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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: sched: Delay task stack freeing on RT
Date: Tue, 28 Sep 2021 14:24:30 +0200
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/5.15/older/patches-5.15.3-rt21.tar.xz
Anything which is done on behalf of a dead task at the end of
finish_task_switch() is preventing the incoming task from doing useful
work. While it is benefitial for fork heavy workloads to recycle the task
stack quickly, this is a latency source for real-time tasks.
Therefore delay the stack cleanup on RT enabled kernels.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210928122411.593486363@linutronix.de
---
kernel/exit.c | 5 +++++
kernel/fork.c | 5 ++++-
kernel/sched/core.c | 8 ++++++--
3 files changed, 15 insertions(+), 3 deletions(-)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -172,6 +172,11 @@ static void delayed_put_task_struct(stru
kprobe_flush_task(tsk);
perf_event_delayed_put(tsk);
trace_sched_process_free(tsk);
+
+ /* RT enabled kernels delay freeing the VMAP'ed task stack */
+ if (IS_ENABLED(CONFIG_PREEMPT_RT))
+ put_task_stack(tsk);
+
put_task_struct(tsk);
}
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -289,7 +289,10 @@ static inline void free_thread_stack(str
return;
}
- vfree_atomic(tsk->stack);
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ vfree_atomic(tsk->stack);
+ else
+ vfree(tsk->stack);
return;
}
#endif
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4845,8 +4845,12 @@ static struct rq *finish_task_switch(str
if (prev->sched_class->task_dead)
prev->sched_class->task_dead(prev);
- /* Task is done with its stack. */
- put_task_stack(prev);
+ /*
+ * Release VMAP'ed task stack immediate for reuse. On RT
+ * enabled kernels this is delayed for latency reasons.
+ */
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ put_task_stack(prev);
put_task_struct_rcu_user(prev);
}
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