From cbf9b4bb76c9ce53b7fdde0dffcd000951b5f0d4 Mon Sep 17 00:00:00 2001 From: Dimitri Sivanich Date: Tue, 26 Sep 2006 10:52:34 +0200 Subject: [PATCH] [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 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 Signed-off-by: Dimitri Sivanich Signed-off-by: Andi Kleen --- arch/x86_64/kernel/time.c | 11 ++++------- 1 file changed, 4 insertions(+), 7 deletions(-) diff --git a/arch/x86_64/kernel/time.c b/arch/x86_64/kernel/time.c index d66c7f750e75..97115e608ed8 100644 --- a/arch/x86_64/kernel/time.c +++ b/arch/x86_64/kernel/time.c @@ -276,6 +276,7 @@ static void set_rtc_mmss(unsigned long nowtime) * 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 @@ unsigned long long monotonic_clock(void) 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 @@ void main_timer_handler(struct pt_regs *regs) 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 @@ void __init time_init(void) 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()); -- 2.34.1