linux-kernel - [PATCH v2 4/7] sched/fair: Introduce a CPU capacity comparison helper

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Date:   Fri, 19 Feb 2021 13:00:00 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org
Cc:     Qais Yousef <qais.yousef@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>
Subject: [PATCH v2 4/7] sched/fair: Introduce a CPU capacity comparison helper

During load-balance, groups classified as group_misfit_task are filtered
out if they do not pass

  group_smaller_max_cpu_capacity(<candidate group>, <local group>);

which itself employs fits_capacity() to compare the sgc->max_capacity of
both groups.

Due to the underlying margin, fits_capacity(X, 1024) will return false for
any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
{261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
CPUs, misfit migration will never intentionally upmigrate it to a CPU of
higher capacity due to the aforementioned margin.

One may argue the 20% margin of fits_capacity() is excessive in the advent
of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
is that fits_capacity() is meant to compare a utilization value to a
capacity value, whereas here it is being used to compare two capacity
values. As CPU capacity and task utilization have different dynamics, a
sensible approach here would be to add a new helper dedicated to comparing
CPU capacities.

Reviewed-by: Qais Yousef <qais.yousef@....com>
Signed-off-by: Valentin Schneider <valentin.schneider@....com>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 24119f9ad191..cc16d0e0b9fb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
  */
 #define fits_capacity(cap, max)	((cap) * 1280 < (max) * 1024)

+/*
+ * The margin used when comparing CPU capacities.
+ * is 'cap1' noticeably greater than 'cap2'
+ *
+ * (default: ~5%)
+ */
+#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
 #endif

 #ifdef CONFIG_CFS_BANDWIDTH
-- 
2.27.0