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Subject[tip: sched/urgent] tasks, sched/core: RCUify the assignment of rq->curr
The following commit has been merged into the sched/urgent branch of tip:

Commit-ID: 5311a98fef7d0dc2e8040ae0e18f5568d6d1dd5a
Gitweb: https://git.kernel.org/tip/5311a98fef7d0dc2e8040ae0e18f5568d6d1dd5a
Author: Eric W. Biederman <ebiederm@xmission.com>
AuthorDate: Sat, 14 Sep 2019 07:35:02 -05:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 25 Sep 2019 17:42:29 +02:00

tasks, sched/core: RCUify the assignment of rq->curr

The current task on the runqueue is currently read with rcu_dereference().

To obtain ordinary RCU semantics for an rcu_dereference() of rq->curr it needs
to be paired with rcu_assign_pointer() of rq->curr. Which provides the
memory barrier necessary to order assignments to the task_struct
and the assignment to rq->curr.

Unfortunately the assignment of rq->curr in __schedule is a hot path,
and it has already been show that additional barriers in that code
will reduce the performance of the scheduler. So I will attempt to
describe below why you can effectively have ordinary RCU semantics
without any additional barriers.

The assignment of rq->curr in init_idle is a slow path called once
per cpu and that can use rcu_assign_pointer() without any concerns.

As I write this there are effectively two users of rcu_dereference() on
rq->curr. There is the membarrier code in kernel/sched/membarrier.c
that only looks at "->mm" after the rcu_dereference(). Then there is
task_numa_compare() in kernel/sched/fair.c. My best reading of the
code shows that task_numa_compare only access: "->flags",
"->cpus_ptr", "->numa_group", "->numa_faults[]",
"->total_numa_faults", and "->se.cfs_rq".

The code in __schedule() essentially does:
rq_lock(...);
smp_mb__after_spinlock();

next = pick_next_task(...);
rq->curr = next;

context_switch(prev, next);

At the start of the function the rq_lock/smp_mb__after_spinlock
pair provides a full memory barrier. Further there is a full memory barrier
in context_switch().

This means that any task that has already run and modified itself (the
common case) has already seen two memory barriers before __schedule()
runs and begins executing. A task that modifies itself then sees a
third full memory barrier pair with the rq_lock();

For a brand new task that is enqueued with wake_up_new_task() there
are the memory barriers present from the taking and release the
pi_lock and the rq_lock as the processes is enqueued as well as the
full memory barrier at the start of __schedule() assuming __schedule()
happens on the same cpu.

This means that by the time we reach the assignment of rq->curr
except for values on the task struct modified in pick_next_task
the code has the same guarantees as if it used rcu_assign_pointer().

Reading through all of the implementations of pick_next_task it
appears pick_next_task is limited to modifying the task_struct fields
"->se", "->rt", "->dl". These fields are the sched_entity structures
of the varies schedulers.

Further "->se.cfs_rq" is only changed in cgroup attach/move operations
initialized by userspace.

Unless I have missed something this means that in practice that the
users of "rcu_dereference(rq->curr)" get normal RCU semantics of
rcu_dereference() for the fields the care about, despite the
assignment of rq->curr in __schedule() ot using rcu_assign_pointer.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20190903200603.GW2349@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/sched/core.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5e5fefb..84c7116 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4033,7 +4033,11 @@ static void __sched notrace __schedule(bool preempt)

if (likely(prev != next)) {
rq->nr_switches++;
- rq->curr = next;
+ /*
+ * RCU users of rcu_dereference(rq->curr) may not see
+ * changes to task_struct made by pick_next_task().
+ */
+ RCU_INIT_POINTER(rq->curr, next);
/*
* The membarrier system call requires each architecture
* to have a full memory barrier after updating
@@ -6060,7 +6064,8 @@ void init_idle(struct task_struct *idle, int cpu)
__set_task_cpu(idle, cpu);
rcu_read_unlock();

- rq->curr = rq->idle = idle;
+ rq->idle = idle;
+ rcu_assign_pointer(rq->curr, idle);
idle->on_rq = TASK_ON_RQ_QUEUED;
#ifdef CONFIG_SMP
idle->on_cpu = 1;
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