While staring at build_sched_domains(), I realized that get_group()
does several duplicate (thus useless) writes.
If you take the Arm Juno r0 (LITTLEs = [0, 3, 4, 5], bigs = [1, 2]), the
sched_group build flow would look like this:
('MC[cpu]->sg' means 'per_cpu_ptr(&tl->data->sg, cpu)' with 'tl == MC')
build_sched_groups(MC[CPU0]->sd, CPU0)
get_group(0) -> MC[CPU0]->sg
get_group(3) -> MC[CPU3]->sg
get_group(4) -> MC[CPU4]->sg
get_group(5) -> MC[CPU5]->sg
build_sched_groups(DIE[CPU0]->sd, CPU0)
get_group(0) -> DIE[CPU0]->sg
get_group(1) -> DIE[CPU1]->sg <-----------------+
|
build_sched_groups(MC[CPU1]->sd, CPU1) |
get_group(1) -> MC[CPU1]->sg |
get_group(2) -> MC[CPU2]->sg |
|
build_sched_groups(DIE[CPU1]->sd, CPU1) ^
get_group(1) -> DIE[CPU1]->sg } We've set up these two up here!
get_group(3) -> DIE[CPU0]->sg }
>From this point on, we will only use sched_groups that have been
previously visited & initialized. The only new operation will
be which group pointer we affect to sd->groups.
On the Juno r0 we get 32 get_group() calls, every single one of them
writing to a sched_group->cpumask. However, all of the data structures
we need are set up after 8 visits (see above).
Return early from get_group() if we've already visited (and thus
initialized) the sched_group we're looking at. Overlapping domains
are not affected as they do not use build_sched_groups().
Tested on a Juno and a 2 * (Xeon E5-2690) system.
Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com>
---
FWIW I initially checked the refs for both sg && sg->sgc, but figured if
they weren't both 0 or > 1 then something must have gone wrong, so I
threw in a WARN_ON().
---
kernel/sched/topology.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 64bec54ded3e..6c0b7326f66e 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1059,6 +1059,7 @@ static struct sched_group *get_group(int cpu, struct
sd_data *sdd)
struct sched_domain *sd = *per_cpu_ptr(sdd->sd, cpu);
struct sched_domain *child = sd->child;
struct sched_group *sg;
+ bool already_visited;
if (child)
cpu = cpumask_first(sched_domain_span(child));
@@ -1066,9 +1067,14 @@ static struct sched_group *get_group(int cpu, struct
sd_data *sdd)
sg = *per_cpu_ptr(sdd->sg, cpu);
sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);
- /* For claim_allocations: */
- atomic_inc(&sg->ref);
- atomic_inc(&sg->sgc->ref);
+ /* Increase refcounts for claim_allocations: */
+ already_visited = atomic_inc_return(&sg->ref) > 1;
+ /* sgc visits should follow a similar trend as sg */
+ WARN_ON(already_visited != (atomic_inc_return(&sg->sgc->ref) > 1));
+
+ /* If we have already visited that group, it's already initialized. */
+ if (already_visited)
+ return sg;
if (child) {
cpumask_copy(sched_group_span(sg), sched_domain_span(child));
--
2.20.1
