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author | 2020-07-17 16:05:44 +0200 | |
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committer | 2020-07-17 21:55:22 +0200 | |
commit | 4468897211628865ee2392acb5ad281f74176f63 (patch) | |
tree | e7ba2e16e4ad15ea6237da10b86c088d8d6184cc /kernel/time | |
parent | timers: Move trigger_dyntick_cpu() to enqueue_timer() (diff) | |
download | linux-4468897211628865ee2392acb5ad281f74176f63.tar.gz linux-4468897211628865ee2392acb5ad281f74176f63.tar.bz2 linux-4468897211628865ee2392acb5ad281f74176f63.zip |
timers: Add comments about calc_index() ceiling work
calc_index() adds 1 unit of the level granularity to the expiry passed
in parameter to ensure that the timer doesn't expire too early. Add a
comment to explain that and the resulting layout in the wheel.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200717140551.29076-6-frederic@kernel.org
Diffstat (limited to 'kernel/time')
-rw-r--r-- | kernel/time/timer.c | 12 |
1 files changed, 11 insertions, 1 deletions
diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 2af08a169564..af1c08b0b168 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -156,7 +156,8 @@ EXPORT_SYMBOL(jiffies_64); /* * The time start value for each level to select the bucket at enqueue - * time. + * time. We start from the last possible delta of the previous level + * so that we can later add an extra LVL_GRAN(n) to n (see calc_index()). */ #define LVL_START(n) ((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT)) @@ -490,6 +491,15 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx) static inline unsigned calc_index(unsigned long expires, unsigned lvl, unsigned long *bucket_expiry) { + + /* + * The timer wheel has to guarantee that a timer does not fire + * early. Early expiry can happen due to: + * - Timer is armed at the edge of a tick + * - Truncation of the expiry time in the outer wheel levels + * + * Round up with level granularity to prevent this. + */ expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl); *bucket_expiry = expires << LVL_SHIFT(lvl); return LVL_OFFS(lvl) + (expires & LVL_MASK); |