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Date: Thu, 6 Feb 2020 19:19:54 +0000
From: Valentin Schneider <valentin.schneider@....com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
dietmar.eggemann@....com, morten.rasmussen@....com,
qperret@...gle.com, adharmap@...eaurora.org,
pkondeti@...eaurora.org
Subject: [PATCH v4 1/4] sched/fair: Add asymmetric CPU capacity wakeup scan
From: Morten Rasmussen <morten.rasmussen@....com>
Issue
=====
On asymmetric CPU capacity topologies, we currently rely on wake_cap() to
drive select_task_rq_fair() towards either
- its slow-path (find_idlest_cpu()) if either the previous or
current (waking) CPU has too little capacity for the waking task
- its fast-path (select_idle_sibling()) otherwise
Commit 3273163c6775 ("sched/fair: Let asymmetric CPU configurations balance
at wake-up") points out that this relies on the assumption that "[...]the
CPU capacities within an SD_SHARE_PKG_RESOURCES domain (sd_llc) are
homogeneous".
This assumption no longer holds on newer generations of big.LITTLE
systems (DynamIQ), which can accommodate CPUs of different compute capacity
within a single LLC domain. To hopefully paint a better picture, a regular
big.LITTLE topology would look like this:
+---------+ +---------+
| L2 | | L2 |
+----+----+ +----+----+
|CPU0|CPU1| |CPU2|CPU3|
+----+----+ +----+----+
^^^ ^^^
LITTLEs bigs
which would result in the following scheduler topology:
DIE [ ] <- sd_asym_cpucapacity
MC [ ] [ ] <- sd_llc
0 1 2 3
Conversely, a DynamIQ topology could look like:
+-------------------+
| L3 |
+----+----+----+----+
| L2 | L2 | L2 | L2 |
+----+----+----+----+
|CPU0|CPU1|CPU2|CPU3|
+----+----+----+----+
^^^^^ ^^^^^
LITTLEs bigs
which would result in the following scheduler topology:
MC [ ] <- sd_llc, sd_asym_cpucapacity
0 1 2 3
What this means is that, on DynamIQ systems, we could pass the wake_cap()
test (IOW presume the waking task fits on the CPU capacities of some LLC
domain), thus go through select_idle_sibling().
This function operates on an LLC domain, which here spans both bigs and
LITTLEs, so it could very well pick a CPU of too small capacity for the
task, despite there being fitting idle CPUs - it very much depends on the
CPU iteration order, on which we have absolutely no guarantees
capacity-wise.
Implementation
==============
Introduce yet another select_idle_sibling() helper function that takes CPU
capacity into account. The policy is to pick the first idle CPU which is
big enough for the task (task_util * margin < cpu_capacity). If no
idle CPU is big enough, we pick the idle one with the highest capacity.
Unlike other select_idle_sibling() helpers, this one operates on the
sd_asym_cpucapacity sched_domain pointer, which is guaranteed to span all
known CPU capacities in the system. As such, this will work for both
"legacy" big.LITTLE (LITTLEs & bigs split at MC, joined at DIE) and for
newer DynamIQ systems (e.g. LITTLEs and bigs in the same MC domain).
Note that this limits the scope of select_idle_sibling() to
select_idle_capacity() for asymmetric CPU capacity systems - the LLC domain
will not be scanned, and no further heuristic will be applied.
Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
Co-developed-by: Valentin Schneider <valentin.schneider@....com>
Signed-off-by: Valentin Schneider <valentin.schneider@....com>
---
kernel/sched/fair.c | 56 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 56 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fe4e0d7753756..9a5a6e9d2375e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5894,6 +5894,40 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
return cpu;
}
+/*
+ * Scan the asym_capacity domain for idle CPUs; pick the first idle one on which
+ * the task fits. If no CPU is big enough, but there are idle ones, try to
+ * maximize capacity.
+ */
+static int
+select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
+{
+ unsigned long best_cap = 0;
+ int cpu, best_cpu = -1;
+ struct cpumask *cpus;
+
+ sync_entity_load_avg(&p->se);
+
+ cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
+ cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+
+ for_each_cpu_wrap(cpu, cpus, target) {
+ unsigned long cpu_cap = capacity_of(cpu);
+
+ if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
+ continue;
+ if (task_fits_capacity(p, cpu_cap))
+ return cpu;
+
+ if (cpu_cap > best_cap) {
+ best_cap = cpu_cap;
+ best_cpu = cpu;
+ }
+ }
+
+ return best_cpu;
+}
+
/*
* Try and locate an idle core/thread in the LLC cache domain.
*/
@@ -5902,6 +5936,28 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
struct sched_domain *sd;
int i, recent_used_cpu;
+ /*
+ * For asymmetric CPU capacity systems, our domain of interest is
+ * sd_asym_cpucapacity rather than sd_llc.
+ */
+ if (static_branch_unlikely(&sched_asym_cpucapacity)) {
+ sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target));
+ /*
+ * On an asymmetric CPU capacity system where an exclusive
+ * cpuset defines a symmetric island (i.e. one unique
+ * capacity_orig value through the cpuset), the key will be set
+ * but the CPUs within that cpuset will not have a domain with
+ * SD_ASYM_CPUCAPACITY. These should follow the usual symmetric
+ * capacity path.
+ */
+ if (!sd)
+ goto symmetric;
+
+ i = select_idle_capacity(p, sd, target);
+ return ((unsigned)i < nr_cpumask_bits) ? i : target;
+ }
+
+symmetric:
if (available_idle_cpu(target) || sched_idle_cpu(target))
return target;
--
2.24.0
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