linux-kernel - Re: [RFC PATCH bpf-next 1/2] Enhance BPF execution timing by excluding IRQ time

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Message-ID: <20250422142352.GA15651@noisy.programming.kicks-ass.net>
Date: Tue, 22 Apr 2025 16:23:52 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jianlin Lv <iecedge@...il.com>
Cc: bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com,
	song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com,
	kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com,
	jolsa@...nel.org, mingo@...hat.com, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, linux-kernel@...r.kernel.org, jianlv@...y.com
Subject: Re: [RFC PATCH  bpf-next 1/2] Enhance BPF execution timing by
 excluding IRQ time

On Tue, Apr 22, 2025 at 09:47:26PM +0800, Jianlin Lv wrote:
> From: Jianlin Lv <iecedge@...il.com>
> 
> This commit excludes IRQ time from the total execution duration of BPF
> programs. When CONFIG_IRQ_TIME_ACCOUNTING is enabled, IRQ time is
> accounted for separately, offering a more accurate assessment of CPU
> usage for BPF programs.
> 
> Signed-off-by: Jianlin Lv <iecedge@...il.com>
> ---
>  include/linux/filter.h | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index f5cf4d35d83e..3e0f975176a6 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -703,12 +703,32 @@ static __always_inline u32 __bpf_prog_run(const struct bpf_prog *prog,
>  	cant_migrate();
>  	if (static_branch_unlikely(&bpf_stats_enabled_key)) {
>  		struct bpf_prog_stats *stats;
> -		u64 duration, start = sched_clock();
> +		u64 duration, start, start_time, end_time, irq_delta;
>  		unsigned long flags;
> +		unsigned int cpu;
>  
> -		ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
> +		#ifdef CONFIG_IRQ_TIME_ACCOUNTING
> +		if (in_task()) {
> +			cpu = get_cpu();
> +			put_cpu();
> +			start_time = irq_time_read(cpu);

This is all sorts of daft.. you don't need get_cpu()/put_cpu().

> +		}
> +		#endif
>  
> +		start = sched_clock();
> +		ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
>  		duration = sched_clock() - start;
> +
> +		#ifdef CONFIG_IRQ_TIME_ACCOUNTING
> +		if (in_task()) {
> +			end_time = irq_time_read(cpu);
> +			if (end_time > start_time) {
> +				irq_delta = end_time - start_time;
> +				duration -= irq_delta;
> +			}
> +		}
> +		#endif

This is really dodgy coding style. Please keep the preprocessor
directives at column 0.

What do you think about steal-time, do you want to remove that from your
BPF runtime too?

If so, perhaps expose the scheduler's clock_task, which does both things
already?