On Mon, Nov 24, 2014 at 10:30:10PM -0800, John Stultz wrote:
> On Mon, Nov 24, 2014 at 10:16 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> wrote:
> >
> >> Hi Josh,
> >>
> >> Today's linux-next merge of the tiny tree got a conflict in
> >> kernel/time/Makefile between commit fd866e2b116b ("time: Rename
> >> udelay_test.c to test_udelay.c") from the tip tree and commit
> >> d1f6d68d03ea ("kernel: time: Compile out NTP support") from the tiny
> >> tree.
> >
> > So I think a timer subsystem commit d1f6d68d03ea with this
> > magnitude of linecount increase:
> >
> > Signed-off-by: Catalina Mocanu <catalina.mocanu@xxxxxxxxx>
> > [josh: Handle CONFIG_COMPAT=y.]
> > Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > ---
> > drivers/pps/Kconfig | 2 +-
> > include/linux/timex.h | 15 +++++++++++++--
> > init/Kconfig | 10 ++++++++++
> > kernel/compat.c | 8 ++++++--
> > kernel/sys_ni.c | 4 ++++
> > kernel/time/Makefile | 3 ++-
> > kernel/time/ntp_internal.h | 28 ++++++++++++++++++++++++++++
> > kernel/time/posix-timers.c | 2 ++
> > kernel/time/time.c | 2 ++
> > kernel/time/timekeeping.c | 2 ++
> > 10 files changed, 70 insertions(+), 6 deletions(-)
> >
> > at minimum needs the ack of timer folks, before it can be
> > committed to Git. Or is the tiny tree plan to submit all
> > patches to the appropriate subsystem or gather acks, before
> > sending it upstream?
>
> Yea. From first glance d1f6d68d03ea looks fairly broken.
>
> Returning 0 for ntp_tick_length() (which should be the current tick
> length in NTP_SCALE_SHIFT shifted ns), seems like it would cause major
> timekeeping problems.
Ouch, yeah; I'm impressed the kernel successfully booted that way (which
I did test).
Computing the tick_length to return seems to require a div_u64; is it
safe to initialize a static const with the result of calling div_u64, or
does the intializer need manual constant-folding to make the expression
compile-time computable?
Going by the logic in ntp_update_frequency, it looks like the stub
ntp_tick_length needs to return:
tick_length_base = 0;
tick_usec = TICK_USEC;
second_length = (u64)(tick_usec * NSEC_PER_USEC * USER_HZ) << NTP_SCALE_SHIFT;
new_base = div_u64(second_length, NTP_INTERVAL_FREQ);
tick_length += new_base - tick_length_base;
(tick_length starts out 0, gets new_base - 0 added initially, and every
subsequent time gets 0 added since tick_length_base won't change.)
Substituting and simplifying:
tick_length = new_base
= div_u64(second_length, NTP_INTERVAL_FREQ)
= div_u64((TICK_USEC * NSEC_PER_USEC * USER_HZ) << NTP_SCALE_SHIFT, NTP_INTERVAL_FREQ)
The numerator there could potentially be simplified, but I don't see an
obvious way around the division by NTP_INTERVAL_FREQ (defined as HZ).
- Josh Triplett
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