Subject: Re: [KVM timekeeping 17/35] Implement getnsboottime kernel API
From: john stultz <johnstul@us.ibm.com>
To: Zachary Amsden <zamsden@redhat.com>
Cc: kvm@vger.kernel.org, Avi Kivity <avi@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Glauber Costa <glommer@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org
In-Reply-To: <4C6F11D6.1050101@redhat.com>
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Date: Fri, 20 Aug 2010 17:02:38 -0700
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On Fri, 2010-08-20 at 13:37 -1000, Zachary Amsden wrote:
> On 08/20/2010 08:39 AM, john stultz wrote:
> > On Thu, 2010-08-19 at 22:07 -1000, Zachary Amsden wrote:
> >    
> >> Add a kernel call to get the number of nanoseconds since boot.  This
> >> is generally useful enough to make it a generic call.
> >>      
> > Few comments here.
> >
> >    
> >> Signed-off-by: Zachary Amsden<zamsden@redhat.com>
> >> ---
> >>   include/linux/time.h      |    1 +
> >>   kernel/time/timekeeping.c |   27 +++++++++++++++++++++++++++
> >>   2 files changed, 28 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/include/linux/time.h b/include/linux/time.h
> >> index ea3559f..5d04108 100644
> >> --- a/include/linux/time.h
> >> +++ b/include/linux/time.h
> >> @@ -145,6 +145,7 @@ extern void getnstimeofday(struct timespec *tv);
> >>   extern void getrawmonotonic(struct timespec *ts);
> >>   extern void getboottime(struct timespec *ts);
> >>   extern void monotonic_to_bootbased(struct timespec *ts);
> >> +extern s64 getnsboottime(void);
> >>      
> > So instead of converting the timespec from getboottime, why did you add
> > a new interface? Also if not a timespec, why did you pick a s64 instead
> > of a ktime_t?
> >    
> 
> The new interface was suggested several times, so I'm proposing it.  I'm 
> indifferent to putting it the kernel API or making it internal to KVM.  
> KVM doesn't want to deal with conversions to / from ktime_t; this code 
> uses a lot (too much) math, and it's easy to get wrong when splitting 
> sec / nsec fields.  So s64 seems a natural type for ns values.  I 
> realize it's not entirely consistent with the kernel API, but s64 
> representation for ns seems to be creeping in.

I can understand wanting that, way back I was pushing for s64 ns
representations for most time values, but the ktime_t was considered a
reasonable compromise to avoid costly 64bit divides to split (sec,nsec)
on 32bit arches.

Maybe call it getboottime_ns() just to distinguish it from
getnstimeofday() which returns a timespec?


> >>   extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
> >>   extern int timekeeping_valid_for_hres(void);
> >> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> >> index caf8d4d..d250f0a 100644
> >> --- a/kernel/time/timekeeping.c
> >> +++ b/kernel/time/timekeeping.c
> >> @@ -285,6 +285,33 @@ void ktime_get_ts(struct timespec *ts)
> >>   }
> >>   EXPORT_SYMBOL_GPL(ktime_get_ts);
> >>
> >> +
> >> +/**
> >> + * getnsboottime - get the bootbased clock in nsec format
> >> + *
> >> + * The function calculates the bootbased clock from the realtime
> >> + * clock and the wall_to_monotonic offset and stores the result
> >> + * in normalized timespec format in the variable pointed to by @ts.
> >> + */
> >> +s64 getnsboottime(void)
> >> +{
> >> +	unsigned int seq;
> >> +	s64 secs, nsecs;
> >> +
> >> +	WARN_ON(timekeeping_suspended);
> >> +
> >> +	do {
> >> +		seq = read_seqbegin(&xtime_lock);
> >> +		secs = xtime.tv_sec + wall_to_monotonic.tv_sec;
> >> +		secs += total_sleep_time.tv_sec;
> >> +		nsecs = xtime.tv_nsec + wall_to_monotonic.tv_nsec;
> >> +		nsecs += total_sleep_time.tv_nsec + timekeeping_get_ns();
> >> +
> >> +	} while (read_seqretry(&xtime_lock, seq));
> >> +	return nsecs + (secs * NSEC_PER_SEC);
> >> +}
> >> +EXPORT_SYMBOL_GPL(getnsboottime);
> >>      
> > You forgot to include the boottime.tv_sec/nsec offset in this. Take a
> > look again at getboottime()
> >    
> 
> I don't think so... boottime is internal to getboottime, and it's just 
> wall_to_monotonic + total_sleep_time -- right?

Right, sorry, some architectures refine boot time even further,
providing an offset from when the machine was actually powered on to
when the timekeeping code was initialized. But that's already adjusted
into wall_to_monotonic at startup. I thought we kept it separately.


> Perhaps I've named the function badly.  What I want is the monotonic 
> clock, adjusted for sleep time - i.e. a clock that counts elapsed real 
> time without accounting for wall clock changes due to time zone, which 
> never goes backwards.

That looks fine then.  Its a little confusing since getboottime()
returns a timespec with the absolute time that the system booted. Where
as your interface is providing the time since boot. 

Maybe gettimefromboot_ns() would be clearer?

thanks
-john


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