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

[Wenwu Hou <hwenwur@gmail.com>]

On Thu, Nov 12, 2026 at 12:28:19PM +0000, sashiko-bot@kernel.org wrote:
> 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:
> 

will fix in v3


> 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