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author | Patrick Bellasi <patrick.bellasi@arm.com> | 2015-06-22 18:32:36 +0100 |
---|---|---|
committer | Punit Agrawal <punit.agrawal@arm.com> | 2016-03-21 14:57:33 +0000 |
commit | 82a3b9a3c6b40c8048b73e04aab04603cecf8ca1 (patch) | |
tree | 277d799103588d31f4e0e029b6d7d9ba92ab8c44 | |
parent | a6332cfa791c2b39c77ef778323963f513694602 (diff) |
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>
-rw-r--r-- | kernel/sched/fair.c | 38 |
1 files changed, 38 insertions, 0 deletions
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. |