linux-kernel - Re: [PATCH v4 0/3] sched,time: fix irq time accounting with nohz

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Date:	Tue, 12 Jul 2016 14:10:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, 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 v4 0/3] sched,time: fix irq time accounting with nohz_idle

On Mon, Jul 11, 2016 at 12:53:54PM -0400, 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.
> 
> 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
> v4: greatly simplify the local_irq_save/restore optimisation, thanks
>     to Paolo pointing out irqs are already blocked by the callers
> 

Thanks Rick!

I'm applying the series with my patches and will do a pull request to
Ingo.