linux-kernel - Re: [PATCH 1/2] cgroup: add cpu.stat

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Date:   Tue, 11 Jan 2022 13:49:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cgroup: add cpu.stat_percpu

On Fri, Jan 07, 2022 at 03:41:37PM -0800, Josh Don wrote:

> +	seq_puts(seq, "usage_usec");
> +	for_each_possible_cpu(cpu) {
> +		cached_bstat = per_cpu_ptr(&cached_percpu_stats, cpu);
> +		val = cached_bstat->cputime.sum_exec_runtime;
> +		do_div(val, NSEC_PER_USEC);
> +		seq_printf(seq, " %llu", val);
> +	}
> +	seq_puts(seq, "\n");
> +
> +	seq_puts(seq, "user_usec");
> +	for_each_possible_cpu(cpu) {
> +		cached_bstat = per_cpu_ptr(&cached_percpu_stats, cpu);
> +		val = cached_bstat->cputime.utime;
> +		do_div(val, NSEC_PER_USEC);
> +		seq_printf(seq, " %llu", val);
> +	}
> +	seq_puts(seq, "\n");
> +
> +	seq_puts(seq, "system_usec");
> +	for_each_possible_cpu(cpu) {
> +		cached_bstat = per_cpu_ptr(&cached_percpu_stats, cpu);
> +		val = cached_bstat->cputime.stime;
> +		do_div(val, NSEC_PER_USEC);
> +		seq_printf(seq, " %llu", val);
> +	}
> +	seq_puts(seq, "\n");

This is an anti-pattern; given enough CPUs (easy) this will trivially
overflow the 1 page seq buffer.

People are already struggling to fix existing ABI, lets not make the
problem worse.