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authorThomas Gleixner <tglx@linutronix.de>2020-12-06 22:46:18 +0100
committerThomas Gleixner <tglx@linutronix.de>2020-12-11 10:40:52 +0100
commitc9e6189fb03123a7dfb93589280347b46f30b161 (patch)
treea03f1a39282bbba9168094f4d190e3a3b4ca93dd /include/linux/timex.h
parent354c796b9270eb4780e59e3bdb83a3ae4930a832 (diff)
ntp: Make the RTC synchronization more reliable
Miroslav reported that the periodic RTC synchronization in the NTP code fails more often than not to hit the specified update window. The reason is that the code uses delayed_work to schedule the update which needs to be in thread context as the underlying RTC might be connected via a slow bus, e.g. I2C. In the update function it verifies whether the current time is correct vs. the requirements of the underlying RTC. But delayed_work is using the timer wheel for scheduling which is inaccurate by design. Depending on the distance to the expiry the wheel gets less granular to allow batching and to avoid the cascading of the original timer wheel. See 500462a9de65 ("timers: Switch to a non-cascading wheel") and the code for further details. The code already deals with this by splitting the 660 seconds period into a long 659 seconds timer and then retrying with a smaller delta. But looking at the actual granularities of the timer wheel (which depend on the HZ configuration) the 659 seconds timer ends up in an outer wheel level and is affected by a worst case granularity of: HZ Granularity 1000 32s 250 16s 100 40s So the initial timer can be already off by max 12.5% which is not a big issue as the period of the sync is defined as ~11 minutes. The fine grained second attempt schedules to the desired update point with a timer expiring less than a second from now. Depending on the actual delta and the HZ setting even the second attempt can end up in outer wheel levels which have a large enough granularity to make the correctness check fail. As this is a fundamental property of the timer wheel there is no way to make this more accurate short of iterating in one jiffies steps towards the update point. Switch it to an hrtimer instead which schedules the actual update work. The hrtimer will expire precisely (max 1 jiffie delay when high resolution timers are not available). The actual scheduling delay of the work is the same as before. The update is triggered from do_adjtimex() which is a bit racy but not much more racy than it was before: if (ntp_synced()) queue_delayed_work(system_power_efficient_wq, &sync_work, 0); which is racy when the work is currently executed and has not managed to reschedule itself. This becomes now: if (ntp_synced() && !hrtimer_is_queued(&sync_hrtimer)) queue_work(system_power_efficient_wq, &sync_work, 0); which is racy when the hrtimer has expired and the work is currently executed and has not yet managed to rearm the hrtimer. Not a big problem as it just schedules work for nothing. The new implementation has a safe guard in place to catch the case where the hrtimer is queued on entry to the work function and avoids an extra update attempt of the RTC that way. Reported-by: Miroslav Lichvar <mlichvar@redhat.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Miroslav Lichvar <mlichvar@redhat.com> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20201206220542.062910520@linutronix.de
Diffstat (limited to 'include/linux/timex.h')
-rw-r--r--include/linux/timex.h1
1 files changed, 0 insertions, 1 deletions
diff --git a/include/linux/timex.h b/include/linux/timex.h
index ce0859763670..9c2e54faf9b7 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -157,7 +157,6 @@ extern int do_clock_adjtime(const clockid_t which_clock, struct __kernel_timex *
extern void hardpps(const struct timespec64 *, const struct timespec64 *);
int read_current_timer(unsigned long *timer_val);
-void ntp_notify_cmos_timer(void);
/* The clock frequency of the i8253/i8254 PIT */
#define PIT_TICK_RATE 1193182ul