[PATCH] x86-64: vsyscall_gtod_data diet and vgettimeofday() fix

Current vsyscall_gtod_data is large (3 or 4 cache lines dirtied at timer
interrupt). We can shrink it to exactly 64 bytes (1 cache line on AMD64)

Instead of copying a whole struct clocksource, we copy only needed fields.

I deleted an unused field : offset_base

This patch fixes one oddity in vgettimeofday(): It can returns a timeval with
tv_usec = 1000000. Maybe not a bug, but why not doing the right thing ?

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 36 insertions, 17 deletions

diff --git a/arch/x86_64/kernel/vsyscall.c b/arch/x86_64/kernel/vsyscall.c
index ba330f870679..dc32cef96195 100644
--- a/arch/x86_64/kernel/vsyscall.c
+++ b/arch/x86_64/kernel/vsyscall.c
@@ -51,13 +51,28 @@
           asm("" : "=r" (v) : "0" (x)); \
           ((v - VSYSCALL_FIRST_PAGE) + __pa_symbol(&__vsyscall_0)); })
 
+/*
+ * vsyscall_gtod_data contains data that is :
+ * - readonly from vsyscalls
+ * - writen by timer interrupt or systcl (/proc/sys/kernel/vsyscall64)
+ * Try to keep this structure as small as possible to avoid cache line ping pongs
+ */
 struct vsyscall_gtod_data_t {
-        seqlock_t lock;
-        int sysctl_enabled;
-        struct timeval wall_time_tv;
+        seqlock_t       lock;
+
+        /* open coded 'struct timespec' */
+        time_t          wall_time_sec;
+        u32             wall_time_nsec;
+
+        int             sysctl_enabled;
         struct timezone sys_tz;
-        cycle_t offset_base;
-        struct clocksource clock;
+        struct { /* extract of a clocksource struct */
+                cycle_t (*vread)(void);
+                cycle_t cycle_last;
+                cycle_t mask;
+                u32     mult;
+                u32     shift;
+        } clock;
 };
 int __vgetcpu_mode __section_vgetcpu_mode;
 
@@ -73,9 +88,13 @@ void update_vsyscall(struct timespec *wall_time, struct clocksource *clock)
 
         write_seqlock_irqsave(&vsyscall_gtod_data.lock, flags);
         /* copy vsyscall data */
-        vsyscall_gtod_data.clock = *clock;
-        vsyscall_gtod_data.wall_time_tv.tv_sec = wall_time->tv_sec;
-        vsyscall_gtod_data.wall_time_tv.tv_usec = wall_time->tv_nsec/1000;
+        vsyscall_gtod_data.clock.vread = clock->vread;
+        vsyscall_gtod_data.clock.cycle_last = clock->cycle_last;
+        vsyscall_gtod_data.clock.mask = clock->mask;
+        vsyscall_gtod_data.clock.mult = clock->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.sys_tz = sys_tz;
         write_sequnlock_irqrestore(&vsyscall_gtod_data.lock, flags);
 }
@@ -110,7 +129,8 @@ static __always_inline long time_syscall(long *t)
 static __always_inline void do_vgettimeofday(struct timeval * tv)
 {
         cycle_t now, base, mask, cycle_delta;
-        unsigned long seq, mult, shift, nsec_delta;
+        unsigned seq;
+        unsigned long mult, shift, nsec;
         cycle_t (*vread)(void);
         do {
                 seq = read_seqbegin(&__vsyscall_gtod_data.lock);
@@ -126,21 +146,20 @@ static __always_inline void do_vgettimeofday(struct timeval * tv)
                 mult = __vsyscall_gtod_data.clock.mult;
                 shift = __vsyscall_gtod_data.clock.shift;
 
-                *tv = __vsyscall_gtod_data.wall_time_tv;
-
+                tv->tv_sec = __vsyscall_gtod_data.wall_time_sec;
+                nsec = __vsyscall_gtod_data.wall_time_nsec;
         } while (read_seqretry(&__vsyscall_gtod_data.lock, seq));
 
         /* calculate interval: */
         cycle_delta = (now - base) & mask;
         /* convert to nsecs: */
-        nsec_delta = (cycle_delta * mult) >> shift;
+        nsec += (cycle_delta * mult) >> shift;
 
-        /* convert to usecs and add to timespec: */
-        tv->tv_usec += nsec_delta / NSEC_PER_USEC;
-        while (tv->tv_usec > USEC_PER_SEC) {
+        while (nsec >= NSEC_PER_SEC) {
                 tv->tv_sec += 1;
-                tv->tv_usec -= USEC_PER_SEC;
+                nsec -= NSEC_PER_SEC;
         }
+        tv->tv_usec = nsec / NSEC_PER_USEC;
 }
 
 int __vsyscall(0) vgettimeofday(struct timeval * tv, struct timezone * tz)
@@ -159,7 +178,7 @@ time_t __vsyscall(1) vtime(time_t *t)
         time_t result;
         if (unlikely(!__vsyscall_gtod_data.sysctl_enabled))
                 return time_syscall(t);
-        result = __vsyscall_gtod_data.wall_time_tv.tv_sec;
+        result = __vsyscall_gtod_data.wall_time_sec;
         if (t)
                 *t = result;
         return result;