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Date: Wed, 11 Dec 2019 16:44:01 +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
Subject: [RFC PATCH 7/7] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan
Reworking select_task_rq_fair()'s domain walk exposed that !want_affine
wakeups only look for highest sched_domain with the required sd_flag
set. This is something we can cache at sched domain build time to slightly
optimize select_task_rq_fair(). Note that this isn't a "free" optimization:
it costs us 3 pointers per CPU.
Add cached per-CPU pointers for the highest domains with SD_BALANCE_WAKE,
SD_BALANCE_EXEC and SD_BALANCE_FORK. Do note that these are exclusively
used in select_task_rq_fair(), which also requires SD_LOAD_BALANCE, so the
cached pointers are also gated by SD_LOAD_BALANCE (which is somewhat icky).
There is another nasty thing that comes out of this: with the
sched_domain/* sysctl knobs, one could e.g. clear SD_LOAD_BALANCE for some
CPU(s) (AFAIK the only way to clear that flag ATM). Before this patch,
that would disable entering find_idlest_cpu() from said CPU(s). With this
patch, writing to the SD flags sysctl wouldn't lead to any cached SD
pointer update, so we'd keep going through the slow-path.
This isn't a new problem (all cached domain pointers are affected by this),
but it is a change in behaviour. It could make sense to either turn this
interface read-only, or add a callback in the flags update to properly
update our cached domain data.
Signed-off-by: Valentin Schneider <valentin.schneider@....com>
---
kernel/sched/fair.c | 25 +++++++++++++------------
kernel/sched/sched.h | 3 +++
kernel/sched/topology.c | 12 ++++++++++++
3 files changed, 28 insertions(+), 12 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ea875c7c82d7..5b72597d6540 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6344,17 +6344,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
int want_affine = 0;
int sd_flag;
- switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
- case WF_TTWU:
- sd_flag = SD_BALANCE_WAKE;
- break;
- case WF_FORK:
- sd_flag = SD_BALANCE_FORK;
- break;
- default:
- sd_flag = SD_BALANCE_EXEC;
- }
-
if (wake_flags & WF_TTWU) {
record_wakee(p);
@@ -6371,7 +6360,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
rcu_read_lock();
- sd = highest_flags_domain(cpu, sd_flag | SD_LOAD_BALANCE);
+ switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
+ case WF_TTWU:
+ sd_flag = SD_BALANCE_WAKE;
+ sd = rcu_dereference(per_cpu(sd_balance_wake, cpu));
+ break;
+ case WF_FORK:
+ sd_flag = SD_BALANCE_FORK;
+ sd = rcu_dereference(per_cpu(sd_balance_fork, cpu));
+ break;
+ default:
+ sd_flag = SD_BALANCE_EXEC;
+ sd = rcu_dereference(per_cpu(sd_balance_exec, cpu));
+ }
/*
* If !want_affine, we just look for the highest domain where
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 233b3d41e347..ba9d55085c7b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1386,6 +1386,9 @@ DECLARE_PER_CPU(struct sched_domain __rcu *, sd_llc);
DECLARE_PER_CPU(int, sd_llc_size);
DECLARE_PER_CPU(int, sd_llc_id);
DECLARE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
+DECLARE_PER_CPU(struct sched_domain __rcu *, sd_balance_wake);
+DECLARE_PER_CPU(struct sched_domain __rcu *, sd_balance_exec);
+DECLARE_PER_CPU(struct sched_domain __rcu *, sd_balance_fork);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_numa);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index f0d2a15fd10b..f76f40b64279 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -619,6 +619,9 @@ DEFINE_PER_CPU(struct sched_domain __rcu *, sd_llc);
DEFINE_PER_CPU(int, sd_llc_size);
DEFINE_PER_CPU(int, sd_llc_id);
DEFINE_PER_CPU(struct sched_domain_shared __rcu *, sd_llc_shared);
+DEFINE_PER_CPU(struct sched_domain __rcu *, sd_balance_wake);
+DEFINE_PER_CPU(struct sched_domain __rcu *, sd_balance_exec);
+DEFINE_PER_CPU(struct sched_domain __rcu *, sd_balance_fork);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_numa);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
DEFINE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
@@ -643,6 +646,15 @@ static void update_top_cache_domain(int cpu)
per_cpu(sd_llc_id, cpu) = id;
rcu_assign_pointer(per_cpu(sd_llc_shared, cpu), sds);
+ sd = highest_flags_domain(cpu, SD_BALANCE_WAKE | SD_LOAD_BALANCE);
+ rcu_assign_pointer(per_cpu(sd_balance_wake, cpu), sd);
+
+ sd = highest_flags_domain(cpu, SD_BALANCE_FORK | SD_LOAD_BALANCE);
+ rcu_assign_pointer(per_cpu(sd_balance_fork, cpu), sd);
+
+ sd = highest_flags_domain(cpu, SD_BALANCE_EXEC | SD_LOAD_BALANCE);
+ rcu_assign_pointer(per_cpu(sd_balance_exec, cpu), sd);
+
sd = lowest_flags_domain(cpu, SD_NUMA);
rcu_assign_pointer(per_cpu(sd_numa, cpu), sd);
--
2.24.0
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