linux-kernel - Re: [PATCH v7 03/17] tracing: add MIPS specific trace_clock

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Date:	Wed, 11 Nov 2009 10:47:16 +0800
From:	Wu Zhangjin <wuzhangjin@...il.com>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	zhangfx@...ote.com, zhouqg@...il.com,
	Ralf Baechle <ralf@...ux-mips.org>, rostedt@...dmis.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Nicholas Mc Guire <der.herr@...r.at>,
	Richard Sandiford <rdsandiford@...glemail.com>,
	Patrik Kluba <kpajko79@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Simek <monstr@...str.eu>
Subject: Re: [PATCH v7 03/17] tracing: add MIPS specific trace_clock_local()

Hi,

On Tue, 2009-11-10 at 09:48 -0800, David Daney wrote:
> Wu Zhangjin wrote:
> [...]
> > + * trace_clock_local(): the simplest and least coherent tracing clock.
> > + *
> > + * Useful for tracing that does not cross to other CPUs nor
> > + * does it go through idle events.
> > + */
> > +u64 trace_clock_local(void)
> > +{
> > +	unsigned long flags;
> > +	u64 clock;
> > +
> > +	raw_local_irq_save(flags);
> > +
> > +	clock = mips_timecounter_read();
> > +
> > +	raw_local_irq_restore(flags);
> > +
> > +	return clock;
> > +}
> 
> Why disable interrupts?
> 

I got it from kernel/trace/trace_clock.c:

/*
 * trace_clock_local(): the simplest and least coherent tracing clock.
 *
 * Useful for tracing that does not cross to other CPUs nor
 * does it go through idle events.
 */
u64 notrace trace_clock_local(void)
{
        unsigned long flags;
        u64 clock;

        /*
         * sched_clock() is an architecture implemented, fast, scalable,
         * lockless clock. It is not guaranteed to be coherent across
         * CPUs, nor across CPU idle events.
         */
        raw_local_irq_save(flags);
        clock = sched_clock();
        raw_local_irq_restore(flags);

        return clock;
}

> Also you call the new function mips_timecounter_read().  Since 
> sched_clock() is a weak function, you can override the definition with a 
> more accurate version when possible.  Then you could just directly call 
> it here, instead of adding the new mips_timecounter_read() that the 
> '[PATCH v7 02/17] tracing: add mips_timecounter_read() for MIPS' adds.
> 

Yes, I have tried to override the sched_clock(), but failed on
booting(just hang there). and also, as you know, this version of
mips_timecounter_read() will bring us some overhead, I think it's not a
good idea to enable it for the whole system, so, I only enable it for
ftrace.

Regards,
	Wu Zhangjin

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