[PATCH] X86_64 monotonic_clock goes backwards
I've noticed some erratic behavior while testing the X86_64 version
of monotonic_clock().
While spinning in a loop reading monotonic clock values (pinned to a
single cpu) I noticed that the difference between subsequent values
occasionally went negative (time going backwards).
I found that in the following code:
this_offset = get_cycles_sync();
/* FIXME: 1000 or 1000000? */
--> offset = (this_offset - last_offset)*1000 / cpu_khz;
}
return base + offset;
the offset sometimes turns out to be 0, even though
this_offset > last_offset.
+Added fix From: Toyo Abe <toyoa@mvista.com>
The x86_64-mm-monotonic-clock.patch in 2.6.18-rc4-mm2 made a change to
the updating of monotonic_base. It now uses cycles_2_ns().
I suggest that a set_cyc2ns_scale() should be done prior to the setup_irq().
Because cycles_2_ns() can be called from the timer ISR right after the irq0
is enabled.
Signed-off-by: Toyo Abe <toyoa@mvista.com>
Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Andi Kleen <ak@suse.de>
diff --git a/arch/x86_64/kernel/time.c b/arch/x86_64/kernel/time.c
index d66c7f7..97115e6 100644
--- a/arch/x86_64/kernel/time.c
+++ b/arch/x86_64/kernel/time.c
@@ -276,6 +276,7 @@
* Note: This function is required to return accurate
* time even in the absence of multiple timer ticks.
*/
+static inline unsigned long long cycles_2_ns(unsigned long long cyc);
unsigned long long monotonic_clock(void)
{
unsigned long seq;
@@ -300,8 +301,7 @@
base = monotonic_base;
} while (read_seqretry(&xtime_lock, seq));
this_offset = get_cycles_sync();
- /* FIXME: 1000 or 1000000? */
- offset = (this_offset - last_offset)*1000 / cpu_khz;
+ offset = cycles_2_ns(this_offset - last_offset);
}
return base + offset;
}
@@ -405,8 +405,7 @@
offset %= USEC_PER_TICK;
}
- /* FIXME: 1000 or 1000000? */
- monotonic_base += (tsc - vxtime.last_tsc) * 1000000 / cpu_khz;
+ monotonic_base += cycles_2_ns(tsc - vxtime.last_tsc);
vxtime.last_tsc = tsc - vxtime.quot * delay / vxtime.tsc_quot;
@@ -929,10 +928,8 @@
vxtime.quot = (USEC_PER_SEC << US_SCALE) / vxtime_hz;
vxtime.tsc_quot = (USEC_PER_MSEC << US_SCALE) / cpu_khz;
vxtime.last_tsc = get_cycles_sync();
- setup_irq(0, &irq0);
-
set_cyc2ns_scale(cpu_khz);
-
+ setup_irq(0, &irq0);
hotcpu_notifier(time_cpu_notifier, 0);
time_cpu_notifier(NULL, CPU_ONLINE, (void *)(long)smp_processor_id());