On Sat, Sep 14 2024 at 13:07, Jeff Layton wrote:
> For multigrain timestamps, we must keep track of the latest timestamp
> that has ever been handed out, and never hand out a coarse time below
> that value.
How is that correct when the clock is off by an hour and then set back
to the correct value? Then you'd get the same stale timestamp for an
hour unless something invokes ktime_get_real_ts64_mg() which will set
the "latest" timestamp back to a time before the previous one.
> Add a static singleton atomic64_t into timekeeper.c that we can use to
> keep track of the latest fine-grained time ever handed out. This is
> tracked as a monotonic ktime_t value to ensure that it isn't affected by
> clock jumps.
That's just wishful thinking.
ktime_get_real_ts64_mg(ts)
ts = Tmono_1 + offset_1; // TReal_1
floor = Tmono_1;
// newtime < TReal_1
clock_settime(REALTIME, newtime);
xtime = newtime; // TReal_2
offset_2 = offset_1 + Treal_2 - TReal(now);
--> offset_2 < offset_1
ktime_get_coarse_real_ts64_mg(ts)
ts = tk_xtime(); // TReal_2
offs = offset_2;
if (Tmono_1 + offset_2 > ts)
ts = Tmono_1 + offset_2; // Not taken
So this returns T_Real_2 because
offset_2 < offset_1
and therefore
Tmono_1 + offset_2 < TReal_2
so the returned time will jump backwards vs. TReal_1 as it should
because that's the actual time, no?
So if that's the intended behaviour then the changelog is misleading at
best.
If the intention is to never return a value < TReal_1 then this does not
work. You can make it work by using the Realtime timestamp as floor, but
that'd be more than questionable vs. clock_settime() making the clock go
backwards.
Thanks,
tglx
