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Message-Id: <20231101160911.394526-1-longman@redhat.com>
Date: Wed, 1 Nov 2023 12:09:11 -0400
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
Johannes Weiner <hannes@...xchg.org>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
Joe Mario <jmario@...hat.com>,
Sebastian Jug <sejug@...hat.com>,
Waiman Long <longman@...hat.com>
Subject: [PATCH] cgroup/rstat: Reduce cpu_lock hold time in cgroup_rstat_flush_locked()
When cgroup_rstat_updated() isn't being called concurrently with
cgroup_rstat_flush_locked(), its run time is pretty short. When
both are called concurrently, the cgroup_rstat_updated() run time
can spike to a pretty high value due to high cpu_lock hold time in
cgroup_rstat_flush_locked(). This can be problematic if the task calling
cgroup_rstat_updated() is a realtime task running on an isolated CPU
with a strict latency requirement. The cgroup_rstat_updated() call can
happens when there is a page fault even though the task is running in
user space most of the time.
The percpu cpu_lock is used to protect the update tree -
updated_next and updated_children. This protection is only needed
when cgroup_rstat_cpu_pop_updated() is being called. The subsequent
flushing operation which can take a much longer time does not need
that protection.
To reduce the cpu_lock hold time, we need to perform all the
cgroup_rstat_cpu_pop_updated() calls up front with the lock
released afterward before doing any flushing. This patch adds a new
cgroup_rstat_flush_list() function to do just that and return a singly
linked list of cgroup_rstat_cpu structures to be flushed.
By adding some instrumentation code to measure the maximum elapsed times
of the new cgroup_rstat_flush_list() function and each cpu iteration
of cgroup_rstat_flush_locked() around the old cpu_lock lock/unlock pair
on a 2-socket x86-64 server running parallel kernel build, the maximum
elapsed times are 31us and 118us respectively. The maximum cpu_lock
hold time is now reduced to about 1/4 of the original.
Signed-off-by: Waiman Long <longman@...hat.com>
---
include/linux/cgroup-defs.h | 7 +++++
kernel/cgroup/rstat.c | 57 +++++++++++++++++++++++++++----------
2 files changed, 49 insertions(+), 15 deletions(-)
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 265da00a1a8b..22adb94ebb74 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -368,6 +368,13 @@ struct cgroup_rstat_cpu {
*/
struct cgroup *updated_children; /* terminated by self cgroup */
struct cgroup *updated_next; /* NULL iff not on the list */
+
+ /*
+ * A singly-linked list of cgroup_rstat_cpu structures to be flushed.
+ * Protected by cgroup_rstat_lock.
+ */
+ struct cgroup_rstat_cpu *flush_next;
+ struct cgroup *cgroup; /* Cgroup back pointer */
};
struct cgroup_freezer_state {
diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index d80d7a608141..93ef2795a68d 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -145,6 +145,42 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos,
return pos;
}
+/*
+ * Return a list of cgroup_rstat_cpu structures to be flushed
+ */
+static struct cgroup_rstat_cpu *cgroup_rstat_flush_list(struct cgroup *root,
+ int cpu)
+{
+ raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock, cpu);
+ struct cgroup_rstat_cpu *head = NULL, *tail, *next;
+ unsigned long flags;
+ struct cgroup *pos;
+
+ /*
+ * The _irqsave() is needed because cgroup_rstat_lock is
+ * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
+ * this lock with the _irq() suffix only disables interrupts on
+ * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
+ * interrupts on both configurations. The _irqsave() ensures
+ * that interrupts are always disabled and later restored.
+ */
+ raw_spin_lock_irqsave(cpu_lock, flags);
+ pos = cgroup_rstat_cpu_pop_updated(NULL, root, cpu);
+ if (!pos)
+ goto unlock;
+
+ head = tail = cgroup_rstat_cpu(pos, cpu);
+ while ((pos = cgroup_rstat_cpu_pop_updated(pos, root, cpu))) {
+ next = cgroup_rstat_cpu(pos, cpu);
+ tail->flush_next = next;
+ tail = next;
+ }
+ tail->flush_next = NULL;
+unlock:
+ raw_spin_unlock_irqrestore(cpu_lock, flags);
+ return head;
+}
+
/*
* A hook for bpf stat collectors to attach to and flush their stats.
* Together with providing bpf kfuncs for cgroup_rstat_updated() and
@@ -179,23 +215,14 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
lockdep_assert_held(&cgroup_rstat_lock);
for_each_possible_cpu(cpu) {
- raw_spinlock_t *cpu_lock = per_cpu_ptr(&cgroup_rstat_cpu_lock,
- cpu);
- struct cgroup *pos = NULL;
- unsigned long flags;
+ struct cgroup_rstat_cpu *rstat_cpu_next;
- /*
- * The _irqsave() is needed because cgroup_rstat_lock is
- * spinlock_t which is a sleeping lock on PREEMPT_RT. Acquiring
- * this lock with the _irq() suffix only disables interrupts on
- * a non-PREEMPT_RT kernel. The raw_spinlock_t below disables
- * interrupts on both configurations. The _irqsave() ensures
- * that interrupts are always disabled and later restored.
- */
- raw_spin_lock_irqsave(cpu_lock, flags);
- while ((pos = cgroup_rstat_cpu_pop_updated(pos, cgrp, cpu))) {
+ rstat_cpu_next = cgroup_rstat_flush_list(cgrp, cpu);
+ while (rstat_cpu_next) {
+ struct cgroup *pos = rstat_cpu_next->cgroup;
struct cgroup_subsys_state *css;
+ rstat_cpu_next = rstat_cpu_next->flush_next;
cgroup_base_stat_flush(pos, cpu);
bpf_rstat_flush(pos, cgroup_parent(pos), cpu);
@@ -205,7 +232,6 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
css->ss->css_rstat_flush(css, cpu);
rcu_read_unlock();
}
- raw_spin_unlock_irqrestore(cpu_lock, flags);
/* play nice and yield if necessary */
if (need_resched() || spin_needbreak(&cgroup_rstat_lock)) {
@@ -281,6 +307,7 @@ int cgroup_rstat_init(struct cgroup *cgrp)
struct cgroup_rstat_cpu *rstatc = cgroup_rstat_cpu(cgrp, cpu);
rstatc->updated_children = cgrp;
+ rstatc->cgroup = cgrp;
u64_stats_init(&rstatc->bsync);
}
--
2.39.3
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