Commit 6ed27145 authored Jun 22, 2015 by Patrick Bellasi Committed by Leo Yan May 10, 2016

sched/fair: add function to convert boost value into "margin"



The basic idea of the boost knob is to "artificially inflate" a signal
to make a task or logical CPU appears more demanding than it actually
is. Independently from the specific signal, a consistent and possibly
simple semantic for the concept of "signal boosting" must define:
1. how we translate the boost percentage into a "margin" value to be added
   to the original signal to inflate
2. what is the meaning of a boost value from a user-space perspective

This patch provides the implementation of a possible boost semantic,
named "Signal Proportional Compensation" (SPC), where the boost
percentage (BP) is used to compute a margin (M) which is proportional to
the complement of the original signal (OS):
  M = BP * (SCHED_LOAD_SCALE - OS)
The computed margin then added to the OS to obtain the Boosted Signal (BS)
  BS = OS + M

The proposed boost semantic has these main features:
- each signal gets a boost which is proportional to its delta with respect
  to the maximum available capacity in the system (i.e. SCHED_LOAD_SCALE)
- a 100% boosting has a clear understanding from a user-space perspective,
  since it means simply to run (possibly) "all" tasks at the max OPP
- each boosting value means to improve the task performance by a quantity
  which is proportional to the maximum achievable performance on that
  system
Thus this semantics is somehow forcing a behaviour which is:

  50% boosting means to run at half-way between the current and the
    maximum performance which a task could achieve on that system

This patch provides the code to implement a fast integer division to
convert a boost percentage (BP) value into a margin (M).

NOTE: this code is suitable for all signals operating in range
      [0..SCHED_LOAD_SCALE]

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>

parent 344f4ec4

kernel/sched/fair.c

+38 −0

Original line number	Diff line number	Diff line
		@@ -5074,6 +5074,44 @@ static bool cpu_overutilized(int cpu)
		return (capacity_of(cpu) * 1024) < (cpu_util(cpu) * capacity_margin);
		}

		#ifdef CONFIG_SCHED_TUNE

		static unsigned long
		schedtune_margin(unsigned long signal, unsigned long boost)
		{
		unsigned long long margin = 0;

		/*
		* Signal proportional compensation (SPC)
		*
		* The Boost (B) value is used to compute a Margin (M) which is
		* proportional to the complement of the original Signal (S):
		* M = B * (SCHED_LOAD_SCALE - S)
		* The obtained M could be used by the caller to "boost" S.
		*/
		margin = SCHED_LOAD_SCALE - signal;
		margin *= boost;

		/*
		* Fast integer division by constant:
		* Constant : (C) = 100
		* Precision : 0.1% (P) = 0.1
		* Reference : C * 100 / P (R) = 100000
		*
		* Thus:
		* Shift bits : ceil(log(R,2)) (S) = 17
		* Mult const : round(2^S/C) (M) = 1311
		*
		*
		*/
		margin *= 1311;
		margin >>= 17;

		return margin;
		}

		#endif /* CONFIG_SCHED_TUNE */

		/*
		* find_idlest_group finds and returns the least busy CPU group within the
		* domain.