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@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>");
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
*
