Re: [lustre-devel] [PATCH] staging: lustre: ldlm: pl_recalc time handling is wrong

[Arnd Bergmann <arnd@arndb.de>]

On Wednesday, November 9, 2016 3:50:29 AM CET Dilger, Andreas wrote:
> On Nov 7, 2016, at 19:47, James Simmons <jsimmons@infradead.org> wrote:
> > 
> > The ldlm_pool field pl_recalc_time is set to the current
> > monotonic clock value but the interval period is calculated
> > with the wall clock. This means the interval period will
> > always be far larger than the pl_recalc_period, which is
> > just a small interval time period. The correct thing to
> > do is to use monotomic clock current value instead of the
> > wall clocks value when calculating recalc_interval_sec.
> 
> It looks like this was introduced by commit 8f83409cf
> "staging/lustre: use 64-bit time for pl_recalc" but that patch changed
> get_seconds() to a mix of ktime_get_seconds() and ktime_get_real_seconds()
> for an unknown reason.  It doesn't appear that there is any difference
> in overhead between the two (on 64-bit at least).

It was meant to use ktime_get_real_seconds() consistently, very sorry
about the mistake. I don't remember exactly how we got there, I assume
I had started out using ktime_get_seconds() and then moved to
ktime_get_real_seconds() later but missed the last three instances.

> Since the ldlm pool recalculation interval is actually driven in response to
> load on the server, it makes sense to use the "real" time instead of the
> monotonic time (if I understand correctly) if the client is in a VM that
> may periodically be blocked and "miss time" compared to the outside world.
> Using the "real" clock, the recalc_interval_sec will correctly reflect the
> actual elapsed time rather than just the number of ticks inside the VM.
> 
> Is my understanding of these different clocks correct?

No, this is not the difference: monotonic and real time always tick
at exactly the same rate, the only difference is the starting point.
monotonic time starts at boot and can not be adjusted, while real
time is set to reflect the UTC time base and gets initialized
from the real time clock at boot, or using settimeofday(2) or
NTP later on.

In my changelog text, I wrote

    keeping the 'real' instead of 'monotonic' time because of the
    debug prints.

the intention here is simply to have the console log keep the
same numbers as "date +%s" for absolute values. The patch that
James suggested converting everything to ktime_get_seconds()
would result in the same intervals that have always been there
(until I broke it by using time domains inconsistently), but
would mean we could use a u32 type for pl_recalc_time and
pl_recalc_period because that doesn't overflow until 136 years
after booting. (signed time_t overflows 68 years after 1970,
i.e 2038).

	Arnd