Re: [PATCH] perf iostat: Do not compute interval deltas a second time

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Nov 05, 2026 at 04:04:29PM +0800, lirongqing wrote:
> From: Li RongQing <lirongqing@baidu.com>
> 
> process_counter_values() now computes deltas directly in evsel->counts
> for global aggregation too. It saves the current cumulative raw count in
> prev_raw_counts before iostat prints the interval.
> 
> Subtracting prev_raw_counts again can therefore underflow the count.
> Worse, replacing the saved raw count with the interval delta corrupts
> the baseline used to calculate the next interval.

Can you share example output before and after this change?

Thanks,
Namhyung

> 
> Use the already computed delta and leave prev_raw_counts untouched.
> 
> Fixes: f976bc6b6bfc ("perf stat: Aggregate events using evsel->stats->aggr")
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>  tools/perf/arch/x86/util/iostat.c | 10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/tools/perf/arch/x86/util/iostat.c b/tools/perf/arch/x86/util/iostat.c
> index 0cf0f93..fac6b4a 100644
> --- a/tools/perf/arch/x86/util/iostat.c
> +++ b/tools/perf/arch/x86/util/iostat.c
> @@ -434,20 +434,12 @@ void iostat_print_metric(struct perf_stat_config *config, struct evsel *evsel,
>  			 struct perf_stat_output_ctx *out)
>  {
>  	double iostat_value = 0;
> -	u64 prev_count_val = 0;
>  	const char *iostat_metric = iostat_metric_by_idx(evsel->core.idx);
>  	u8 die = ((struct iio_root_port *)evsel->priv)->die;
>  	struct perf_counts_values *count = perf_counts(evsel->counts, die, 0);
>  
>  	if (count && count->run && count->ena) {
> -		if (evsel->prev_raw_counts && !out->force_header) {
> -			struct perf_counts_values *prev_count =
> -				perf_counts(evsel->prev_raw_counts, die, 0);
> -
> -			prev_count_val = prev_count->val;
> -			prev_count->val = count->val;
> -		}
> -		iostat_value = (count->val - prev_count_val) /
> +		iostat_value = count->val /
>  			       ((double) count->run / count->ena);
>  	}
>  	out->print_metric(config, out->ctx, METRIC_THRESHOLD_UNKNOWN, "%8.0f", iostat_metric,
> -- 
> 2.9.4
>