linux-kernel - Re: [PATCH] sched: reduce /proc/schedstat access times

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Date:	Fri, 3 Feb 2012 08:52:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH] sched: reduce /proc/schedstat access times


* Eric Dumazet <eric.dumazet@...il.com> wrote:

> On a 16 cpus NUMA machine, we can have quite a long /proc/schedstat
> 
> # wc -c /proc/schedstat 
> 8355 /proc/schedstat

Btw., the long-term goal would be to make the schedstats info 
fully available via perf and integrate it into 'perf sched' - or 
'perf stat --sched' or 'perf schedstat' (whichever variant suits 
the person who first implements it).

> @@ -47,9 +43,9 @@ static int show_schedstat(struct seq_file *seq, void *v)
>  		for_each_domain(cpu, sd) {
>  			enum cpu_idle_type itype;
>  
> -			cpumask_scnprintf(mask_str, mask_len,
> -					  sched_domain_span(sd));
> -			seq_printf(seq, "domain%d %s", dcount++, mask_str);
> +			seq_printf(seq, "domain%d ", dcount++);
> +			seq_bitmap(seq, cpumask_bits(sched_domain_span(sd)),
> +				   nr_cpumask_bits);
>  			for (itype = CPU_IDLE; itype < CPU_MAX_IDLE_TYPES;
>  					itype++) {
>  				seq_printf(seq, " %u %u %u %u %u %u %u %u",

that way, via perf, all information gets passed in a binary 
fashion through the perf ring-buffer, so there's no formatting 
overhead (only during post-processing), no restart artifacts due 
to seqfile limitations, etc.

Thanks,

	Ingo
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