From: Steve Muckle <steve.muckle@linaro.org>
Subject: PELT initial task load and wake_up_new_task()
To: "yuyang.du@intel.com" <yuyang.du@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Vincent Guittot <vincent.guittot@linaro.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Message-ID: <566B8009.2090006@linaro.org>
Date: Fri, 11 Dec 2015 18:01:45 -0800
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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.

If last_update_time is initialized to cfs_rq_clock_task() the load will
not go away, but it will also then be subtracted from the original CPU
in remove_entity_load_avg() if the task is placed on a different CPU,
which is bad since it was never added there before.

Thinking about this more it seemed questionable to treat the assignment
of a task to a new CPU in wake_up_new_task() as a migration given that
the task has never executed previously. Would it make sense to call
__set_task_cpu() there instead of set_task_cpu()?

thanks,
Steve
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