From 5d01bbd111d6ff9ea9d9847774f66dff39633776 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Mon, 16 Jul 2012 10:42:35 +0000 Subject: rcu: Yield simpler The rcu_yield() code is amazing. It's there to avoid starvation of the system when lots of (boosting) work is to be done. Now looking at the code it's functionality is: Make the thread SCHED_OTHER and very nice, i.e. get it out of the way Arm a timer with 2 ticks schedule() Now if the system goes idle the rcu task returns, regains SCHED_FIFO and plugs on. If the systems stays busy the timer fires and wakes a per node kthread which in turn makes the per cpu thread SCHED_FIFO and brings it back on the cpu. For the boosting thread the "make it FIFO" bit is missing and it just runs some magic boost checks. Now this is a lot of code with extra threads and complexity. It's way simpler to let the tasks when they detect overload schedule away for 2 ticks and defer the normal wakeup as long as they are in yielded state and the cpu is not idle. That solves the same problem and the only difference is that when the cpu goes idle it's not guaranteed that the thread returns right away, but it won't be longer out than two ticks, so no harm is done. If that's an issue than it is way simpler just to wake the task from idle as RCU has callbacks there anyway. Signed-off-by: Thomas Gleixner Cc: Peter Zijlstra Cc: Srivatsa S. Bhat Cc: Rusty Russell Cc: Namhyung Kim Reviewed-by: Paul E. McKenney Link: http://lkml.kernel.org/r/20120716103948.131256723@linutronix.de Signed-off-by: Thomas Gleixner --- kernel/rcutree.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) (limited to 'kernel/rcutree.c') diff --git a/kernel/rcutree.c b/kernel/rcutree.c index f280e54..f08ee3b 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -139,7 +139,7 @@ DEFINE_PER_CPU(char, rcu_cpu_has_work); #endif /* #ifdef CONFIG_RCU_BOOST */ -static void rcu_node_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu); +static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu); static void invoke_rcu_core(void); static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp); @@ -1469,7 +1469,7 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp) /* Adjust any no-longer-needed kthreads. */ rcu_stop_cpu_kthread(cpu); - rcu_node_kthread_setaffinity(rnp, -1); + rcu_boost_kthread_setaffinity(rnp, -1); /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */ @@ -2594,11 +2594,11 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self, break; case CPU_ONLINE: case CPU_DOWN_FAILED: - rcu_node_kthread_setaffinity(rnp, -1); + rcu_boost_kthread_setaffinity(rnp, -1); rcu_cpu_kthread_setrt(cpu, 1); break; case CPU_DOWN_PREPARE: - rcu_node_kthread_setaffinity(rnp, cpu); + rcu_boost_kthread_setaffinity(rnp, cpu); rcu_cpu_kthread_setrt(cpu, 0); break; case CPU_DYING: -- cgit v1.1 From 62ab7072476ae1600e877cc62b43758e485f4f1e Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Mon, 16 Jul 2012 10:42:38 +0000 Subject: rcu: Use smp_hotplug_thread facility for RCUs per-CPU kthread Bring RCU into the new-age CPU-hotplug fold by modifying RCU's per-CPU kthread code to use the new smp_hotplug_thread facility. [ tglx: Adapted it to use callbacks and to the simplified rcu yield ] Signed-off-by: Paul E. McKenney Cc: Peter Zijlstra Cc: Srivatsa S. Bhat Cc: Rusty Russell Cc: Namhyung Kim Link: http://lkml.kernel.org/r/20120716103948.673354828@linutronix.de Signed-off-by: Thomas Gleixner --- kernel/rcutree.c | 4 ---- 1 file changed, 4 deletions(-) (limited to 'kernel/rcutree.c') diff --git a/kernel/rcutree.c b/kernel/rcutree.c index f08ee3b..11a4fdc 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -133,7 +133,6 @@ static int rcu_scheduler_fully_active __read_mostly; */ static DEFINE_PER_CPU(struct task_struct *, rcu_cpu_kthread_task); DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status); -DEFINE_PER_CPU(int, rcu_cpu_kthread_cpu); DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops); DEFINE_PER_CPU(char, rcu_cpu_has_work); @@ -1468,7 +1467,6 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp) struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */ /* Adjust any no-longer-needed kthreads. */ - rcu_stop_cpu_kthread(cpu); rcu_boost_kthread_setaffinity(rnp, -1); /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */ @@ -2595,11 +2593,9 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self, case CPU_ONLINE: case CPU_DOWN_FAILED: rcu_boost_kthread_setaffinity(rnp, -1); - rcu_cpu_kthread_setrt(cpu, 1); break; case CPU_DOWN_PREPARE: rcu_boost_kthread_setaffinity(rnp, cpu); - rcu_cpu_kthread_setrt(cpu, 0); break; case CPU_DYING: case CPU_DYING_FROZEN: -- cgit v1.1