linux-kernel - Re: [RFC PATCH v1 15/31] ARC: Process/scheduling/clock/Timers/Delay Management

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Date:	Wed, 2 Jan 2013 14:15:56 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	James Hogan <james.hogan@...tec.com>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [RFC PATCH v1 15/31] ARC: Process/scheduling/clock/Timers/Delay
 Management

On Wednesday 02 January 2013 12:43 PM, Vineet Gupta wrote:
> On Tuesday 13 November 2012 01:59 AM, Thomas Gleixner wrote:
>> On Wed, 7 Nov 2012, Vineet Gupta wrote:
>>> +static int arc_finished_booting;
>>> +
>>> +/*
>>> + * Scheduler clock - returns current time in nanosec units.
>>> + * It's return value must NOT wrap around.
>>> + *
>>> + * Although the return value is nanosec units based, what's more important
>>> + * is whats the "source" of this value. The orig jiffies based computation
>>> + * was only as granular as jiffies itself (10ms on ARC).
>>> + * We need something that is more granular, so use the same mechanism as
>>> + * gettimeofday(), which uses ARC Timer T1 wrapped as a clocksource.
>>> + * Unfortunately the first call to sched_clock( ) is way before that subsys
>>> + * is initialiased, thus use the jiffies based value in the interim.
>>> + */
>>> +unsigned long long sched_clock(void)
>>> +{
>>> +	if (!arc_finished_booting) {
>>> +		return (unsigned long long)(jiffies - INITIAL_JIFFIES)
>>> +		    * (NSEC_PER_SEC / HZ);
>>> +	} else {
>>> +		struct timespec ts;
>>> +		getrawmonotonic(&ts);
>>
>> This can live lock. sched_clock() is used by the tracer. So assume you
>> are function tracing and you trace a function called from within the
>> timekeeping seqcount write "locked" region. You spin forever in
>> getrawmonotonic(). Not what you want, right ?
> 
> Correct- so that means we need an equivalent of partially open-code
> getrawmonotonic w/o any locks here - read the clocksource directly just as other
> arches.

Spoke too soon. The desired semantics here are monotonically increasing value
using a 32-bit overflowing hardware counter (and not Jiffies based since Android
CTS has a test case which requires that back-back calls to
clock_gettime(CLOCK_THREAD_CPUTIME_ID)  - which simply use sched_clock( ) return
different values.

Since counter is overflowing, we need additional state (last snapshot) provided by
likes of clocksource. But IMHO, opencoding clocksource code w/o xtime seqlock will
be wrong.

I don't see a way out - except to keep using jiffies based versions and use the 64
bit Time-stamp counter available in newer ARC CPUs.

-Vineet


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