Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

From: Paul E. McKenney
Date: Tue Jun 23 2015 - 22:24:24 EST

On Tue, Jun 23, 2015 at 12:05:06PM -0700, Paul E. McKenney wrote:
> On Tue, Jun 23, 2015 at 11:26:26AM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 23, 2015 at 08:04:11PM +0200, Peter Zijlstra wrote:
> > > On Tue, Jun 23, 2015 at 10:30:38AM -0700, Paul E. McKenney wrote:
> > > > Good, you don't need this because you can check for dynticks later.
> > > > You will need to check for offline CPUs.
> > >
> > > get_online_cpus()
> > > for_each_online_cpus() {
> > > ...
> > > }
> > >
> > > is what the new code does.
> >
> > Ah, I missed that this was not deleted.
>
> But get_online_cpus() will re-introduce a deadlock.

And here is an untested patch that applies the gist of your approach,
the series of stop_one_cpu() calls, but without undoing the rest.
I forged your Signed-off-by, please let me know if that doesn't work
for you. There are a number of simplifications that can be made, but
the basic approach gets a good testing first.

And I just noticed that I forgot to get rid of try_stop_cpus().
Well, there will probably be a test failure or two to handle, so
I can add that in the next version. ;-)

Thanx, Paul

------------------------------------------------------------------------

commit 1de96c34b39d840c5fe2689640345ed26f78b8f8
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue Jun 23 19:03:45 2015 -0700

rcu: Switch synchronize_sched_expedited() to stop_one_cpu()

The synchronize_sched_expedited() currently invokes try_stop_cpus(),
which schedules the stopper kthreads on each online non-idle CPU,
and waits until all those kthreads are running before letting any
of them stop. This is disastrous for real-time workloads, which
get hit with a preemption that is as long as the longest scheduling
latency on any CPU, including any non-realtime housekeeping CPUs.
This commit therefore switches to using stop_one_cpu() on each CPU
in turn. This avoids inflicting the worst-case scheduling latency
on the worst-case CPU onto all other CPUs, and also simplifies the
code a little bit.

Follow-up commits will simplify the counter-snapshotting algorithm
and convert a number of the counters that are now protected by the
new ->expedited_mutex to non-atomic.

Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
[ paulmck: Kept stop_one_cpu(), dropped disabling of "guardrails". ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 78d0a87ff354..a30971474134 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -103,6 +103,7 @@ struct rcu_state sname##_state = { \
.orphan_nxttail = &sname##_state.orphan_nxtlist, \
.orphan_donetail = &sname##_state.orphan_donelist, \
.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
+ .expedited_mutex = __MUTEX_INITIALIZER(sname##_state.expedited_mutex), \
.name = RCU_STATE_NAME(sname), \
.abbr = sabbr, \
}
@@ -3357,8 +3358,6 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
*/
void synchronize_sched_expedited(void)
{
- cpumask_var_t cm;
- bool cma = false;
int cpu;
long firstsnap, s, snap;
int trycount = 0;
@@ -3394,28 +3393,11 @@ void synchronize_sched_expedited(void)
}
WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));

- /* Offline CPUs, idle CPUs, and any CPU we run on are quiescent. */
- cma = zalloc_cpumask_var(&cm, GFP_KERNEL);
- if (cma) {
- cpumask_copy(cm, cpu_online_mask);
- cpumask_clear_cpu(raw_smp_processor_id(), cm);
- for_each_cpu(cpu, cm) {
- struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
-
- if (!(atomic_add_return(0, &rdtp->dynticks) & 0x1))
- cpumask_clear_cpu(cpu, cm);
- }
- if (cpumask_weight(cm) == 0)
- goto all_cpus_idle;
- }
-
/*
* Each pass through the following loop attempts to force a
* context switch on each CPU.
*/
- while (try_stop_cpus(cma ? cm : cpu_online_mask,
- synchronize_sched_expedited_cpu_stop,
- NULL) == -EAGAIN) {
+ while (!mutex_trylock(&rsp->expedited_mutex)) {
put_online_cpus();
atomic_long_inc(&rsp->expedited_tryfail);

@@ -3425,7 +3407,6 @@ void synchronize_sched_expedited(void)
/* ensure test happens before caller kfree */
smp_mb__before_atomic(); /* ^^^ */
atomic_long_inc(&rsp->expedited_workdone1);
- free_cpumask_var(cm);
return;
}

@@ -3435,7 +3416,6 @@ void synchronize_sched_expedited(void)
} else {
wait_rcu_gp(call_rcu_sched);
atomic_long_inc(&rsp->expedited_normal);
- free_cpumask_var(cm);
return;
}

@@ -3445,7 +3425,6 @@ void synchronize_sched_expedited(void)
/* ensure test happens before caller kfree */
smp_mb__before_atomic(); /* ^^^ */
atomic_long_inc(&rsp->expedited_workdone2);
- free_cpumask_var(cm);
return;
}

@@ -3460,16 +3439,23 @@ void synchronize_sched_expedited(void)
/* CPU hotplug operation in flight, use normal GP. */
wait_rcu_gp(call_rcu_sched);
atomic_long_inc(&rsp->expedited_normal);
- free_cpumask_var(cm);
return;
}
snap = atomic_long_read(&rsp->expedited_start);
smp_mb(); /* ensure read is before try_stop_cpus(). */
}
- atomic_long_inc(&rsp->expedited_stoppedcpus);

-all_cpus_idle:
- free_cpumask_var(cm);
+ /* Stop each CPU that is online, non-idle, and not us. */
+ for_each_online_cpu(cpu) {
+ struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+
+ /* Skip our CPU and any idle CPUs. */
+ if (raw_smp_processor_id() == cpu ||
+ !(atomic_add_return(0, &rdtp->dynticks) & 0x1))
+ continue;
+ stop_one_cpu(cpu, synchronize_sched_expedited_cpu_stop, NULL);
+ }
+ atomic_long_inc(&rsp->expedited_stoppedcpus);

/*
* Everyone up to our most recent fetch is covered by our grace
@@ -3488,6 +3474,7 @@ all_cpus_idle:
}
} while (atomic_long_cmpxchg(&rsp->expedited_done, s, snap) != s);
atomic_long_inc(&rsp->expedited_done_exit);
+ mutex_unlock(&rsp->expedited_mutex);

put_online_cpus();
}
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index de22d6d06bf9..b04ffa0dea58 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -478,6 +478,7 @@ struct rcu_state {
/* _rcu_barrier(). */
/* End of fields guarded by barrier_mutex. */

+ struct mutex expedited_mutex; /* Serializes expediting. */
atomic_long_t expedited_start; /* Starting ticket. */
atomic_long_t expedited_done; /* Done ticket. */
atomic_long_t expedited_wrap; /* # near-wrap incidents. */

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