x86-64: Simplify and optimize vdso clock_gettime monotonic variants

We used to store the wall-to-monotonic offset and the realtime base.
It's faster to precompute the monotonic base.

This is about a 3% speedup on Sandy Bridge for CLOCK_MONOTONIC.
It's much more impressive for CLOCK_MONOTONIC_COARSE.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: John Stultz <john.stultz@linaro.org>

3 files changed, 26 insertions, 37 deletions

diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index 1f007178c813..8b38be2de9e1 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -7,11 +7,6 @@
 struct vsyscall_gtod_data {
         seqcount_t      seq;
 
-        /* open coded 'struct timespec' */
-        time_t          wall_time_sec;
-        u32             wall_time_nsec;
-
-        struct timezone sys_tz;
         struct { /* extract of a clocksource struct */
                 int vclock_mode;
                 cycle_t cycle_last;
@@ -19,8 +14,16 @@ struct vsyscall_gtod_data {
                 u32     mult;
                 u32     shift;
         } clock;
-        struct timespec wall_to_monotonic;
+
+        /* open coded 'struct timespec' */
+        time_t          wall_time_sec;
+        u32             wall_time_nsec;
+        u32             monotonic_time_nsec;
+        time_t          monotonic_time_sec;
+
+        struct timezone sys_tz;
         struct timespec wall_time_coarse;
+        struct timespec monotonic_time_coarse;
 };
 extern struct vsyscall_gtod_data vsyscall_gtod_data;
 
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index cdc95a707cd1..4285f1f404c2 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -84,6 +84,7 @@ void update_vsyscall(struct timespec *wall_time, struct timespec *wtm,
                         struct clocksource *clock, u32 mult)
 {
         write_seqcount_begin(&vsyscall_gtod_data.seq);
+        struct timespec monotonic;
 
         /* copy vsyscall data */
         vsyscall_gtod_data.clock.vclock_mode    = clock->archdata.vclock_mode;
@@ -91,10 +92,17 @@ void update_vsyscall(struct timespec *wall_time, struct timespec *wtm,
         vsyscall_gtod_data.clock.mask           = clock->mask;
         vsyscall_gtod_data.clock.mult           = mult;
         vsyscall_gtod_data.clock.shift          = clock->shift;
+
         vsyscall_gtod_data.wall_time_sec        = wall_time->tv_sec;
         vsyscall_gtod_data.wall_time_nsec       = wall_time->tv_nsec;
-        vsyscall_gtod_data.wall_to_monotonic    = *wtm;
+
+        monotonic = timespec_add(*wall_time, *wtm);
+        vsyscall_gtod_data.monotonic_time_sec   = monotonic.tv_sec;
+        vsyscall_gtod_data.monotonic_time_nsec  = monotonic.tv_nsec;
+
         vsyscall_gtod_data.wall_time_coarse     = __current_kernel_time();
+        vsyscall_gtod_data.monotonic_time_coarse =
+                timespec_add(vsyscall_gtod_data.wall_time_coarse, *wtm);
 
         write_seqcount_end(&vsyscall_gtod_data.seq);
 }
diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 944c5e5d6b6a..6eea70b8f384 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -113,27 +113,17 @@ notrace static noinline int do_realtime(struct timespec *ts)
 
 notrace static noinline int do_monotonic(struct timespec *ts)
 {
-        unsigned long seq, ns, secs;
+        unsigned long seq, ns;
         int mode;
 
         do {
                 seq = read_seqcount_begin(&gtod->seq);
                 mode = gtod->clock.vclock_mode;
-                secs = gtod->wall_time_sec;
-                ns = gtod->wall_time_nsec + vgetns();
-                secs += gtod->wall_to_monotonic.tv_sec;
-                ns += gtod->wall_to_monotonic.tv_nsec;
+                ts->tv_sec = gtod->monotonic_time_sec;
+                ts->tv_nsec = gtod->monotonic_time_nsec;
+                ns = vgetns();
         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
-
-        /* 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;
+        timespec_add_ns(ts, ns);
 
         return mode;
 }
@@ -151,25 +141,13 @@ notrace static noinline int do_realtime_coarse(struct timespec *ts)
 
 notrace static noinline int do_monotonic_coarse(struct timespec *ts)
 {
-        unsigned long seq, ns, secs;
+        unsigned long seq;
         do {
                 seq = read_seqcount_begin(&gtod->seq);
-                secs = gtod->wall_time_coarse.tv_sec;
-                ns = gtod->wall_time_coarse.tv_nsec;
-                secs += gtod->wall_to_monotonic.tv_sec;
-                ns += gtod->wall_to_monotonic.tv_nsec;
+                ts->tv_sec = gtod->monotonic_time_coarse.tv_sec;
+                ts->tv_nsec = gtod->monotonic_time_coarse.tv_nsec;
         } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 
-        /* 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;
 }