linux-kernel - Re: [KVM timekeeping 17/35] Implement getnsboottime kernel API

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Message-ID: <4C6F11D6.1050101@redhat.com>
Date:	Fri, 20 Aug 2010 13:37:58 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	john stultz <johnstul@...ibm.com>
CC:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [KVM timekeeping 17/35] Implement getnsboottime kernel API

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@...hat.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.

>
>    
>>   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?

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.

Zach
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