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Commit cee43939 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Rename cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs() 

Commit e31d28b6 ("trace: Eliminate cond_resched_rcu_qs() in favor
of cond_resched()") substituted cond_resched() for the earlier call
to cond_resched_rcu_qs().  However, the new-age cond_resched() does
not do anything to help RCU-tasks grace periods because (1) RCU-tasks
is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
complete no-op when preemption is enabled.  This situation results
in hangs when running the trace benchmarks.

A number of potential fixes were discussed on LKML
(https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home

),
including making cond_resched() not be a no-op; making cond_resched()
not be a no-op, but only when running tracing benchmarks; reverting
the aforementioned commit (which works because cond_resched_rcu_qs()
does provide an RCU-tasks quiescent state; and adding a call to the
scheduler/RCU rcu_note_voluntary_context_switch() function.  All were
deemed unsatisfactory, either due to added cond_resched() overhead or
due to magic functions inviting cargo culting.

This commit renames cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which provides a clear hint as to what this function is doing and
why and where it should be used, and then replaces the call to
cond_resched() with cond_resched_tasks_rcu_qs() in the trace benchmark's
benchmark_event_kthread() function.

Reported-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: default avatarNicholas Piggin <npiggin@gmail.com>
parent 6fba2b37

include/linux/rcupdate.h

+2 −2
Original line number Diff line number Diff line
@@ -188,13 +188,13 @@ static inline void exit_tasks_rcu_finish(void) { } 
#endif /* #else #ifdef CONFIG_TASKS_RCU */



/**

 * cond_resched_rcu_qs - Report potential quiescent states to RCU

 * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU

 *

 * This macro resembles cond_resched(), except that it is defined to

 * report potential quiescent states to RCU-tasks even if the cond_resched()

 * machinery were to be shut off, as some advocate for PREEMPT kernels.

 */

#define cond_resched_rcu_qs() \

#define cond_resched_tasks_rcu_qs() \

do { \

	if (!cond_resched()) \

		rcu_note_voluntary_context_switch_lite(current); \

kernel/rcu/rcuperf.c

+1 −1
Original line number Diff line number Diff line
@@ -369,7 +369,7 @@ static bool __maybe_unused torturing_tasks(void) 
 */

static void rcu_perf_wait_shutdown(void)

{

	cond_resched_rcu_qs();

	cond_resched_tasks_rcu_qs();

	if (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters)

		return;

	while (!torture_must_stop())

kernel/rcu/tree.c

+10 −10
Original line number Diff line number Diff line
@@ -1234,10 +1234,10 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp) 
	}



	/*

	 * Has this CPU encountered a cond_resched_rcu_qs() since the

	 * beginning of the grace period?  For this to be the case,

	 * the CPU has to have noticed the current grace period.  This

	 * might not be the case for nohz_full CPUs looping in the kernel.

	 * Has this CPU encountered a cond_resched() since the beginning

	 * of the grace period?  For this to be the case, the CPU has to

	 * have noticed the current grace period.  This might not be the

	 * case for nohz_full CPUs looping in the kernel.

	 */

	jtsq = jiffies_till_sched_qs;

	ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
@@ -2049,7 +2049,7 @@ static bool rcu_gp_init(struct rcu_state *rsp) 
					    rnp->level, rnp->grplo,

					    rnp->grphi, rnp->qsmask);

		raw_spin_unlock_irq_rcu_node(rnp);

		cond_resched_rcu_qs();

		cond_resched_tasks_rcu_qs();

		WRITE_ONCE(rsp->gp_activity, jiffies);

	}
@@ -2151,7 +2151,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp) 
		sq = rcu_nocb_gp_get(rnp);

		raw_spin_unlock_irq_rcu_node(rnp);

		rcu_nocb_gp_cleanup(sq);

		cond_resched_rcu_qs();

		cond_resched_tasks_rcu_qs();

		WRITE_ONCE(rsp->gp_activity, jiffies);

		rcu_gp_slow(rsp, gp_cleanup_delay);

	}
@@ -2202,7 +2202,7 @@ static int __noreturn rcu_gp_kthread(void *arg) 
			/* Locking provides needed memory barrier. */

			if (rcu_gp_init(rsp))

				break;

			cond_resched_rcu_qs();

			cond_resched_tasks_rcu_qs();

			WRITE_ONCE(rsp->gp_activity, jiffies);

			WARN_ON(signal_pending(current));

			trace_rcu_grace_period(rsp->name,
@@ -2247,7 +2247,7 @@ static int __noreturn rcu_gp_kthread(void *arg) 
				trace_rcu_grace_period(rsp->name,

						       READ_ONCE(rsp->gpnum),

						       TPS("fqsend"));

				cond_resched_rcu_qs();

				cond_resched_tasks_rcu_qs();

				WRITE_ONCE(rsp->gp_activity, jiffies);

				ret = 0; /* Force full wait till next FQS. */

				j = jiffies_till_next_fqs;
@@ -2260,7 +2260,7 @@ static int __noreturn rcu_gp_kthread(void *arg) 
				}

			} else {

				/* Deal with stray signal. */

				cond_resched_rcu_qs();

				cond_resched_tasks_rcu_qs();

				WRITE_ONCE(rsp->gp_activity, jiffies);

				WARN_ON(signal_pending(current));

				trace_rcu_grace_period(rsp->name,
@@ -2782,7 +2782,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp)) 
	struct rcu_node *rnp;



	rcu_for_each_leaf_node(rsp, rnp) {

		cond_resched_rcu_qs();

		cond_resched_tasks_rcu_qs();

		mask = 0;

		raw_spin_lock_irqsave_rcu_node(rnp, flags);

		if (rnp->qsmask == 0) {

kernel/rcu/tree_plugin.h

+2 −2
Original line number Diff line number Diff line
@@ -1598,7 +1598,7 @@ static int rcu_oom_notify(struct notifier_block *self, 


	for_each_online_cpu(cpu) {

		smp_call_function_single(cpu, rcu_oom_notify_cpu, NULL, 1);

		cond_resched_rcu_qs();

		cond_resched_tasks_rcu_qs();

	}



	/* Unconditionally decrement: no need to wake ourselves up. */
@@ -2227,7 +2227,7 @@ static int rcu_nocb_kthread(void *arg) 
				cl++;

			c++;

			local_bh_enable();

			cond_resched_rcu_qs();

			cond_resched_tasks_rcu_qs();

			list = next;

		}

		trace_rcu_batch_end(rdp->rsp->name, c, !!list, 0, 0, 1);

kernel/rcu/update.c

+1 −1
Original line number Diff line number Diff line
@@ -624,7 +624,7 @@ EXPORT_SYMBOL_GPL(call_rcu_tasks); 
 * grace period has elapsed, in other words after all currently

 * executing rcu-tasks read-side critical sections have elapsed.  These

 * read-side critical sections are delimited by calls to schedule(),

 * cond_resched_rcu_qs(), idle execution, userspace execution, calls

 * cond_resched_tasks_rcu_qs(), idle execution, userspace execution, calls

 * to synchronize_rcu_tasks(), and (in theory, anyway) cond_resched().

 *

 * This is a very specialized primitive, intended only for a few uses in