Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | [RFC PATCH 7/7] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan | Date | Wed, 11 Dec 2019 16:44:01 +0000 |
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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@arm.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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