Re: Stolen and degraded time and schedulers

[Dan Hecht]

On 03/14/2007 02:18 PM, Jeremy Fitzhardinge wrote:
> Dan Hecht wrote:
> > On 03/14/2007 01:31 PM, Jeremy Fitzhardinge wrote:
> > > Dan Hecht wrote:
> > > > Sounds good.  I don't see this in your patchset you sent yesterday
> > > > though; did you add it after sending out those patches?
> > > Yes.
> > >
> > > >   if so, could you forward the new patch?  does it explicitly prevent
> > > > stolen time from getting accounted as  user/system time or does it
> > > > just rely on NO_HZ mode sort of happening to work that way (since the
> > > > one shot timer is skipped ahead for missed ticks)?
> > > Hm, not sure.  It doesn't care how often it gets called; it just
> > > accumulates results up to that point, but I'm not sure if the time would
> > > get double accounted.  Perhaps it doesn't matter when using
> > > xen_sched_clock().
> > I think you might be double counting time in some cases. 
> > sched_clock() isn't really relevant to stolen time accounting (i.e.
> > cpustat->steal).
> >
> > I think what you want is to make sure that the sum of the cputime
> > passed to all of:
> >
> > account_user_time
> > account_system_time
> > account_steal_time
> >
> > adds up to the total amount of time that has passed.  I think it is
> > sort of working for you (i.e. doesn't always double count stolen
> > ticks) since in NO_HZ mode, update_process_time (which calls
> > account_user_time & account_system_time) happens to be skipped during
> > periods of stolen time due to the hrtimer_forward()'ing of the one
> > shot expiry. 
>
> OK, this will need a closer look.
>
> BTW, what are the properties of the vmi read_available_cycles()?  Is
> that a per-cpu timer?  If its used as the timebase for sched_clock, how
> does recalc_task_prio not get a -ve sleep time?

Available time is defined to be (real_time - stolen_time).  i.e. time in 
which the vcpu is either running or not ready to run [because it is 
halted, and nothing is pending]).

So, yes, it is per-vcpu.  But, the sched_clock() samples are rebased 
when processes are migrated between runqueues; search sched.c for 
most_recent_timestamp.  It's not perfect since most_recent_timestamp 
between cpu0 and cpu1 do not correspond to the exact same instant, but 
does prevent negative sleep time and is fairly close.

Dan
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