Messages in this thread | | | Date | Mon, 01 Dec 2014 21:14:08 +0100 | From | Christian Borntraeger <> | Subject | Re: [RFC PATCH 07/30] cputime: Convert kcpustat to nsecs |
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Am 01.12.2014 um 18:15 schrieb Thomas Gleixner: > On Mon, 1 Dec 2014, Martin Schwidefsky wrote: >> On Mon, 1 Dec 2014 17:10:34 +0100 >> Frederic Weisbecker <fweisbec@gmail.com> wrote: >> >>> Speaking about the degradation in s390: >>> >>> s390 is really a special case. And it would be a shame if we prevent from a >>> real core cleanup just for this special case especially as it's fairly possible >>> to keep a specific treatment for s390 in order not to impact its performances >>> and time precision. We could simply accumulate the cputime in per-cpu values: >>> >>> struct s390_cputime { >>> cputime_t user, sys, softirq, hardirq, steal; >>> } >>> >>> DEFINE_PER_CPU(struct s390_cputime, s390_cputime); >>> >>> Then on irq entry/exit, just add the accumulated time to the relevant buffer >>> and account for real (through any account_...time() functions) only on tick >>> and task switch. There the costly operations (unit conversion and call to >>> account_...._time() functions) are deferred to a rarer yet periodic enough >>> event. This is what s390 does already for user/system time and kernel >>> boundaries. >>> >>> This way we should even improve the situation compared to what we have >>> upstream. It's going to be faster because calling the accounting functions >>> can be costlier than simple per-cpu ops. And also we keep the cputime_t >>> granularity. For archs like s390 which have a granularity higher than nsecs, >>> we can have: >>> >>> u64 cputime_to_nsecs(cputime_t time, u64 *rem); >>> >>> And to avoid remainder losses, we can do that from the tick: >>> >>> delta_cputime = this_cpu_read(s390_cputime.hardirq); >>> delta_nsec = cputime_to_nsecs(delta_cputime, &rem); >>> account_system_time(delta_nsec, HARDIRQ_OFFSET); >>> this_cpu_write(s390_cputime.hardirq, rem); >>> >>> Although I doubt that remainders below one nsec lost each tick matter that much. >>> But if it does, it's fairly possible to handle like above. >> >> To make that work we would have to move some of the logic from account_system_time >> to the architecture code. The decision if a system time delta is guest time, >> irq time, softirq time or simply system time is currently done in >> kernel/sched/cputime.c. >> >> As the conversion + the accounting is delayed to a regular tick we would have >> to split the accounting code into decision functions which bucket a system time >> delta should go to and introduce new function to account to the different buckets. >> >> Instead of a single account_system_time we would have account_guest_time, >> account_system_time, account_system_time_irq and account_system_time_softirq. >> >> In principle not a bad idea, that would make the interrupt path for s390 faster >> as we would not have to call account_system_time, only the decision function >> which could be an inline function. > > Why make this s390 specific? > > We can decouple the accounting from the time accumulation for all > architectures. > > struct cputime_record { > u64 user, sys, softirq, hardirq, steal; > };
Wont we need guest, nice, guest_nice as well?
> > DEFINE_PER_CPU(struct cputime_record, cputime_record); > > Now let account_xxx_time() just work on that per cpu data > structures. That would just accumulate the deltas based on whatever > the architecture uses as a cputime source with whatever resolution it > provides. > > Then we collect that accumulated results for the various buckets on a > regular base and convert them to nano seconds. This is not even > required to be at the tick, it could be done by some async worker and > on idle enter/exit. > > Thanks, > > tglx > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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