b5eb31ef0ca8b02c68fc0666337a94df8c67b9
Author: Wenwu Hou <hwenwur@gmail.com>

tracing/hwlat: Do the latency calculations in nanoseconds

This commit changes get_sample() to perform its latency calculations in
nanoseconds so tr->max_latency is stored in nanoseconds, converting the
durations to microseconds only when recording the hwlat event.

> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> index 3fe274b84f1c2..b230cf14de93e 100644
> --- a/kernel/trace/trace_hwlat.c
> +++ b/kernel/trace/trace_hwlat.c
> @@ -203,14 +203,12 @@ static int get_sample(void)
>  	time_type start, t1, t2, last_t2;
>  	s64 diff, outer_diff, total, last_total = 0;
>  	u64 sample = 0;
> -	u64 sample_width = READ_ONCE(hwlat_data.sample_width);
> +	u64 sample_width = READ_ONCE(hwlat_data.sample_width) * NSEC_PER_USEC;
>  	u64 thresh = tracing_thresh;
>  	u64 outer_sample = 0;
>  	int ret = -1;
>  	unsigned int count = 0;
>  
> -	do_div(thresh, NSEC_PER_USEC); /* modifies interval value */
> -
>  	kdata->nmi_total_ts = 0;
>  	kdata->nmi_count = 0;
>  	/* Make sure NMIs see this first */
> @@ -229,7 +227,7 @@ static int get_sample(void)
>  
>  		if (time_u64(last_t2)) {
>  			/* Check the delta from outer loop (t2 to next t1) */
> -			outer_diff = time_to_us(time_sub(t1, last_t2));
> +			outer_diff = time_sub(t1, last_t2);
>  			/* This shouldn't happen */
>  			if (outer_diff < 0) {
>  				hwlat_err(BANNER "time running backwards\n");
> @@ -240,7 +238,7 @@ static int get_sample(void)
>  		}
>  		last_t2 = t2;
>  
> -		total = time_to_us(time_sub(t2, start)); /* sample width */
> +		total = time_sub(t2, start); /* sample width */
>  
>  		/* Check for possible overflows */
>  		if (total < last_total) {
> @@ -250,7 +248,7 @@ static int get_sample(void)
>  		last_total = total;
>  
>  		/* This checks the inner loop (t1 to t2) */
> -		diff = time_to_us(time_sub(t2, t1));     /* current diff */
> +		diff = time_sub(t2, t1);     /* current diff */

[Severity: Medium]
Can the "goto out" paths when outer_diff < 0 or diff < 0 leave
trace_hwlat_callback_enabled set to true?

Previously, time_to_us() passed the delta through unsigned div_u64(), which
cleared the sign bit and made the negative checks unreachable.  Assigning
time_sub() directly makes those error paths reachable on a backward clock
reading, and jumping to out skips clearing trace_hwlat_callback_enabled:

kernel/trace/trace_hwlat.c:get_sample() {
	...
	trace_hwlat_callback_enabled = true;
	...
	do {
		...
			if (outer_diff < 0) {
				hwlat_err(BANNER "time running backwards\n");
				goto out;
			}
		...
		if (diff < 0) {
			hwlat_err(BANNER "time running backwards\n");
			goto out;
		}
		...
	} while (total <= sample_width);

	barrier(); /* finish the above in the view for NMIs */
	trace_hwlat_callback_enabled = false;
	...
out:
	return ret;
}

Because nothing else resets trace_hwlat_callback_enabled, NMI tracking stays
active while kthread_fn() re-enables interrupts and sleeps, and remains enabled
even after the tracer is stopped if kthread_stop() wakes the thread before
another sample finishes normally.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261112121502.2070755-1-hwenwur@gmail.com?part=1