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Date:	Sun, 02 Feb 2014 17:02:52 +0100
From:	Stefani Seibold <stefani@...bold.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	ak@...ux.intel.com, aarcange@...hat.com, john.stultz@...aro.org,
	luto@...capital.net, xemul@...allels.com, gorcunov@...nvz.org,
	andriy.shevchenko@...ux.intel.com, Martin.Runge@...de-schwarz.com,
	Andreas.Brief@...de-schwarz.com
Subject: Re: [PATCH 6/8] cleanup __vdso_gettimeofday

On Sun, 2014-02-02 at 07:43 -0800, H. Peter Anvin wrote:
> Even reading the timezone via gettimeofday is the uncommon case... never mind only the timezome.  Whatever you do please don't slow down the common case.
> 
> On February 2, 2014 3:27:13 AM PST, stefani@...bold.net wrote:
> >From: Stefani Seibold <stefani@...bold.net>
> >
> >This patch do a little cleanup for the __vdso_gettimeofday() function.
> >
> >It kick out an unneeded ret local variable and makes the code faster if
> >only the timezone is needed.
> >
> >Signed-off-by: Stefani Seibold <stefani@...bold.net>
> >---
> > arch/x86/vdso/vclock_gettime.c | 7 ++-----
> > 1 file changed, 2 insertions(+), 5 deletions(-)
> >
> >diff --git a/arch/x86/vdso/vclock_gettime.c
> >b/arch/x86/vdso/vclock_gettime.c
> >index 743f277..bf969a0 100644
> >--- a/arch/x86/vdso/vclock_gettime.c
> >+++ b/arch/x86/vdso/vclock_gettime.c
> >@@ -259,13 +259,12 @@ int clock_gettime(clockid_t, struct timespec *)
> > 
> >notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone
> >*tz)
> > {
> >-	long ret = VCLOCK_NONE;
> >-
> > 	if (likely(tv != NULL)) {
> > 		BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
> > 			     offsetof(struct timespec, tv_nsec) ||
> > 			     sizeof(*tv) != sizeof(struct timespec));
> >-		ret = do_realtime((struct timespec *)tv);
> >+		if (do_realtime((struct timespec *)tv) == VCLOCK_NONE)
> >+			return vdso_fallback_gtod(tv, tz);
> > 		tv->tv_usec /= 1000;
> > 	}
> > 	if (unlikely(tz != NULL)) {
> >@@ -274,8 +273,6 @@ notrace int __vdso_gettimeofday(struct timeval *tv,
> >struct timezone *tz)
> > 		tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
> > 	}
> > 
> >-	if (ret == VCLOCK_NONE)
> >-		return vdso_fallback_gtod(tv, tz);
> > 	return 0;
> > }
> > int gettimeofday(struct timeval *, struct timezone *)
> 

I don't see where i slow down the common case! It is exactly the same
behaviour as the original code. Only in the uncommon case where the tv
== NULL and tz != NULL is a bit faster.


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