On Wed, Jul 03, 2019 at 11:24:54PM -0400, Joel Fernandes wrote:
> 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:
[ . . . ]
> > > 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.
My eyes seem to be working better today.
Here is rcu_perf_wait_shutdown():
static void rcu_perf_wait_shutdown(void)
{
cond_resched_tasks_rcu_qs();
if (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters)
return;
while (!torture_must_stop())
schedule_timeout_uninterruptible(1);
}
Take a close look at the "while" loop. It is effectively ending your
test prematurely and thus rendering the code no longer CPU-bound. ;-)
Thanx, Paul
> 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.
>
