Re: [PATCH] kernel: use ktime_get_real_ts64() to calculate acct.ac_btime

Thomas Gleixner
2019-11-07T14:37:11Z

On Thu, 7 Nov 2019, Peter Zijlstra wrote:
> On Thu, Nov 07, 2019 at 01:40:47PM +0100, Thomas Gleixner wrote:
> > On Thu, 7 Nov 2019, Peter Zijlstra wrote:
>
> > > + mono = ktime_get_ns();
> > > + real = ktime_get_real_ns();
> > > + /*
> > > + * Compute btime by subtracting the elapsed time from the current
> > > + * CLOCK_REALTIME.
> > > + *
> > > + * XXX totally buggered, because it changes results across
> > > + * adjtime() calls and suspend/resume.
> > > + */
> > > + delta = mono - tsk->start_time; // elapsed in ns
> > > + btime = real - delta; // real ns - elapsed ns
> > > + do_div(btime, NSEC_PER_SEC); // truncated to seconds
> > > + stats->ac_btime = btime;
> >
> > That has pretty much the same problem as just storing the CLOCK_REALTIME
> > start time at fork and additionally it is wreckaged vs. suspend resume.
>
> It's wrecked in general. It also jumps around for any REALTIME
> adjustment.
>
> > So a CLOCK_REALTIME time stamp at fork would at least be correct
> > vs. suspend resume.
>
> But still wrecked vs REALTIME jumps, as in, when DST flips the clock
> back an hour, your timestamp is in the future.
>
> Any which way around the whole thing is buggered. The only real fix is
> not using REALTIME anything. Which is why I'm loath to add that REALTIME
> timestamp at fork(), it just encourages more use.

Fair enough. You have a sane alternative though: CLOCK_TAI

Thanks,

tglx