linux-kernel - [PATCH 2/2] sched/debug: Use sched_debug_lock to serialize use of cgroup

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Date:   Sat, 27 Mar 2021 19:25:29 -0400
From:   Waiman Long <longman@...hat.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Bharata B Rao <bharata@...ux.vnet.ibm.com>,
        Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org,
        Waiman Long <longman@...hat.com>
Subject: [PATCH 2/2] sched/debug: Use sched_debug_lock to serialize use of cgroup_path[] only

The purpose of sched_debug_lock is to serialize the use of the global
cgroup_path[] buffer in print_cpu(). The rests of the printf calls
don't need serialization from sched_debug_lock. The printing of sched
debug data to console can take quite a while, taking sched_debug_lock
at the print_cpu() level will unnecessarily block the progress of other
print_cpu() users. Fix that by holding sched_debug_lock only when using
cgroup_path[] via task_group_path().

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/sched/debug.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index c4ae8a0853a1..87168d13714f 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -8,8 +8,6 @@
  */
 #include "sched.h"
 
-static DEFINE_SPINLOCK(sched_debug_lock);
-
 /*
  * This allows printing both to /proc/sched_debug and
  * to the console
@@ -470,6 +468,7 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group
 #endif
 
 #ifdef CONFIG_CGROUP_SCHED
+static DEFINE_SPINLOCK(sched_debug_lock);
 static char group_path[PATH_MAX];
 
 static char *task_group_path(struct task_group *tg)
@@ -506,7 +505,9 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
 	SEQ_printf(m, " %d %d", task_node(p), task_numa_group_id(p));
 #endif
 #ifdef CONFIG_CGROUP_SCHED
+	spin_lock(&sched_debug_lock);
 	SEQ_printf(m, " %s", task_group_path(task_group(p)));
+	spin_unlock(&sched_debug_lock);
 #endif
 
 	SEQ_printf(m, "\n");
@@ -543,7 +544,9 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	SEQ_printf(m, "\n");
+	spin_lock(&sched_debug_lock);
 	SEQ_printf(m, "cfs_rq[%d]:%s\n", cpu, task_group_path(cfs_rq->tg));
+	spin_unlock(&sched_debug_lock);
 #else
 	SEQ_printf(m, "\n");
 	SEQ_printf(m, "cfs_rq[%d]:\n", cpu);
@@ -614,7 +617,9 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq)
 {
 #ifdef CONFIG_RT_GROUP_SCHED
 	SEQ_printf(m, "\n");
+	spin_lock(&sched_debug_lock);
 	SEQ_printf(m, "rt_rq[%d]:%s\n", cpu, task_group_path(rt_rq->tg));
+	spin_unlock(&sched_debug_lock);
 #else
 	SEQ_printf(m, "\n");
 	SEQ_printf(m, "rt_rq[%d]:\n", cpu);
@@ -716,13 +721,11 @@ do {									\
 	}
 #undef P
 
-	spin_lock(&sched_debug_lock);
 	print_cfs_stats(m, cpu);
 	print_rt_stats(m, cpu);
 	print_dl_stats(m, cpu);
 
 	print_rq(m, rq, cpu);
-	spin_unlock(&sched_debug_lock);
 	SEQ_printf(m, "\n");
 }
 
-- 
2.18.1