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SubjectRe: [RFC][PATCH 4/4] time: Do leapsecond adjustment in gettime fastpaths
On Mon, 8 Jun 2015, Thomas Gleixner wrote:
> On Mon, 8 Jun 2015, John Stultz wrote:
> > Now, It could be possible to do a lighter weight version of my patch,
> > which just does the adjustment only for the hrtimer_interrupt code
> > (leaving the rest of the read paths alone).
> Yes, that should work. As long as I can keep the cached values in the
> hrtimer cpu bases and the whole thing keeps the clock_was_set_seq
> logic intact.
> If we do not do the conditional version, then on every hrtimer
> interrupt we write THREE cachelines for nothing.
> And if we cannot cache the offsets, then we end up calling into the
> timekeeping code for every timer which is not CLOCK_MONOTONIC based
> and retrieve the offset. That hurts especially on 32bit machines,
> because we need to protect the readout with the timekeeper sequence
> counter.

Below is a patch which just has the required data in the right place
and the changes to ktime_get_update_offsets_now().

It's simpler than your version as:

- there is no requirement to do the add in the first place as we
know the monotonic time at which the leap second happens.

- we can precalculate the leap adjusted offset and just replace
the non adjusted offset for the window.

When the whole thing is over you just need to increment the
clock_was_set_seq counter so the next timer interrupt will cache the
normal values again.



diff --git a/include/linux/timekeeper_internal.h b/include/linux/timekeeper_internal.h
index e1f5a1136554..ecd193c23676 100644
--- a/include/linux/timekeeper_internal.h
+++ b/include/linux/timekeeper_internal.h
@@ -90,6 +90,9 @@ struct timekeeper {
ktime_t offs_tai;
s32 tai_offset;
unsigned int clock_was_set_seq;
+ ktime_t next_leap_ktime;
+ ktime_t offs_real_leap_adjusted;
struct timespec64 raw_time;

/* The following members are for timekeeping internal use */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 90ed5db67c1d..0de85bf9e331 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1952,15 +1952,21 @@ ktime_t ktime_get_update_offsets_now(unsigned int *cwsseq, ktime_t *offs_real,

base = tk->tkr_mono.base;
nsecs = timekeeping_get_ns(&tk->tkr_mono);
+ base = ktime_add_ns(base, nsecs);
if (*cwsseq != tk->clock_was_set_seq) {
*cwsseq = tk->clock_was_set_seq;
*offs_real = tk->offs_real;
*offs_boot = tk->offs_boot;
*offs_tai = tk->offs_tai;
+ if (base.tv64 >= tk->next_leap_ktime.tv64)
+ *offs_real = tk->offs_real_leap_adjusted;
} while (read_seqcount_retry(&tk_core.seq, seq));

- return ktime_add_ns(base, nsecs);
+ return base;


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