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Subject: Re: [PATCH 04/10] tile: convert to use clocksource_register_hz
From: Peter Zijlstra <peterz@infradead.org>
To: john stultz <johnstul@us.ibm.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>, lkml <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>
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Date: Thu, 11 Nov 2010 23:22:13 +0100
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On Thu, 2010-11-11 at 14:06 -0800, john stultz wrote:
> 1) How often is sched_clock guaranteed to be called? Once each tick, (so
> the maximum time in nohz mode would be reasonable?)

Never,.. sparc64 for example can stay in nohz mode for hours. We have a
nohz_exit hook for the kernel/sched_clock.c code though which resyncs us
against the GTOD.

> 2) What considerations for sched_clock wrapping is there in generic
> code? I see some considerations in kernel/sched_clock.c, but its not
> obvious the limits. On x86, the 64-bit TSC won't wrap (but might jump on
> non-synced systems, or halt in idle modes). Do architectures that have
> faster-wrapping counters need to handle the cycle accumulation
> internally? 

Basically all code assumes we wrap on the u64 boundary.

So the whole kernel/sched_clock.c machinery tries to make a crummy arch
sched_clock() usable, it syncs against the GTOD code (on tick, idle_exit
and nohz_exit) and only assumes the arch sched_clock() wraps at the u64
boundary, jumps, inter-cpu drift etc are all taken care of.