Messages in this thread | | | Date | Thu, 7 Nov 2019 13:40:47 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH] kernel: use ktime_get_real_ts64() to calculate acct.ac_btime |
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On Thu, 7 Nov 2019, Peter Zijlstra wrote: > Lets start by saying this accounting stuff is terrible crap and it > deserves to fail and burn.
No argument about that.
> And what does btime want? As implemented it jumps around if you ask the > question twice with an adjtime() call or suspend in between. Of course, > if we take an actual CLOCK_REALTIME timestamp at fork() the value > doesn't change, but then it can be in the future (DST,adjtime()), which > is exactly the reason why CLOCK_REALTIME is absolute shit for timestamps > (logging, accounting, etc.). > > And your 'fix' is pretty terible too. Arguably ktime_get_seconds() wants > fixing for not having the ns accumulation and actually differing from > tv_sec, but now you accrue one source of ns while still disregarding > another (also, I friggin hate timespec, it's a terrible interface for > time). > > All in all, I'm tempted to just declare this stuff broken and -EWONTFIX, > but if we have to do something, something like the below is at least > internally consistent.
Kinda :)
> + 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.
So a CLOCK_REALTIME time stamp at fork would at least be correct vs. suspend resume.
The same result is achieved by:
boot = ktime_get_boot_ns(); delta = boot = tsk->real_start_time;
Typing real_start_time makes me really cringe.
Thanks,
tglx
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