Re: [PATCH v2 5/7] KVM-GST: KVM Steal time accounting

[Glauber Costa]

On 06/19/2011 07:04 AM, Avi Kivity wrote:
> On 06/17/2011 01:20 AM, Glauber Costa wrote:
> > This patch accounts steal time time in kernel/sched.
> > I kept it from last proposal, because I still see advantages
> > in it: Doing it here will give us easier access from scheduler
> > variables such as the cpu rq. The next patch shows an example of
> > usage for it.
> >
> > Since functions like account_idle_time() can be called from
> > multiple places, not only account_process_tick(), steal time
> > grabbing is repeated in each account function separatedely.
> >
> > /*
> > + * We have to at flush steal time information every time something else
> > + * is accounted. Since the accounting functions are all visible to
> > the rest
> > + * of the kernel, it gets tricky to do them in one place. This helper
> > function
> > + * helps us.
> > + *
> > + * When the system is idle, the concept of steal time does not apply.
> > We just
> > + * tell the underlying hypervisor that we grabbed the data, but skip
> > steal time
> > + * accounting
> > + */
> > +static inline bool touch_steal_time(int is_idle)
> > +{
> > + u64 steal, st = 0;
> > +
> > + if (static_branch(&paravirt_steal_enabled)) {
> > +
> > + steal = paravirt_steal_clock(smp_processor_id());
> > +
> > + steal -= this_rq()->prev_steal_time;
> > + if (is_idle) {
> > + this_rq()->prev_steal_time += steal;
> > + return false;
> > + }
> > +
> > + while (steal>= TICK_NSEC) {
> > + /*
> > + * Inline assembly required to prevent the compiler
> > + * optimising this loop into a divmod call.
> > + * See __iter_div_u64_rem() for another example of this.
> > + */
>
> Why not use said function?

because here we want to do work during each loop. The said function
would have to be adapted for that, possibly using a macro, to run 
arbitrary code during each loop iteration, in a way that I don't think
it is worthy given the current number of callers (2 counting this new one)

> > + asm("" : "+rm" (steal));
> > +
> > + steal -= TICK_NSEC;
> > + this_rq()->prev_steal_time += TICK_NSEC;
> > + st++;
>
> Suppose a live migration or SIGSTOP causes lots of steal time. How long
> will we spend here?
Silly me. I actually used this same argument with Peter to cap it with 
"delta" in the next patch in this series. So I think you are 100 % 
right. Here, however, we do want to account all that time, I believe.

How about we do a slow division if we're > 10 sec (unlikely), and 
account everything as steal time in this scenario ?

> > + }
> > +
> > + account_steal_time(st);
> > + return !!st;
>
> !! !needed, you're returning a bool.

ah, sure thing.

> > + }
> > + return false;
> > +}
> > +
>
> I'll need Peter's (or another sched maintainer's) review to apply this.

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