Commit 6193c76a authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Kick CPU halfway to RCU CPU stall warning 



When an RCU CPU stall warning occurs, the CPU invokes resched_cpu() on
itself.  This can help move the grace period forward in some situations,
but it would be even better to do this -before- the RCU CPU stall warning.
This commit therefore causes resched_cpu() to be called every five jiffies
once the system is halfway to an RCU CPU stall warning.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent dc1ccc48

kernel/rcu/tree.c

+25 −1
Original line number Diff line number Diff line
@@ -754,6 +754,12 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp, 
	return (rdp->dynticks_snap & 0x1) == 0;

}



/*

 * This function really isn't for public consumption, but RCU is special in

 * that context switches can allow the state machine to make progress.

 */

extern void resched_cpu(int cpu);



/*

 * Return true if the specified CPU has passed through a quiescent

 * state by virtue of being in or having passed through an dynticks
@@ -812,16 +818,34 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp, 
	 */

	rcu_kick_nohz_cpu(rdp->cpu);



	/*

	 * Alternatively, the CPU might be running in the kernel

	 * for an extended period of time without a quiescent state.

	 * Attempt to force the CPU through the scheduler to gain the

	 * needed quiescent state, but only if the grace period has gone

	 * on for an uncommonly long time.  If there are many stuck CPUs,

	 * we will beat on the first one until it gets unstuck, then move

	 * to the next.  Only do this for the primary flavor of RCU.

	 */

	if (rdp->rsp == rcu_state &&

	    ULONG_CMP_GE(ACCESS_ONCE(jiffies), rdp->rsp->jiffies_resched)) {

		rdp->rsp->jiffies_resched += 5;

		resched_cpu(rdp->cpu);

	}



	return 0;

}



static void record_gp_stall_check_time(struct rcu_state *rsp)

{

	unsigned long j = ACCESS_ONCE(jiffies);

	unsigned long j1;



	rsp->gp_start = j;

	smp_wmb(); /* Record start time before stall time. */

	rsp->jiffies_stall = j + rcu_jiffies_till_stall_check();

	j1 = rcu_jiffies_till_stall_check();

	rsp->jiffies_stall = j + j1;

	rsp->jiffies_resched = j + j1 / 2;

}



/*

kernel/rcu/tree.h

+2 −0
Original line number Diff line number Diff line
@@ -453,6 +453,8 @@ struct rcu_state { 
						/*  but in jiffies. */

	unsigned long jiffies_stall;		/* Time at which to check */

						/*  for CPU stalls. */

	unsigned long jiffies_resched;		/* Time at which to resched */

						/*  a reluctant CPU. */

	unsigned long gp_max;			/* Maximum GP duration in */

						/*  jiffies. */

	const char *name;			/* Name of structure. */

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