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sched/topology: Propagate SD_ASYM_CPUCAPACITY upwards

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We currently set this flag *only* on domains whose topology level exactly
match the level where we detect asymmetry (as returned by
asym_cpu_capacity_level()). This is rather problematic.

Say there are two clusters in the system, one with a lone big CPU and the
other with a mix of big and LITTLE CPUs (as is allowed by DynamIQ):

  DIE [                ]
  MC  [             ][ ]
       0   1   2   3  4
       L   L   B   B  B

asym_cpu_capacity_level() will figure out that the MC level is the one
where all CPUs can see a CPU of max capacity, and we will thus set
SD_ASYM_CPUCAPACITY at MC level for all CPUs.

That lone big CPU will degenerate its MC domain, since it would be alone in
there, and will end up with just a DIE domain. Since the flag was only set
at MC, this CPU ends up not seeing any SD with the flag set, which is
broken.

Rather than clearing dflags at every topology level, clear it before
entering the topology level loop. This will properly propagate upwards
flags that are set starting from a certain level.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Quentin Perret <qperret@google.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20200817113003.20802-11-valentin.schneider@arm.com
This commit is contained in:
Valentin Schneider 2020-08-17 12:29:56 +01:00 committed by Ingo Molnar
parent ab65afb094
commit c200191d4c
2 changed files with 4 additions and 3 deletions
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4
include/linux/sched/sd_flags.h
View file

@ -83,9 +83,11 @@ SD_FLAG(SD_WAKE_AFFINE, SDF_SHARED_CHILD)
/*
* Domain members have different CPU capacities
*
* SHARED_PARENT: Set from the topmost domain down to the first domain where
* asymmetry is detected.
* NEEDS_GROUPS: Per-CPU capacity is asymmetric between groups.
*/
SD_FLAG(SD_ASYM_CPUCAPACITY, SDF_NEEDS_GROUPS)
SD_FLAG(SD_ASYM_CPUCAPACITY, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
/*
* Domain members share CPU capacity (i.e. SMT)

3
kernel/sched/topology.c
View file

@ -1988,11 +1988,10 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
/* Set up domains for CPUs specified by the cpu_map: */
for_each_cpu(i, cpu_map) {
struct sched_domain_topology_level *tl;
int dflags = 0;
sd = NULL;
for_each_sd_topology(tl) {
int dflags = 0;
if (tl == tl_asym) {
dflags |= SD_ASYM_CPUCAPACITY;
has_asym = true;