On Mon, Jan 08, 2018 at 02:52:38PM -0800, Paul E. McKenney wrote: > On Mon, Jan 08, 2018 at 04:28:23AM -0800, Tejun Heo wrote: > > Hello, Paul. > > > > Sorry about the delay. Travel followed by cold. :( > > > > On Tue, Jan 02, 2018 at 10:01:19AM -0800, Paul E. McKenney wrote: > > > Actually, after taking a quick look, could you please supply me with > > > a way of mark a statically allocated workqueue as WQ_MEM_RECLAIM after > > > the fact? Otherwise, I end up having to check for the workqueue having > > > > Hmmm... there is no statically allocated workqueue tho. If you're > > referring to the system-wide workqueues (system*_wq), they're just > > created dynamically early during boot. > > Good point, I was confused. But yes, they are conveniently allocated > just before the call to rcu_init(), which does work out well. ;-) > > > > been allocated pretty much each time I use it, which is going to be an > > > open invitation for bugs. Plus it looks like there are ways that RCU's > > > workqueue wakeups can be executed during very early boot, which can be > > > handled, but again in a rather messy fashion. > > > > > > In contrast, given a way of mark a statically allocated workqueue > > > as WQ_MEM_RECLAIM after the fact, I simply continue initializing the > > > workqueue at early boot, and then add the WQ_MEM_RECLAIM marking some > > > arbitrarily chosen time after the scheduler has been initialized. > > > > > > The required change to workqueues looks easy, just move the body of > > > the "if (flags & WQ_MEM_RECLAIM) {" statement in __alloc_workqueue_key() > > > to a separate function, right? > > > > Ah, okay, yes, currently, workqueue init is kinda silly in that while > > it allows init of non-mem-reclaiming workqueues way before workqueue > > is actually online, it doesn't allow the same for mem-reclaiming ones. > > As you pointed out, it's just an oversight on my part as the init path > > split was done initially to accomodate early init of system > > workqueues. > > > > I'll update the code so that rescuers can be added later too; however, > > please note that while the work items may be queued, they won't be > > executed until workqueue_init() is run (the same as now) as there > > can't be worker threads anyway before that point. > > Thank you! I added the following patch to allow RCU access to the > init_rescuer() function. Does that work for you, or did you have some > other arrangement in mind? And here are the corresponding changes to RCU, which pass light rcutorture testing. Thanx, Paul ------------------------------------------------------------------------ commit d0d6626927faf3421df6a1db875ad7099f7d49cd Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Date: Mon Jan 8 14:35:52 2018 -0800 rcu: Create RCU-specific workqueues with rescuers RCU's expedited grace periods can participate in out-of-memory deadlocks due to all available system_wq kthreads being blocked and there not being memory available to create more. This commit prevents such deadlocks by allocating an RCU-specific workqueue_struct at early boot time, and providing it with a rescuer to ensure forward progress. This uses the shiny new init_rescuer() function provided by Tejun. This commit also causes SRCU to use this new RCU-specific workqueue_struct. Note that SRCU's use of workqueues never blocks them waiting for readers, so this should be safe from a forward-progress viewpoint. Reported-by: Prateek Sood <prsood@xxxxxxxxxxxxxx> Reported-by: Tejun Heo <tj@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 59c471de342a..acabc4781b08 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -493,6 +493,7 @@ void show_rcu_gp_kthreads(void); void rcu_force_quiescent_state(void); void rcu_bh_force_quiescent_state(void); void rcu_sched_force_quiescent_state(void); +extern struct workqueue_struct *rcu_gp_workqueue; #endif /* #else #ifdef CONFIG_TINY_RCU */ #ifdef CONFIG_RCU_NOCB_CPU diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 6d5880089ff6..89f0f6b3ce9a 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -465,7 +465,7 @@ static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq, */ static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay) { - srcu_queue_delayed_work_on(sdp->cpu, system_power_efficient_wq, + srcu_queue_delayed_work_on(sdp->cpu, rcu_gp_workqueue, &sdp->work, delay); } @@ -664,7 +664,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp, rcu_seq_state(sp->srcu_gp_seq) == SRCU_STATE_IDLE) { WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed)); srcu_gp_start(sp); - queue_delayed_work(system_power_efficient_wq, &sp->work, + queue_delayed_work(rcu_gp_workqueue, &sp->work, srcu_get_delay(sp)); } raw_spin_unlock_irqrestore_rcu_node(sp, flags); @@ -1198,7 +1198,7 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay) raw_spin_unlock_irq_rcu_node(sp); if (pushgp) - queue_delayed_work(system_power_efficient_wq, &sp->work, delay); + queue_delayed_work(rcu_gp_workqueue, &sp->work, delay); } /* diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index f9c0ca2ccf0c..99c12650b9db 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4272,6 +4272,15 @@ static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp) pr_cont("\n"); } +struct workqueue_struct *rcu_gp_workqueue; + +static int __init rcu_init_wq_rescuer(void) +{ + WARN_ON(init_rescuer(rcu_gp_workqueue)); + return 0; +} +core_initcall(rcu_init_wq_rescuer); + void __init rcu_init(void) { int cpu; @@ -4298,6 +4307,10 @@ void __init rcu_init(void) rcu_cpu_starting(cpu); rcutree_online_cpu(cpu); } + + /* Create workqueue for expedited GPs and for Tree SRCU. */ + rcu_gp_workqueue = alloc_workqueue("rcu_gp", 0, 0); + WARN_ON(!rcu_gp_workqueue); } #include "tree_exp.h" diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 46d61b597731..3ba3ef4d4796 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -606,7 +606,7 @@ static void _synchronize_rcu_expedited(struct rcu_state *rsp, rew.rew_rsp = rsp; rew.rew_s = s; INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp); - schedule_work(&rew.rew_work); + queue_work(rcu_gp_workqueue, &rew.rew_work); } /* Wait for expedited grace period to complete. */ -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html