[PATCH 7/9] sched/psi: cache parent psi_group to speed up groups iterate

Chengming Zhou
2022-07-21T04:06:08Z

We use iterate_groups() to iterate each level psi_group to update
PSI stats, which is a very hot path.

In current code, iterate_groups() have to use multiple branches and
cgroup_parent() to get parent psi_group for each level, which is not
very efficient.

This patch cache parent psi_group, only need to get psi_group of task
itself first, then just use group->parent to iterate.

And this patch is preparation for the following patch, in which we can
configure PSI to only account for leaf cgroups and system-wide.

Performance test on Intel Xeon Platinum with 3 levels of cgroup:

1. before the patch:

$ perf bench sched all
# Running sched/messaging benchmark...
# 20 sender and receiver processes per group
# 10 groups == 400 processes run

Total time: 0.032 [sec]

# Running sched/pipe benchmark...
# Executed 1000000 pipe operations between two processes

Total time: 8.077 [sec]

8.077648 usecs/op
123798 ops/sec

2. after the patch:

$ perf bench sched all
# Running sched/messaging benchmark...
# 20 sender and receiver processes per group
# 10 groups == 400 processes run

Total time: 0.032 [sec]

# Running sched/pipe benchmark...
# Executed 1000000 pipe operations between two processes

Total time: 7.758 [sec]

7.758354 usecs/op
128893 ops/sec

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
include/linux/psi_types.h | 2 ++
kernel/sched/psi.c | 48 ++++++++++++++++++++-------------------
2 files changed, 27 insertions(+), 23 deletions(-)

diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
index c7fe7c089718..c124f7d186d0 100644
--- a/include/linux/psi_types.h
+++ b/include/linux/psi_types.h
@@ -147,6 +147,8 @@ struct psi_trigger {
};

struct psi_group {
+ struct psi_group *parent;
+
/* Protects data used by the aggregator */
struct mutex avgs_lock;

diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index aa40bf888102..2228cbf3bdd3 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -758,30 +758,22 @@ static void psi_group_change(struct psi_group *group, int cpu,
schedule_delayed_work(&group->avgs_work, PSI_FREQ);
}

-static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
+static inline struct psi_group *task_psi_group(struct task_struct *task)
{
- if (*iter == &psi_system)
- return NULL;
-
#ifdef CONFIG_CGROUPS
if (static_branch_likely(&psi_cgroups_enabled)) {
- struct cgroup *cgroup = NULL;
-
- if (!*iter)
- cgroup = task->cgroups->dfl_cgrp;
- else
- cgroup = cgroup_parent(*iter);
+ struct cgroup *cgroup = task_dfl_cgroup(task);

- if (cgroup && cgroup_parent(cgroup)) {
- *iter = cgroup;
+ if (cgroup && cgroup_parent(cgroup))
return cgroup_psi(cgroup);
- }
}
#endif
- *iter = &psi_system;
return &psi_system;
}

+#define for_each_psi_group(group) \
+ for (; group; group = group->parent)
+
static void psi_flags_change(struct task_struct *task, int clear, int set)
{
if (((task->psi_flags & set) ||
@@ -799,12 +791,11 @@ static void psi_flags_change(struct task_struct *task, int clear, int set)

void psi_change_groups(struct task_struct *task, int clear, int set)
{
+ struct psi_group *group = task_psi_group(task);
int cpu = task_cpu(task);
- struct psi_group *group;
- void *iter = NULL;
u64 now = cpu_clock(cpu);

- while ((group = iterate_groups(task, &iter)))
+ for_each_psi_group(group)
psi_group_change(group, cpu, clear, set, now, true);
}

@@ -822,7 +813,6 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
{
struct psi_group *group, *common = NULL;
int cpu = task_cpu(prev);
- void *iter;
u64 now = cpu_clock(cpu);

if (next->pid) {
@@ -833,8 +823,8 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
* we reach the first common ancestor. Iterate @next's
* ancestors only until we encounter @prev's ONCPU.
*/
- iter = NULL;
- while ((group = iterate_groups(next, &iter))) {
+ group = task_psi_group(next);
+ for_each_psi_group(group) {
if (per_cpu_ptr(group->pcpu, cpu)->tasks[NR_ONCPU]) {
common = group;
break;
@@ -874,9 +864,12 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,

psi_flags_change(prev, clear, set);

- iter = NULL;
- while ((group = iterate_groups(prev, &iter)) && group != common)
+ group = task_psi_group(prev);
+ for_each_psi_group(group) {
+ if (group == common)
+ break;
psi_group_change(group, cpu, clear, set, now, wake_clock);
+ }

/*
* TSK_ONCPU is handled up to the common ancestor. If we're tasked
@@ -884,7 +877,8 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
*/
if (sleep || unlikely(prev->in_memstall != next->in_memstall)) {
clear &= ~TSK_ONCPU;
- for (; group; group = iterate_groups(prev, &iter))
+
+ for_each_psi_group(group)
psi_group_change(group, cpu, clear, set, now, true);
}
}
@@ -953,6 +947,8 @@ void psi_memstall_leave(unsigned long *flags)
#ifdef CONFIG_CGROUPS
int psi_cgroup_alloc(struct cgroup *cgroup)
{
+ struct cgroup *parent;
+
if (!static_branch_likely(&psi_cgroups_enabled))
return 0;

@@ -960,6 +956,12 @@ int psi_cgroup_alloc(struct cgroup *cgroup)
if (!cgroup->psi.pcpu)
return -ENOMEM;
group_init(&cgroup->psi);
+
+ parent = cgroup_parent(cgroup);
+ if (parent && cgroup_parent(parent))
+ cgroup->psi.parent = cgroup_psi(parent);
+ else
+ cgroup->psi.parent = &psi_system;
return 0;
}

--
2.36.1