linux-kernel - Re: [PATCH v3 0/4] sched,time: fix irq time accounting with nohz

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Date:	Tue, 5 Jul 2016 16:02:41 +0300
From:	Nikolay Borisov <kernel@...p.com>
To:	riel@...hat.com, linux-kernel@...r.kernel.org
Cc:	peterz@...radead.org, mingo@...nel.org, pbonzini@...hat.com,
	fweisbec@...hat.com, wanpeng.li@...mail.com, efault@....de,
	tglx@...utronix.de, rkrcmar@...hat.com
Subject: Re: [PATCH v3 0/4] sched,time: fix irq time accounting with nohz_idle



On 06/30/2016 10:35 PM, riel@...hat.com wrote:
> Currently irq time accounting only works in these cases:
> 1) purely ticke based accounting
> 2) nohz_full accounting, but only on housekeeping & nohz_full CPUs
> 3) architectures with native vtime accounting
> 
> On nohz_idle CPUs, which are probably the majority nowadays,
> irq time accounting is currently broken. This leads to systems
> reporting a dramatically lower amount of irq & softirq time than
> is actually spent handling them, with all the time spent while the
> system is in the idle task being accounted as idle.

Is this supposed to alleviate the issue reported here:
http://www.gossamer-threads.com/lists/linux/kernel/2169180

> 
> This patch set seems to bring the amount of irq time reported by
> top (and /proc/stat) roughly in line with that measured when I do
> a "perf record -g -a" run to see what is using all that time.
> 
> The amount of irq time used, especially softirq, is shockingly high,
> to the point of me thinking this patch set may be wrong, but the
> numbers seem to match what perf is giving me...
> 
> These patches apply on top of Wanpeng Li's steal time patches.
> 
> CONFIG_IRQ_TIME_ACCOUNTING is now a config option that is available
> as a separate choice from tick based / nohz_idle / nohz_full mode,
> a suggested by Frederic Weisbecker.
> 
> Next up: look at the things that are using CPU time on an otherwise
> idle system, and see if I can make those a little faster :)
> 
> v2: address Peterz's concerns, some more cleanups
> v3: rewrite the code along Frederic's suggestions, now cputime_t
>     is used everywhere
>