linux-kernel - Re: [PATCH 4/6] x86-64: vclock_gettime(CLOCK

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Date:	Tue, 29 Mar 2011 08:21:12 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andy Lutomirski <luto@....EDU>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 4/6] x86-64: vclock_gettime(CLOCK_MONOTONIC) can't ever
 see nsec < 0


* Andy Lutomirski <luto@....EDU> wrote:

> vclock_gettime's do_monotonic helper can't ever generate a negative
> nsec value, so it doesn't need to check whether it's negative.  This
> saves a single easily-predicted branch.
> 
> Signed-off-by: Andy Lutomirski <luto@....edu>
> ---
>  arch/x86/vdso/vclock_gettime.c |   40 ++++++++++++++++++++++------------------
>  1 files changed, 22 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
> index ee55754..67d54bb 100644
> --- a/arch/x86/vdso/vclock_gettime.c
> +++ b/arch/x86/vdso/vclock_gettime.c
> @@ -56,22 +56,6 @@ notrace static noinline int do_realtime(struct timespec *ts)
>  	return 0;
>  }
>  
> -/* Copy of the version in kernel/time.c which we cannot directly access */
> -notrace static void
> -vset_normalized_timespec(struct timespec *ts, long sec, long nsec)
> -{
> -	while (nsec >= NSEC_PER_SEC) {
> -		nsec -= NSEC_PER_SEC;
> -		++sec;
> -	}
> -	while (nsec < 0) {
> -		nsec += NSEC_PER_SEC;
> -		--sec;
> -	}
> -	ts->tv_sec = sec;
> -	ts->tv_nsec = nsec;
> -}
> -
>  notrace static noinline int do_monotonic(struct timespec *ts)
>  {
>  	unsigned long seq, ns, secs;
> @@ -82,7 +66,17 @@ notrace static noinline int do_monotonic(struct timespec *ts)
>  		secs += gtod->wall_to_monotonic.tv_sec;
>  		ns += gtod->wall_to_monotonic.tv_nsec;
>  	} while (unlikely(read_seqretry(&gtod->lock, seq)));
> -	vset_normalized_timespec(ts, secs, ns);
> +
> +	/* wall_time_nsec, vgetns(), and wall_to_monotonic.tv_nsec
> +	 * are all guaranteed to be nonnegative.
> +	 */
> +	while (ns >= NSEC_PER_SEC) {
> +		ns -= NSEC_PER_SEC;
> +		++secs;
> +	}
> +	ts->tv_sec = secs;
> +	ts->tv_nsec = ns;
> +
>  	return 0;
>  }
>  
> @@ -107,7 +101,17 @@ notrace static noinline int do_monotonic_coarse(struct timespec *ts)
>  		secs += gtod->wall_to_monotonic.tv_sec;
>  		ns += gtod->wall_to_monotonic.tv_nsec;
>  	} while (unlikely(read_seqretry(&gtod->lock, seq)));
> -	vset_normalized_timespec(ts, secs, ns);
> +
> +	/* wall_time_nsec and wall_to_monotonic.tv_nsec are
> +	 * guaranteed to be between 0 and NSEC_PER_SEC.
> +	 */
> +	if (ns >= NSEC_PER_SEC) {
> +		ns -= NSEC_PER_SEC;
> +		++secs;
> +	}
> +	ts->tv_sec = secs;
> +	ts->tv_nsec = ns;
> +
>  	return 0;

Beyond the change you describe in the changelog, you also uninlined the helper 
function.

You can use __always_inline instead and still keep the code maintainable.

Thanks,

	Ingo
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