On Wed, Jul 03, 2019 at 05:50:09PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 03, 2019 at 08:32:13PM -0400, Joel Fernandes wrote:
> > On Wed, Jul 03, 2019 at 08:21:30PM -0400, Joel Fernandes wrote:
> > > On Wed, Jul 03, 2019 at 04:01:03PM -0700, Paul E. McKenney wrote:
> > > > On Wed, Jul 03, 2019 at 06:24:06PM -0400, Joel Fernandes wrote:
> > > > > On Wed, Jul 03, 2019 at 02:57:14PM -0700, Paul E. McKenney wrote:
> > > > > > On Wed, Jul 03, 2019 at 05:24:26PM -0400, Joel Fernandes wrote:
> > > > > > > On Wed, Jul 03, 2019 at 10:39:35AM -0700, Paul E. McKenney wrote:
> > > > > > > > On Wed, Jul 03, 2019 at 12:41:34PM -0400, Joel Fernandes wrote:
> > > > > > > > > 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
> > > > > > > >
> > > > > > > > Both of these results are within the design range for normal
> > > > > > > > RCU grace-period durations on busy systems. See the code in
> > > > > > > > adjust_jiffies_till_sched_qs(), which is setting one of the "panic
> > > > > > > > durations" at which RCU starts taking more aggressive actions to end
> > > > > > > > the current grace period. See especially:
> > > > > > > >
> > > > > > > > if (j < HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV)
> > > > > > > > j = HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
> > > > > > > > pr_info("RCU calculated value of scheduler-enlistment delay is %ld jiffies.\n", j);
> > > > > > > > WRITE_ONCE(jiffies_to_sched_qs, j);
> > > > > > > >
> > > > > > > > This usually gets you about 100 milliseconds, and if you are starting
> > > > > > > > grace periods in quick succession from a single thread while other threads
> > > > > > > > are doing likewise, each grace-period wait gets to wait about two grace
> > > > > > > > periods worth due to the end of the previous grace period having started
> > > > > > > > a new grace period before the thread is awakened.
> > > > > > > >
> > > > > > > > Of course, if this is causing trouble for some use case, it would not
> > > > > > > > be hard to create a tunable to override this panic duration. But that
> > > > > > > > would of course require a real use case in real use, given that RCU isn't
> > > > > > > > exactly short on tunables at the moment. Significantly shortening this
> > > > > > > > panic duration caused 0day to complain about slowness last I tried it,
> > > > > > > > just so you know.
> > > > > > >
> > > > > > > Thanks a lot for the explanation.
> > > > > > > Indeed this code in the tick is doing a good job and I just had to drop
> > > > > > > jiffies_till_first_fqs to bring down the latencies. With a
> > > > > > > jiffies_till_first_fqs of 50 instead of the default of 100, the latencies
> > > > > > > drop by 4 fold.
> > > > > >
> > > > > > You lost me on this one. The normal value of jiffies_till_first_fqs
> > > > > > is but three, for systems with 255 or fewer CPUs and HZ=1000. So I
> > > > > > have to ask... What did you do to get jiffies_till_first_fqs=100?
> > > > > > The normal default automatic settings would need something like 8,000
> > > > > > CPUs to get it up to that level.
> > > > > >
> > > > > > Or did you instead mean replacing the "HZ / 10" in the code snippet
> > > > > > above with "HZ / 20" or similar?
> > > > > >
> > > > >
> > > > > I meant jiffies_to_sched_qs.
> > > >
> > > > Whew!!! ;-)
> > > >
> > > > > Without any changes, it is 100 jiffies on my
> > > > > system. Setting rcutree.jiffies_till_sched_qs sets the jiffies_to_sched_qs. I
> > > > > had set it to 50 and observed dramatic improvements.
> > > > >
> > > > > /* If jiffies_till_sched_qs was specified, respect the request. */
> > > > > if (jiffies_till_sched_qs != ULONG_MAX) {
> > > > > WRITE_ONCE(jiffies_to_sched_qs, jiffies_till_sched_qs);
> > > > > return;
> > > > > }
> > > > >
> > > > > > Or did you mean jiffies_to_sched_qs instead of jiffies_till_first_fqs?
> > > > > > -That- does default to 100, and you could set it using the
> > > > > > rcutree.jiffies_to_sched_qs kernel boot parameter. But even then, I
> > > > > > must admit that I would naively expect halving jiffies_till_first_fqs to
> > > > > > halve the latencies. But I have not looked at it closely, and there are
> > > > > > lots of moving parts in RCU's grace-period encouragement code, so maybe
> > > > > > that is the effect.
> > > > >
> > > > > It could also be my sloppy testing. Now I tried again with 50 and it cuts the
> > > > > latencies by around half as you said. However my histogram does have several
> > > > > really nasty outliers..
> > > >
> > > > OK, that is more what I would expect.
> > > >
> > > > > (rcu-kvm is my wrapper where I pass the -net qemu args I need)
> > > > >
> > > > > rcu-kvm --rcuperf --boot-args "rcuperf.pd_test=1 rcuperf.pd_busy_wait=5000
> > > > > rcuperf.holdout=5 rcuperf.pd_resched=0 rcutree.jiffies_till_sched_qs=50"
> > > > > --kvm-args "--duration 1
> > > > >
> > > > > Log says:
> > > > > 0.087440] rcu: Boot-time adjustment of scheduler-enlistment delay to 50 jiffies.
> > > > >
> > > > > Output:
> > > > > Histogram bucket size: 1000
> > > > > 57000 4
> > > > > 58000 1
> > > > > 59000 1
> > > > > 60000 2
> > > > > 103000 1
> > > > > 104000 2
> > > > > 105000 8
> > > > > 106000 44
> > > > > 107000 60
> > > > > 108000 131
> > > > > 109000 164
> > > > > 110000 143 <---------- most of the time its ~100ms.
> > > > > 111000 136
> > > > > 112000 51
> > > > > 113000 45
> > > > > 114000 11
> > > > > 115000 4
> > > > > 12464000 1
> > > > > 12466000 2 <--- But what are these :(
> > > > > 12467000 2
> > > > > 12468000 1
> > > > > 12470000 1
> > > >
> > > > Well, those are a bit over one second, which is when .b.need_qs is set
> > > > in CONFIG_PREEMPT=y systems. Are you possibly seeing vCPU preeemption?
> > > > (Not that .b.need_qs does anything about vCPU preemption just yet.)
> > >
> > > Actually, I just realized, there is an extra 0 on those outliers. So it is
> > > really 12 seconds, for example: 12,466,000 microseconds is ~12.4 seconds. So
> > > these outliers are really odd since I don't see any RCU stalls, possible theories:
> > >
> > > Looking closer into the logs, it is always the last iteration of a writer:
> > >
> > > Third last iteration..
> > > [ 62.157951] rcu-perf: 0 writer-duration: 99 109999933
> > > [ 62.258131] rcu-perf: 1 writer-duration: 99 110999466
> > > [ 62.353607] rcu-perf: 2 writer-duration: 99 112000830
> > > [ 62.433249] rcu-perf: 3 writer-duration: 99 114000321
> > > [ 62.510405] rcu-perf: 4 writer-duration: 99 109000240
> > > [ 62.603643] rcu-perf: 5 writer-duration: 99 108999164
> > > [ 62.702109] rcu-perf: 6 writer-duration: 99 111000721
> > > [ 62.799970] rcu-perf: 7 writer-duration: 99 109000536
> > >
> > > Second last iteration..
> > > [ 62.158920] rcu-perf: 0 writer-duration: 100 109998415
> > > [ 62.259061] rcu-perf: 1 writer-duration: 100 104024906
> > > [ 62.354395] rcu-perf: 2 writer-duration: 100 105019175
> > > [ 62.434072] rcu-perf: 3 writer-duration: 100 108019926
> > > [ 62.511154] rcu-perf: 4 writer-duration: 100 109998839
> > > [ 62.604572] rcu-perf: 5 writer-duration: 100 109000969
> > > [ 62.703005] rcu-perf: 6 writer-duration: 100 107015416
> > > [ 62.800894] rcu-perf: 7 writer-duration: 100 111018680
> > >
> > > And last one which is over blown over..
> > > [ 62.161123] rcu-perf: 0 writer-duration: 101 42665751175
> > > [ 62.261115] rcu-perf: 1 writer-duration: 101 42693148470
> > > [ 62.357093] rcu-perf: 2 writer-duration: 101 42692066685
> > > [ 62.436059] rcu-perf: 3 writer-duration: 101 42692831737
> > > [ 62.513063] rcu-perf: 4 writer-duration: 101 42693195036
> > > [ 62.705105] rcu-perf: 6 writer-duration: 101 42692114079
> > > [ 62.803104] rcu-perf: 7 writer-duration: 101 42693153435
> > >
> > > I am wondering if this because the torture_stop() is having a hard time
> > > stopping my preempt disable thread, which is odd because I am checking for
> > > torture_must_stop() to break out of the loop as are other threads.
> > >
> > > Anyway this is clearly more of a test issue than an RCU one ;-)
> > >
> > > > > Average grace-period duration: 215642 microseconds <-- avg ended up skewed
> > > > > Minimum grace-period duration: 57979.7
> > > > > 50th percentile grace-period duration: 110000
> > > > > 90th percentile grace-period duration: 112999 <-- but this reduced.
> > > > > 99th percentile grace-period duration: 115000
> > > >
> > > > Indeed, medians and percentiles are often more stable than are averages.
> > > > (But yes, you can cook up distributions that work the other way around.)
> >
> > If I add an rcu_perf_wait_shutdown() to the end of the loop, the outliers go away.
> >
> > Still can't explain that :)
> >
> > do {
> > ...
> > ...
> > + rcu_perf_wait_shutdown();
> > } while (!torture_must_stop());
>
> Might it be the cond_resched_tasks_rcu_qs() invoked from within
> rcu_perf_wait_shutdown()? So I have to ask... What happens if you
> use cond_resched_tasks_rcu_qs() at the end of that loop instead of
> rcu_perf_wait_shutdown()?
I don't think it is, if I call cond_resched_tasks_rcu_qs(), it still doesn't
help. Only calling rcu_perf_wait_shutdown() cures it. If you would like try
it out, below is the diff and the command is:
tools/testing/selftests/rcutorture/bin/kvm.sh --qemu-args -net nic,model=e1000
--bootargs "rcuperf.pd_test=1 rcuperf.pd_busy_wait=5000 rcuperf.holdout=5
rcuperf.pd_resched=0 rcutree.jiffies_till_sched_qs=50 rcutree.kthread_prio=10
ftrace_dump_on_oops trace_event=rcu:rcu_grace_period"
(The diff is just for debugging and not meant for upstreaming, atleast not yet)
------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..08ff48ffa065 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 +478,68 @@ 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
+ rcu_perf_wait_shutdown();
+ } 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 +624,11 @@ 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 +652,9 @@ static int compute_real(int n)
if (nr <= 0)
nr = 1;
}
- return nr;
+
+ // Reserve 2 cpus for testing
+ return nr - 2;
}
/*
@@ -681,6 +764,20 @@ 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
*
diff --git a/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh b/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh
index db0375a57f28..c8fd6bb6d8cb 100755
--- a/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh
+++ b/tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh
@@ -1,4 +1,5 @@
#!/bin/bash
+set -x
# SPDX-License-Identifier: GPL-2.0+
#
# Analyze a given results directory for rcuperf performance measurements.
@@ -78,6 +79,7 @@ END {
print "90th percentile grace-period duration: " gptimes[pct90];
print "99th percentile grace-period duration: " gptimes[pct99];
print "Maximum grace-period duration: " gptimes[newNR];
+ print "Batches " nbatches " done";
print "Grace periods: " ngps + 0 " Batches: " nbatches + 0 " Ratio: " ngps / nbatches;
print "Computed from rcuperf printk output.";
}'
diff --git a/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh b/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh
index 33c669619736..f6ec8de276b7 100755
--- a/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh
+++ b/tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh
@@ -1,4 +1,5 @@
#!/bin/bash
+set -x
# SPDX-License-Identifier: GPL-2.0+
#
# Run a kvm-based test of the specified tree on the specified configs.
