linux-kernel - Re: [RFC PATCH 2/4] x86: Add IRQ_TIME_ACCOUNTING, finer accounting of irq time to task

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Date:	Tue, 25 May 2010 08:35:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [RFC PATCH 2/4] x86: Add IRQ_TIME_ACCOUNTING, finer accounting
 of irq time to task

On Mon, 2010-05-24 at 17:11 -0700, Venkatesh Pallipadi wrote:
> +void account_system_vtime(struct task_struct *tsk)
> +{
> +       unsigned long flags;
> +       int cpu;
> +       u64 now;
> +
> +       local_irq_save(flags);
> +       cpu = task_cpu(tsk);
> +       now = sched_clock_cpu(cpu);
> +       if (hardirq_count())
> +               tsk->hi_time += now - per_cpu(irq_start_time, cpu);
> +       else if (softirq_count())
> +               tsk->si_time += now - per_cpu(irq_start_time, cpu);
> +
> +       per_cpu(irq_start_time, cpu) = now;
> +       local_irq_restore(flags);
> +} 

Right, so this gets called from irq_enter/exit() and __do_softirq().

The reason I never pressed onwards with this (I had patches to add IRQ
time accounting) is that it sucks terribly for anything falling back to
jiffies -- maybe find some smart way to disable the whole call when
there's no TSC available, preferably without adding conditionals, using
alternatives maybe?

I guess you mostly side-stepped that by adding a IRQ_TIME_ACCOUNTING
config (but forgot to make it depend on X86_TSC).

Another thing I dislike about this account_system_vtime() is that its
the same call for both IRQ and SoftIRQ, leaving us to add conditionals
inside the call to figure out what context we got called from.

[ Adedd the s390 and ppc guys who already use this stuff ]

Anyway, once we have this, please also add it to sched_rt_avg_update()
(which should really be called sched_!fair_avg_update()).

Also, did you measure the overhead of doing this? sched_clock_cpu() adds
a cmpxchg64 on all systems that don't have a rock solid TSC (ie. most of
todays machines).

Another thing that would be real nice is if you could find a way to not
make all of this x86 specific.
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