BACKPORT: sched/fair: Initiate a new task's util avg to a bounded value

BACKPORT: sched/fair: Initiate a new task's util avg to a bounded value

A new task's util_avg is set to full utilization of a CPU (100% time
running). This accelerates a new task's utilization ramp-up, useful to
boost its execution in early time. However, it may result in
(insanely) high utilization for a transient time period when a flood
of tasks are spawned. Importantly, it violates the "fundamentally
bounded" CPU utilization, and its side effect is negative if we don't
take any measure to bound it.

This patch proposes an algorithm to address this issue. It has
two methods to approach a sensible initial util_avg:

(1) An expected (or average) util_avg based on its cfs_rq's util_avg:

  util_avg = cfs_rq->util_avg / (cfs_rq->load_avg + 1) * se.load.weight

(2) A trajectory of how successive new tasks' util develops, which
gives 1/2 of the left utilization budget to a new task such that
the additional util is noticeably large (when overall util is low) or
unnoticeably small (when overall util is high enough). In the meantime,
the aggregate utilization is well bounded:

  util_avg_cap = (1024 - cfs_rq->avg.util_avg) / 2^n

where n denotes the nth task.

If util_avg is larger than util_avg_cap, then the effective util is
clamped to the util_avg_cap.

Change-Id: Idafe989b24d9e70911666f09800bf1d5a011e1f4
Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Yuyang Du <yuyang.du@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bsegall@google.com
Cc: morten.rasmussen@arm.com
Cc: pjt@google.com
Cc: steve.muckle@linaro.org
Link: http://lkml.kernel.org/r/1459283456-21682-1-git-send-email-yuyang.du@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 2b8c41daba327c633228169e8bd8ec067ab443f8)
[integrate with schedfreq - schedfreq has a tuneable for init task util
 but this commit removes the use of the tuneable since we have a new
 algorithm for calculating an initial utilisation. I've left the tuneable
 in place, but it is no longer used even when schedfreq is the CPUFreq
 governor]
Signed-off-by: Chris Redpath <chris.redpath@arm.com>