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Date: Mon, 8 Jan 2018 16:31:27 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Tejun Heo <tj@...nel.org>
Cc: Prateek Sood <prsood@...eaurora.org>,
Peter Zijlstra <peterz@...radead.org>, avagin@...il.com,
mingo@...nel.org, linux-kernel@...r.kernel.org,
cgroups@...r.kernel.org, sramana@...eaurora.org
Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency
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@...ux.vnet.ibm.com>
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@...eaurora.org>
Reported-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
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. */
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