Re: Normal RCU grace period can be stalled for long because need-resched flags not set?

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jul 03, 2019 at 11:30:36AM -0400, Steven Rostedt wrote:
> On Wed, 3 Jul 2019 11:25:20 -0400
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> 
> > I am sorry if this is not a realistic real-life problem, but more a
> > "doctor it hurts if I do this" problem as Steven once said ;-)
> > 
> > I'll keep poking ;-)
> 
> Hi Joel,
> 
> Can you also share the tests you are performing as well as any
> module/code changes you made so that we can duplicate the results?

Sure thing. Below is the diff that I applied to Paul's /dev branch. But I
believe Linus's tree should have same results.

After applying the diff below, I run it like this:
tools/testing/selftests/rcutorture/bin/kvm.sh --bootargs rcuperf.pd_test=1 rcuperf.pd_busy_wait=5000 rcuperf.holdout=5 rcuperf.pd_resched=0 --duration 1 --torture rcuperf

Some new options I added:
pd_test=1 runs the preempt disable loop test
pd_busy_wait is the busy wait time each pass through the loop in microseconds
pd_resched is whether the loop should set the need-resched flag periodically.

If your qemu is a bit old or from debian, then you may also need to pass: --qemu-args "-net nic,model=e1000"

With pd_resched = 0, I get quite high average grace-period latencies. The
preempt-disable loop thread is running on its own CPU. Enabling the rcu:*
tracepoints, I see that for long periods of time, the FQS rcu loop can be
running while the scheduler tick learns from rcu_preempt_deferred_qs() that
there's nothing to worry about (at least this is what I remember tracing).

With pd_resched = 0, the output of the command above:
Average grace-period duration: 195629 microseconds
Minimum grace-period duration: 30111.7
50th percentile grace-period duration: 211000
90th percentile grace-period duration: 218000
99th percentile grace-period duration: 222999
Maximum grace-period duration: 236351

With pd_resched = 1, you get more like twice (10ms) the busy-wait time (5ms).
I wonder why its twice, but that's still Ok. It is as follows:
Average grace-period duration: 12302.2 microseconds
Minimum grace-period duration: 5998.35
50th percentile grace-period duration: 12000.4
90th percentile grace-period duration: 15996.4
99th percentile grace-period duration: 18000.6
Maximum grace-period duration: 20998.6

The config for the test is in the kernel sources itself:
tools/testing/selftests/rcutorture/configs/rcuperf/TREE

(Sorry for any commented out dead code)
(Also I hardcoded the default number of readers/writers as 10 but you can
 pass them as you like to the rcuperf options)
-------8<---------

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 8fd4f82c9b3d..5d30dbc7000b 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -429,12 +429,14 @@ static inline void srcu_init(void) { }
 static inline bool rcu_gp_is_normal(void) { return true; }
 static inline bool rcu_gp_is_expedited(void) { return false; }
 static inline void rcu_expedite_gp(void) { }
+static inline bool rcu_expedite_gp_called(void) { }
 static inline void rcu_unexpedite_gp(void) { }
 static inline void rcu_request_urgent_qs_task(struct task_struct *t) { }
 #else /* #ifdef CONFIG_TINY_RCU */
 bool rcu_gp_is_normal(void);     /* Internal RCU use. */
 bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
 void rcu_expedite_gp(void);
+bool rcu_expedite_gp_called(void);
 void rcu_unexpedite_gp(void);
 void rcupdate_announce_bootup_oddness(void);
 void rcu_request_urgent_qs_task(struct task_struct *t);
diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
index 4513807cd4c4..67208b844128 100644
--- a/kernel/rcu/rcuperf.c
+++ b/kernel/rcu/rcuperf.c
@@ -34,6 +34,7 @@
 #include <linux/stat.h>
 #include <linux/srcu.h>
 #include <linux/slab.h>
+#include <linux/trace_clock.h>
 #include <asm/byteorder.h>
 #include <linux/torture.h>
 #include <linux/vmalloc.h>
@@ -79,13 +80,16 @@ MODULE_AUTHOR("Paul E. McKenney <paulmck@linux.ibm.com>");
 torture_param(bool, gp_async, false, "Use asynchronous GP wait primitives");
 torture_param(int, gp_async_max, 1000, "Max # outstanding waits per reader");
 torture_param(bool, gp_exp, false, "Use expedited GP wait primitives");
-torture_param(int, holdoff, 10, "Holdoff time before test start (s)");
-torture_param(int, nreaders, -1, "Number of RCU reader threads");
-torture_param(int, nwriters, -1, "Number of RCU updater threads");
+torture_param(int, holdoff, 5, "Holdoff time before test start (s)");
+torture_param(int, nreaders, 10, "Number of RCU reader threads");
+torture_param(int, nwriters, 10, "Number of RCU updater threads");
 torture_param(bool, shutdown, RCUPERF_SHUTDOWN,
 	      "Shutdown at end of performance tests.");
 torture_param(int, verbose, 1, "Enable verbose debugging printk()s");
 torture_param(int, writer_holdoff, 0, "Holdoff (us) between GPs, zero to disable");
+torture_param(int, pd_test, 0, "Do the preempt disable loop test");
+torture_param(int, pd_busy_wait, 5000, "Preempt-disable per-loop wait in usecs");
+torture_param(int, pd_resched, 1, "Preempt-disable per-loop wait in usecs");
 
 static char *perf_type = "rcu";
 module_param(perf_type, charp, 0444);
@@ -96,6 +100,7 @@ static int nrealwriters;
 static struct task_struct **writer_tasks;
 static struct task_struct **reader_tasks;
 static struct task_struct *shutdown_task;
+static struct task_struct *pd_task;
 
 static u64 **writer_durations;
 static int *writer_n_durations;
@@ -375,6 +380,11 @@ rcu_perf_writer(void *arg)
 	if (holdoff)
 		schedule_timeout_uninterruptible(holdoff * HZ);
 
+	// Wait for rcu_unexpedite_gp() to be called from init to avoid
+	// doing expedited GPs if we are not supposed to
+	while (!gp_exp && rcu_expedite_gp_called())
+		schedule_timeout_uninterruptible(1);
+
 	t = ktime_get_mono_fast_ns();
 	if (atomic_inc_return(&n_rcu_perf_writer_started) >= nrealwriters) {
 		t_rcu_perf_writer_started = t;
@@ -413,6 +423,9 @@ rcu_perf_writer(void *arg)
 		} else {
 			rcu_perf_writer_state = RTWS_SYNC;
 			cur_ops->sync();
+			if (i % 20 == 0) {
+				//pr_err("writer %ld loop=%d gpseq=%lu\n", me, i, cur_ops->get_gp_seq());
+			}
 		}
 		rcu_perf_writer_state = RTWS_IDLE;
 		t = ktime_get_mono_fast_ns();
@@ -452,6 +465,7 @@ rcu_perf_writer(void *arg)
 			alldone = true;
 		if (started && !alldone && i < MAX_MEAS - 1)
 			i++;
+		//pr_err("writer %d i now %d\n", me, i);
 		rcu_perf_wait_shutdown();
 	} while (!torture_must_stop());
 	if (gp_async) {
@@ -464,6 +478,67 @@ rcu_perf_writer(void *arg)
 	return 0;
 }
 
+static void busy_wait(int time_us)
+{
+	u64 start, end;
+	start = trace_clock_local();
+	do {
+		end = trace_clock_local();
+		if (kthread_should_stop())
+			break;
+	} while ((end - start) < (time_us * 1000));
+}
+
+static int
+rcu_perf_preempt_disable(void *arg)
+{
+	struct sched_param sp;
+
+	VERBOSE_PERFOUT_STRING("rcu_perf_preempt_disable task thread enter");
+
+	// Create pd thread on last CPU
+	set_cpus_allowed_ptr(current, cpumask_of(nr_cpu_ids - 1));
+	sp.sched_priority = 1;
+	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
+
+	// Wait for holdoff
+	if (holdoff)
+		schedule_timeout_uninterruptible(holdoff * HZ);
+
+	// Wait for rcu_unexpedite_gp() to be called from init to avoid
+	// doing expedited GPs if we are not supposed to
+	while (!gp_exp && rcu_expedite_gp_called())
+		schedule_timeout_uninterruptible(1);
+
+	pr_err("PD test started on cpu %d\n", smp_processor_id());
+
+	do {
+		preempt_disable();
+		busy_wait(pd_busy_wait);
+
+		/* Prevent stalls and unnecessary extension of grace period */
+		if (pd_resched) {
+			set_tsk_need_resched(current);
+			set_preempt_need_resched();
+		}
+
+		preempt_enable();
+
+		/* Just in case preempt_enable didn't resched ;-) */
+		if (pd_resched)
+			cond_resched();
+#if 0
+		if (i++ % 1000 == 0){
+			pr_err("pd: looped once in 1000, i = %d\n", i);
+			trace_printk("pd: looped once in 1000, i = %d\n", i);
+		}
+#endif
+	} while (!torture_must_stop());
+
+	torture_kthread_stopping("rcu_perf_preempt_disable");
+	return 0;
+}
+
 static void
 rcu_perf_print_module_parms(struct rcu_perf_ops *cur_ops, const char *tag)
 {
@@ -548,6 +623,11 @@ rcu_perf_cleanup(void)
 		kfree(writer_n_durations);
 	}
 
+	if (pd_task) {
+		torture_stop_kthread(rcu_perf_preempt_disable, pd_task);
+		kfree(pd_task);
+	}
+
 	/* Do torture-type-specific cleanup operations.  */
 	if (cur_ops->cleanup != NULL)
 		cur_ops->cleanup();
@@ -571,7 +651,9 @@ static int compute_real(int n)
 		if (nr <= 0)
 			nr = 1;
 	}
-	return nr;
+
+	// Reserve 2 cpus for testing
+	return nr - 2;
 }
 
 /*
@@ -681,6 +763,20 @@ rcu_perf_init(void)
 		if (firsterr)
 			goto unwind;
 	}
+
+	if (pd_test) {
+		pd_task = kzalloc(sizeof(*pd_task), GFP_KERNEL);
+		if (!pd_task) {
+			firsterr = -ENOMEM;
+			goto unwind;
+		}
+
+		firsterr = torture_create_kthread(rcu_perf_preempt_disable,
+				NULL, pd_task);
+		if (firsterr)
+			goto unwind;
+	}
+
 	torture_init_end();
 	return 0;
 
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 249517058b13..840f62805d62 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -154,6 +154,15 @@ void rcu_expedite_gp(void)
 }
 EXPORT_SYMBOL_GPL(rcu_expedite_gp);
 
+/**
+ * rcu_expedite_gp_called - Was there a prior call to rcu_expedite_gp()?
+ */
+bool rcu_expedite_gp_called(void)
+{
+	return (atomic_read(&rcu_expedited_nesting) != 0);
+}
+EXPORT_SYMBOL_GPL(rcu_expedite_gp_called);
+
 /**
  * rcu_unexpedite_gp - Cancel prior rcu_expedite_gp() invocation
  *