Commit ad7c946b authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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 (but indirectly).

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.  Note that this moves SRCU from system_power_efficient_wq
to a normal workqueue.  In the unlikely event that this results in
measurable degradation, a separate power-efficient workqueue will be
creates for SRCU.

Reported-by: default avatarPrateek Sood <prsood@codeaurora.org>
Reported-by: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
parent 65963d24

kernel/rcu/rcu.h

+1 −0
Original line number Diff line number Diff line
@@ -485,6 +485,7 @@ 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_wq;

#endif /* #else #ifdef CONFIG_TINY_RCU */



#ifdef CONFIG_RCU_NOCB_CPU

kernel/rcu/srcutree.c

+3 −5
Original line number Diff line number Diff line
@@ -492,8 +492,7 @@ 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,

				   &sdp->work, delay);

	srcu_queue_delayed_work_on(sdp->cpu, rcu_gp_wq, &sdp->work, delay);

}



/*
@@ -691,8 +690,7 @@ 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,

				   srcu_get_delay(sp));

		queue_delayed_work(rcu_gp_wq, &sp->work, srcu_get_delay(sp));

	}

	spin_unlock_irqrestore_rcu_node(sp, flags);

}
@@ -1225,7 +1223,7 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay) 
	spin_unlock_irq_rcu_node(sp);



	if (pushgp)

		queue_delayed_work(system_power_efficient_wq, &sp->work, delay);

		queue_delayed_work(rcu_gp_wq, &sp->work, delay);

}



/*

kernel/rcu/tree.c

+6 −0
Original line number Diff line number Diff line
@@ -4167,6 +4167,8 @@ static void __init rcu_dump_rcu_node_tree(struct rcu_state *rsp) 
	pr_cont("\n");

}



struct workqueue_struct *rcu_gp_wq;



void __init rcu_init(void)

{

	int cpu;
@@ -4193,6 +4195,10 @@ void __init rcu_init(void) 
		rcu_cpu_starting(cpu);

		rcutree_online_cpu(cpu);

	}



	/* Create workqueue for expedited GPs and for Tree SRCU. */

	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);

	WARN_ON(!rcu_gp_wq);

}



#include "tree_exp.h"

kernel/rcu/tree_exp.h

+1 −1
Original line number Diff line number Diff line
@@ -626,7 +626,7 @@ 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_wq, &rew.rew_work);

	}



	/* Wait for expedited grace period to complete. */

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