Messages in this thread | | | Date | Mon, 1 Dec 2014 20:59:43 +0100 | From | Frederic Weisbecker <> | Subject | Re: [RFC PATCH 07/30] cputime: Convert kcpustat to nsecs |
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On Mon, Dec 01, 2014 at 06:27:38PM +0100, Martin Schwidefsky wrote: > On Mon, 1 Dec 2014 18:15:36 +0100 (CET) > Thomas Gleixner <tglx@linutronix.de> wrote: > > > 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; > > }; > > > > 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. > > And leave the decision making in kernel/sched/cputime.c. Yes, that is good. > This would make the arch and the account_xxx_time() function care about > cputime_t and all other common code would use nano-seconds. With the added > benefit that I do not have to change the low level code too much ;-)
Yes that sounds really good. Besides, this whole machinery can also benefit for CONFIG_VIRT_CPU_ACCOUNTING_GEN which is the same context entry/exit based cputime accounting, just it's based on context tracking.
Note the current differences between those two CONFIG_VIRT_CPU_ACCOUNTING flavours:
_ CONFIG_VIRT_CPU_ACCOUNTING_NATIVE: * user/kernel boundaries: accumulate * irq boundaries: account * context_switch: account * tick: account (pending user time if any) * task_cputime(): direct access
_ CONFIG_VIRT_CPU_ACCOUNTING_GEN: * user/kernel boundaries: account * irq boundaries: account * context_switch: account * tick: ignore * task_cputime(): direct access + accumulate
More details about task_cputime(): tsk->[us]time are fetched through task_cputime(), in NATIVE tsk->[us]time are periodically accounted (thanks to the tick) so task_cputime() simply return the fields as is. In GEN the tick maybe off so task_cputime() returns tsk->[us]time + pending accumulated time.
I'm recalling that because Thomas suggests that we don't _have_ to account the accumulated time from the tick and indeed, this can be done from calls to task_cputime() like we do for GEN. Of course this comes at the cost of overhead on any access to utime and stime fields of a task_struct. Thus the pros and cons must be carefully considered between tick overhead and task_cputime() overhead, I'd personally be cautious and flush from the tick at least as a first step.
That is:
_ CONFIG_VIRT_CPU_ACCOUNTING (GEN || NATIVE) * user/kernel boundaries: accumulate * irq boundaries: accumulate * context_switch: account * tick: account on NATIVE * task_cputime: return accumulated on GEN
I can take care of that as a preclude before the conversion of cputime to nsecs.
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