From 82a3b9a3c6b40c8048b73e04aab04603cecf8ca1 Mon Sep 17 00:00:00 2001
From: Patrick Bellasi <patrick.bellasi@arm.com>
Date: Mon, 22 Jun 2015 18:32:36 +0100
Subject: [PATCH] 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>
---
kernel/sched/fair.c | 38 ++++++++++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 651e160cdd67..1b6a57837433 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -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.
--
2.34.1