linux-kernel - Re: [PATCH] optimized ktime_get[_ts] for GENERIC

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Date:	Tue, 7 Jul 2009 09:40:18 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [PATCH] optimized ktime_get[_ts] for GENERIC_TIME=y

On Mon, 6 Jul 2009 22:31:39 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Mon, 6 Jul 2009, Martin Schwidefsky wrote:
> > +ktime_t ktime_get(void)
> > +{
> > +	cycle_t cycle_now, cycle_delta;
> > +	struct timespec time;
> > +	unsigned long seq;
> > +	s64 nsecs;
> > +
> > +	do {
> > +		seq = read_seqbegin(&xtime_lock);
> > +		time.tv_sec = xtime.tv_sec + wall_to_monotonic.tv_sec;
> > +		time.tv_nsec = xtime.tv_nsec + wall_to_monotonic.tv_nsec;
> 
> That's actually a violation of the timespec semantics. tv_nsec can end
> up greater than (10^9 - 1). Please use separate sec and nsec variables.

Well the tv_sec/tv_nsec fields of a timespec are long values. But its
no problem to switch to local variables.

>   	        secs = xtime.tv_sec + wall_to_monotonic.tv_sec;
>   	     	nsecs = xtime.tv_nsec + wall_to_monotonic.tv_nsec;
> 
> > +		/* read clocksource: */
> > +		cycle_now = clocksource_read(clock);
> > +
> > +		/* calculate the delta since the last update_wall_time: */
> > +		cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
> > +
> > +		/* convert to nanoseconds: */
> > +		nsecs = cyc2ns(clock, cycle_delta);
> 
> So that needs to be changed to:
> 
> 		nsecs += cyc2ns(clock, cycle_delta);

Ok.

> > +
> > +	} while (read_seqretry(&xtime_lock, seq));
> > +	nsecs += time.tv_sec * 1000000000ULL + time.tv_nsec;
> > +	return (ktime_t) { .tv64 = nsecs };
> 
> This will break all 32bit architectures which do not have
> CONFIG_KTIME_SCALAR set.

Yep, that is a bug.
 
> With the above changes applied:
> 
>   return ktime_add_ns(ktime_set(secs, 0), nsecs);
> 
> will do the trick. Might need some comments though :)

Ok, will update the patch. Thanks for the review.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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