Re: PELT initial task load and wake_up_new_task()

[Yuyang Du]

On Mon, Dec 14, 2015 at 04:41:14PM -0800, Steve Muckle wrote:
> Hi Yuyang,
> 
> On 12/13/2015 11:13 AM, Yuyang Du wrote:
> > Hi Steve,
> > 
> > On Fri, Dec 11, 2015 at 06:01:45PM -0800, Steve Muckle wrote:
> >> In init_entity_runnable_average() the last_update_time is initialized to
> >> zero. The task is given max load and utilization as a pessimistic
> >> initial estimate.
> >>
> >> But if in wake_up_new_task() the task is placed on a CPU other than
> >> where it was created, __update_load_avg() will be called via
> >> set_task_cpu() -> migrate_task_rq_fair() -> remove_entity_load_avg().
> >>
> >> Since last_update_time is zero the delta will be huge and the task's
> >> load will be entirely decayed away before it is enqueued at the
> >> destination CPU.
> >  
> > Since the new task's last_update_time is equal to 0, it will not be decayed.
> 
> Can you point me to the code for that logic? I don't see anything that
> prevents the decay when a newly woken task is placed on a different CPU
> via the call chain I mentioned above. My testing also shows the load
> being decayed to zero.
> 
You may search the last_update_time, and see it would be treated differently
if it is 0. Hope this may be helpful.
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