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SubjectRe: [PATCH tip/core/rcu 44/55] rcu: wire up RCU_BOOST_PRIO for rcutree
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Hi Paul,

This patch causes RCU thread priority funnies, with some help from rcun.

On Tue, 2011-09-06 at 11:00 -0700, Paul E. McKenney wrote:
> return 0;
> @@ -1466,6 +1474,7 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg)
> {
> struct sched_param sp;
> struct timer_list yield_timer;
> + int prio = current->normal_prio;
>
> setup_timer_on_stack(&yield_timer, f, arg);
> mod_timer(&yield_timer, jiffies + 2);

There's a thinko there, prio either needs to be inverted before feeding
it to __setscheduler().. or just use ->rt_priority. I did the latter,
and twiddled rcun to restore it's priority instead of RCU_KTHREAD_PRIO.
RCU threads now stay put.

rcu: wire up RCU_BOOST_PRIO for rcutree

RCU boost threads start life at RCU_BOOST_PRIO, while others remain at
RCU_KTHREAD_PRIO. Adjust rcu_yield() to preserve priority across the
yield, and if the node thread restores RT policy for a yielding thread,
it sets priority to it's own priority. This sets the stage for user
controlled runtime changes to priority in the -rt tree.

While here, change thread names to match other kthreads.

Signed-off-by: Mike Galbraith <efault@gmx.de>

---
kernel/rcutree.c | 2 --
kernel/rcutree_plugin.h | 22 ++++++++++++++++------
2 files changed, 16 insertions(+), 8 deletions(-)

Index: linux-3.0-tip/kernel/rcutree.c
===================================================================
--- linux-3.0-tip.orig/kernel/rcutree.c
+++ linux-3.0-tip/kernel/rcutree.c
@@ -128,8 +128,6 @@ static void rcu_node_kthread_setaffinity
static void invoke_rcu_core(void);
static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);

-#define RCU_KTHREAD_PRIO 1 /* RT priority for per-CPU kthreads. */
-
/*
* Track the rcutorture test sequence number and the update version
* number within a given test. The rcutorture_testseq is incremented
Index: linux-3.0-tip/kernel/rcutree_plugin.h
===================================================================
--- linux-3.0-tip.orig/kernel/rcutree_plugin.h
+++ linux-3.0-tip/kernel/rcutree_plugin.h
@@ -27,6 +27,14 @@
#include <linux/delay.h>
#include <linux/stop_machine.h>

+#define RCU_KTHREAD_PRIO 1
+
+#ifdef CONFIG_RCU_BOOST
+#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
+#else
+#define RCU_BOOST_PRIO RCU_KTHREAD_PRIO
+#endif
+
/*
* Check the RCU kernel configuration parameters and print informative
* messages about anything out of the ordinary. If you like #ifdef, you
@@ -1345,13 +1353,13 @@ static int __cpuinit rcu_spawn_one_boost
if (rnp->boost_kthread_task != NULL)
return 0;
t = kthread_create(rcu_boost_kthread, (void *)rnp,
- "rcub%d", rnp_index);
+ "rcub/%d", rnp_index);
if (IS_ERR(t))
return PTR_ERR(t);
raw_spin_lock_irqsave(&rnp->lock, flags);
rnp->boost_kthread_task = t;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
- sp.sched_priority = RCU_KTHREAD_PRIO;
+ sp.sched_priority = RCU_BOOST_PRIO;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
return 0;
@@ -1446,6 +1454,7 @@ static void rcu_yield(void (*f)(unsigned
{
struct sched_param sp;
struct timer_list yield_timer;
+ int prio = current->rt_priority;

setup_timer_on_stack(&yield_timer, f, arg);
mod_timer(&yield_timer, jiffies + 2);
@@ -1453,7 +1462,8 @@ static void rcu_yield(void (*f)(unsigned
sched_setscheduler_nocheck(current, SCHED_NORMAL, &sp);
set_user_nice(current, 19);
schedule();
- sp.sched_priority = RCU_KTHREAD_PRIO;
+ set_user_nice(current, 0);
+ sp.sched_priority = prio;
sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
del_timer(&yield_timer);
}
@@ -1562,7 +1572,7 @@ static int __cpuinit rcu_spawn_one_cpu_k
if (!rcu_scheduler_fully_active ||
per_cpu(rcu_cpu_kthread_task, cpu) != NULL)
return 0;
- t = kthread_create(rcu_cpu_kthread, (void *)(long)cpu, "rcuc%d", cpu);
+ t = kthread_create(rcu_cpu_kthread, (void *)(long)cpu, "rcuc/%d", cpu);
if (IS_ERR(t))
return PTR_ERR(t);
if (cpu_online(cpu))
@@ -1608,7 +1618,7 @@ static int rcu_node_kthread(void *arg)
continue;
}
per_cpu(rcu_cpu_has_work, cpu) = 1;
- sp.sched_priority = RCU_KTHREAD_PRIO;
+ sp.sched_priority = current->rt_priority;
sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
preempt_enable();
}
@@ -1671,7 +1681,7 @@ static int __cpuinit rcu_spawn_one_node_
return 0;
if (rnp->node_kthread_task == NULL) {
t = kthread_create(rcu_node_kthread, (void *)rnp,
- "rcun%d", rnp_index);
+ "rcun/%d", rnp_index);
if (IS_ERR(t))
return PTR_ERR(t);
raw_spin_lock_irqsave(&rnp->lock, flags);



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