linux-kernel - Re: [PATCH 3/7] Add IRQ_TIME_ACCOUNTING, finer accounting of irq time -v3

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Date:	Sat, 02 Oct 2010 01:14:33 +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>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, Paul Turner <pjt@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH 3/7] Add IRQ_TIME_ACCOUNTING, finer accounting of irq
 time -v3

On Fri, 2010-10-01 at 10:29 -0700, Venkatesh Pallipadi wrote:
> So, on x86, sched_clock_stable is not set on all other kind of CPUs
> and my test system happens to be one of them. So, sched_clock_cpu()
> falls back to tick based even when TSC is not marked unstable and
> clocksource is using TSC for timing. 

It is never tick based!! It's tick augmented! Because TSC is such a
piece of crap we use external (slow) means of determining a window in
which the TSC should live and then use the TSC to generate high
resolution offsets inside that.

So even if your usage is in the hardirq context that moves that window
it should all work out.
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