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8d165db107
I have been looking at sources of OS jitter and notice that after a long
NO_HZ idle period we wakeup too early:
relative time (us) event
timer irq exit
999946.405 timer irq entry
4.835 timer irq exit
21.685 timer irq entry
3.540 timer (tick_sched_timer) entry
Here we slept for just under a second then took a timer interrupt that did
nothing. 21.685 us later we wake up again and do the work.
We set a rather low shift value of 16 for the decrementer clockevent, which I
think is causing this issue. On this box we have a 207MHz decrementer and see:
clockevent: decrementer mult[3501] shift[16] cpu[0]
For calculations of large intervals this mult/shift combination could be
off by a significant amount. I notice the sparc code has a loop that iterates
to find a mult/shift combination that maximises the shift value while
keeping mult under 32bit. With the patch below we get:
clockevent: decrementer mult[35015c20] shift[32] cpu[15]
And we no longer see the spurious wakeups.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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