Re: [RFC PATCH 07/30] cputime: Convert kcpustat to nsecs

[Thomas Gleixner <tglx@linutronix.de>]

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.

Thanks,

	tglx