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sched: Move mmdrop to RCU on RT

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mmdrop() is invoked from finish_task_switch() by the incoming task to drop
the mm which was handed over by the previous task. mmdrop() can be quite
expensive which prevents an incoming real-time task from getting useful
work done.

Provide mmdrop_sched() which maps to mmdrop() on !RT kernels. On RT kernels
it delagates the eventually required invocation of __mmdrop() to RCU.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210928122411.648582026@linutronix.de
This commit is contained in:
Thomas Gleixner 2021-09-28 14:24:32 +02:00 committed by Peter Zijlstra
parent d07b2eee45
commit 8d491de6ed
3 changed files with 34 additions and 1 deletions
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4
include/linux/mm_types.h
View file

@ -12,6 +12,7 @@
#include <linux/completion.h>
#include <linux/cpumask.h>
#include <linux/uprobes.h>
#include <linux/rcupdate.h>
#include <linux/page-flags-layout.h>
#include <linux/workqueue.h>
#include <linux/seqlock.h>
@ -572,6 +573,9 @@ struct mm_struct {
bool tlb_flush_batched;
#endif
struct uprobes_state uprobes_state;
#ifdef CONFIG_PREEMPT_RT
struct rcu_head delayed_drop;
#endif
#ifdef CONFIG_HUGETLB_PAGE
atomic_long_t hugetlb_usage;
#endif

29
include/linux/sched/mm.h
View file

@ -49,6 +49,35 @@ static inline void mmdrop(struct mm_struct *mm)
__mmdrop(mm);
}
#ifdef CONFIG_PREEMPT_RT
/*
* RCU callback for delayed mm drop. Not strictly RCU, but call_rcu() is
* by far the least expensive way to do that.
*/
static inline void __mmdrop_delayed(struct rcu_head *rhp)
{
struct mm_struct *mm = container_of(rhp, struct mm_struct, delayed_drop);
__mmdrop(mm);
}
/*
* Invoked from finish_task_switch(). Delegates the heavy lifting on RT
* kernels via RCU.
*/
static inline void mmdrop_sched(struct mm_struct *mm)
{
/* Provides a full memory barrier. See mmdrop() */
if (atomic_dec_and_test(&mm->mm_count))
call_rcu(&mm->delayed_drop, __mmdrop_delayed);
}
#else
static inline void mmdrop_sched(struct mm_struct *mm)
{
mmdrop(mm);
}
#endif
/**
* mmget() - Pin the address space associated with a &struct mm_struct.
* @mm: The address space to pin.

2
kernel/sched/core.c
View file

@ -4836,7 +4836,7 @@ static struct rq *finish_task_switch(struct task_struct *prev)
*/
if (mm) {
membarrier_mm_sync_core_before_usermode(mm);
mmdrop(mm);
mmdrop_sched(mm);
}
if (unlikely(prev_state == TASK_DEAD)) {
if (prev->sched_class->task_dead)