linux-kernel - [PATCH v3 1/2] x86/vdso: Move mult and shift into struct vgtod

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Date:   Wed, 5 Jun 2019 14:41:44 +0000
From:   "Sverdlin, Alexander (Nokia - DE/Ulm)" <alexander.sverdlin@...ia.com>
To:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Sverdlin, Alexander (Nokia - DE/Ulm)" <alexander.sverdlin@...ia.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Vas Dias <jason.vas.dias@...il.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v3 1/2] x86/vdso: Move mult and shift into struct vgtod_ts

From: Alexander Sverdlin <alexander.sverdlin@...ia.com>

This is a preparation for CLOCK_MONOTONIC_RAW vDSO implementation.
Coincidentally it had a slight performance improvement as well:

---- Test code ----
 #include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
 #include <unistd.h>

 #define CLOCK_TYPE CLOCK_MONOTONIC_RAW
 #define DURATION_SEC 10

int main(int argc, char **argv)
{
	struct timespec t, end;
	unsigned long long cnt = 0;

	clock_gettime(CLOCK_TYPE, &end);
	end.tv_sec += DURATION_SEC;

	do {
		clock_gettime(CLOCK_TYPE, &t);
		++cnt;
	} while (t.tv_sec < end.tv_sec || t.tv_nsec < end.tv_nsec);

	dprintf(STDOUT_FILENO, "%llu", cnt);

	return EXIT_SUCCESS;
}
-------------------

The results from the above test program:

Clock                   Before  After   Diff
-----                   ------  -----   ----
CLOCK_MONOTONIC         355.5M  359.6M  +1%
CLOCK_REALTIME          355.5M  359.6M  +1%

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Jason Vas Dias <jason.vas.dias@...il.com>
Suggested-by: Andy Lutomirski <luto@...nel.org>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...ia.com>
---
 arch/x86/entry/vdso/vclock_gettime.c    | 4 ++--
 arch/x86/entry/vsyscall/vsyscall_gtod.c | 8 ++++++--
 arch/x86/include/asm/vgtod.h            | 4 ++--
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 0f82a70..a99eaf5 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -153,8 +153,8 @@ notrace static int do_hres(clockid_t clk, struct timespec *ts)
 		if (unlikely((s64)cycles < 0))
 			return vdso_fallback_gettime(clk, ts);
 		if (cycles > last)
-			ns += (cycles - last) * gtod->mult;
-		ns >>= gtod->shift;
+			ns += (cycles - last) * base->mult;
+		ns >>= base->shift;
 		sec = base->sec;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 
diff --git a/arch/x86/entry/vsyscall/vsyscall_gtod.c b/arch/x86/entry/vsyscall/vsyscall_gtod.c
index cfcdba0..64b1e7c 100644
--- a/arch/x86/entry/vsyscall/vsyscall_gtod.c
+++ b/arch/x86/entry/vsyscall/vsyscall_gtod.c
@@ -44,16 +44,18 @@ void update_vsyscall(struct timekeeper *tk)
 	vdata->vclock_mode	= vclock_mode;
 	vdata->cycle_last	= tk->tkr_mono.cycle_last;
 	vdata->mask		= tk->tkr_mono.mask;
-	vdata->mult		= tk->tkr_mono.mult;
-	vdata->shift		= tk->tkr_mono.shift;
 
 	base = &vdata->basetime[CLOCK_REALTIME];
 	base->sec = tk->xtime_sec;
 	base->nsec = tk->tkr_mono.xtime_nsec;
+	base->mult = tk->tkr_mono.mult;
+	base->shift = tk->tkr_mono.shift;
 
 	base = &vdata->basetime[CLOCK_TAI];
 	base->sec = tk->xtime_sec + (s64)tk->tai_offset;
 	base->nsec = tk->tkr_mono.xtime_nsec;
+	base->mult = tk->tkr_mono.mult;
+	base->shift = tk->tkr_mono.shift;
 
 	base = &vdata->basetime[CLOCK_MONOTONIC];
 	base->sec = tk->xtime_sec + tk->wall_to_monotonic.tv_sec;
@@ -64,6 +66,8 @@ void update_vsyscall(struct timekeeper *tk)
 		base->sec++;
 	}
 	base->nsec = nsec;
+	base->mult = tk->tkr_mono.mult;
+	base->shift = tk->tkr_mono.shift;
 
 	base = &vdata->basetime[CLOCK_REALTIME_COARSE];
 	base->sec = tk->xtime_sec;
diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index 913a133..b1f6df3 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -25,6 +25,8 @@ typedef unsigned long gtod_long_t;
 struct vgtod_ts {
 	u64		sec;
 	u64		nsec;
+	u32		mult;
+	u32		shift;
 };
 
 #define VGTOD_BASES	(CLOCK_TAI + 1)
@@ -41,8 +43,6 @@ struct vsyscall_gtod_data {
 	int		vclock_mode;
 	u64		cycle_last;
 	u64		mask;
-	u32		mult;
-	u32		shift;
 
 	struct vgtod_ts	basetime[VGTOD_BASES];
 
-- 
2.4.6