Re: Stolen and degraded time and schedulers

Dan Hecht
2007-03-15T19:09:30Z

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