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Date: Mon, 22 Sep 2014 22:32:10 +0400
From: Kirill Tkhai <tkhai@...dex.ru>
To: linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Kirill Tkhai <ktkhai@...allels.com>
Subject: [PATCH v2 2/6] sched: Do not pick a task which is switching on
other cpu
From: Kirill Tkhai <ktkhai@...allels.com>
Architectures, which define __ARCH_WANT_UNLOCKED_CTXSW,
may pull a task when it's in the middle of schedule().
CPU1(task1 calls schedule) CPU2
... schedule()
... idle_balance()
... load_balance()
... ...
schedule() ...
prepare_lock_switch() ...
raw_spin_unlock(&rq1->lock) ...
... raw_spin_lock(&rq1->lock)
... detach_tasks();
... can_migrate_task(task1)
... attach_tasks(); <--- move task1 to rq2
... raw_spin_unlock(&rq1->lock)
... context_switch() <--- switch to task1's stack
... ...
(using task1's stack) (using task1's stack)
... ...
context_switch() ...
Parallel use of a single stack is not a good idea.
Signed-off-by: Kirill Tkhai <ktkhai@...allels.com>
Cc: <stable@...r.kernel.org> # Should this go to stable?
---
kernel/sched/core.c | 11 +++--------
kernel/sched/deadline.c | 7 ++++++-
kernel/sched/fair.c | 3 +++
kernel/sched/rt.c | 7 ++++++-
kernel/sched/sched.h | 16 ++++++++++++++++
5 files changed, 34 insertions(+), 10 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2a93b87..5b864e9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1700,15 +1700,10 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
#ifdef CONFIG_SMP
/*
- * If the owning (remote) cpu is still in the middle of schedule() with
- * this task as prev, wait until its done referencing the task.
+ * Note, that p is dequeued at the moment. But it still
+ * may be "prev" in the middle of schedule() on other cpu.
*/
- while (p->on_cpu)
- cpu_relax();
- /*
- * Pairs with the smp_wmb() in finish_lock_switch().
- */
- smp_rmb();
+ cpu_relax__while_on_cpu(p);
p->sched_contributes_to_load = !!task_contributes_to_load(p);
p->state = TASK_WAKING;
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index aaa5abb..ea0ba33 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1364,7 +1364,9 @@ static int push_dl_task(struct rq *rq)
next_task = task;
goto retry;
}
-
+#ifdef __ARCH_WANT_UNLOCKED_CTXSW
+ cpu_relax__while_on_cpu(next_task);
+#endif
deactivate_task(rq, next_task, 0);
set_task_cpu(next_task, later_rq->cpu);
activate_task(later_rq, next_task, 0);
@@ -1451,6 +1453,9 @@ static int pull_dl_task(struct rq *this_rq)
ret = 1;
+#ifdef __ARCH_WANT_UNLOCKED_CTXSW
+ cpu_relax__while_on_cpu(p);
+#endif
deactivate_task(src_rq, p, 0);
set_task_cpu(p, this_cpu);
activate_task(this_rq, p, 0);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 420bc98..80c5064 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5298,6 +5298,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
schedstat_inc(env->sd, lb_hot_gained[env->idle]);
schedstat_inc(p, se.statistics.nr_forced_migrations);
}
+#ifdef __ARCH_WANT_UNLOCKED_CTXSW
+ cpu_relax__while_on_cpu(p);
+#endif
return 1;
}
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 2e6a774..de356b0 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1734,7 +1734,9 @@ static int push_rt_task(struct rq *rq)
next_task = task;
goto retry;
}
-
+#ifdef __ARCH_WANT_UNLOCKED_CTXSW
+ cpu_relax__while_on_cpu(next_task);
+#endif
deactivate_task(rq, next_task, 0);
set_task_cpu(next_task, lowest_rq->cpu);
activate_task(lowest_rq, next_task, 0);
@@ -1823,6 +1825,9 @@ static int pull_rt_task(struct rq *this_rq)
ret = 1;
+#ifdef __ARCH_WANT_UNLOCKED_CTXSW
+ cpu_relax__while_on_cpu(p);
+#endif
deactivate_task(src_rq, p, 0);
set_task_cpu(p, this_cpu);
activate_task(this_rq, p, 0);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 1bc6aad..9c07d72 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1034,6 +1034,22 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
#endif /* __ARCH_WANT_UNLOCKED_CTXSW */
/*
+ * If the owning (remote) cpu is still in the middle of schedule() with
+ * this task as prev, wait until its done referencing the task.
+ */
+static inline void cpu_relax__while_on_cpu(struct task_struct *p)
+{
+#ifdef CONFIG_SMP
+ while (p->on_cpu)
+ cpu_relax();
+ /*
+ * Pairs with the smp_wmb() in finish_lock_switch().
+ */
+ smp_rmb();
+#endif
+}
+
+/*
* wake flags
*/
#define WF_SYNC 0x01 /* waker goes to sleep after wakeup */
--
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