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Date: Fri, 4 Nov 2022 18:09:26 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Connor O'Brien <connoro@...gle.com>, linux-kernel@...r.kernel.org,
kernel-team@...roid.com, John Stultz <jstultz@...gle.com>,
Qais Yousef <qais.yousef@....com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Will Deacon <will@...nel.org>,
Waiman Long <longman@...hat.com>,
Boqun Feng <boqun.feng@...il.com>,
"Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [RFC PATCH 07/11] sched: Add proxy execution
On 31/10/2022 19:00, Joel Fernandes wrote:
> On Mon, Oct 31, 2022 at 05:39:45PM +0100, Dietmar Eggemann wrote:
>> On 29/10/2022 05:31, Joel Fernandes wrote:
>>> Hello Dietmar,
>>>
>>>> On Oct 24, 2022, at 6:13 AM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>
>>>> On 03/10/2022 23:44, Connor O'Brien wrote:
>>>>> From: Peter Zijlstra <peterz@...radead.org>
[...]
>>>>> + rq_unpin_lock(rq, rf);
>>>>> + raw_spin_rq_unlock(rq);
>>>>
>>>> Don't we run into rq_pin_lock()'s:
>>>>
>>>> SCHED_WARN_ON(rq->balance_callback && rq->balance_callback !=
>>>> &balance_push_callback)
>>>>
>>>> by releasing rq lock between queue_balance_callback(, push_rt/dl_tasks)
>>>> and __balance_callbacks()?
>>>
>>> Apologies, I’m a bit lost here. The code you are responding to inline does not call rq_pin_lock, it calls rq_unpin_lock. So what scenario does the warning trigger according to you?
>>
>> True, but the code which sneaks in between proxy()'s
>> raw_spin_rq_unlock(rq) and raw_spin_rq_lock(rq) does.
>>
>
> Got it now, thanks a lot for clarifying. Can this be fixed by do a
> __balance_callbacks() at:
I tried the:
head = splice_balance_callbacks(rq)
task_rq_unlock(rq, p, &rf);
...
balance_callbacks(rq, head);
separation known from __sched_setscheduler() in __schedule() (right
after pick_next_task()) but it doesn't work. Lot of `BUG: scheduling
while atomic:`
[ 0.384135] BUG: scheduling while atomic: swapper/0/1/0x00000002
[ 0.384198] INFO: lockdep is turned off.
[ 0.384241] Modules linked in:
[ 0.384289] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G W 6.1.0-rc2-00023-g8a4c0a9d97ce-dirty #166
[ 0.384375] Hardware name: ARM Juno development board (r0) (DT)
[ 0.384426] Call trace:
[ 0.384454] dump_backtrace.part.0+0xe4/0xf0
[ 0.384501] show_stack+0x18/0x40
[ 0.384540] dump_stack_lvl+0x8c/0xb8
[ 0.384582] dump_stack+0x18/0x34
[ 0.384622] __schedule_bug+0x88/0xa0
[ 0.384666] __schedule+0xae0/0xba4
[ 0.384711] schedule+0x5c/0xfc
[ 0.384754] schedule_timeout+0xcc/0x10c
[ 0.384798] __wait_for_common+0xe4/0x1f4
[ 0.384847] wait_for_completion+0x20/0x2c
[ 0.384897] kthread_park+0x58/0xd0
...
>
>> __schedule()
>>
>> rq->proxy = next = pick_next_task()
>>
>> __pick_next_task()
>>
>> pick_next_task_rt()
>>
>> set_next_task_rt()
>>
>> rt_queue_push_tasks()
>>
>> queue_balance_callback(..., push_rt_tasks); <-- queue rt cb
>>
>> proxy()
>>
>
> ... here, before doing the following unlock?
>
>> raw_spin_rq_unlock(rq)
>>
>> ... <-- other thread does rq_lock_XXX(rq)
>> raw_spin_rq_lock_XXX(rq)
>> rq_pin_lock(rq)
>>
>> raw_spin_rq_lock(rq)
>>
>> context_switch()
>>
>> finish_task_switch()
>>
>> finish_lock_switch()
>>
>> __balance_callbacks(rq) <-- run rt cb here
>>
>> __balance_callbacks(rq)() <-- or run rt cb here
>
>
> Hmm also Connor, does locktorture do hotplug? Maybe it should to reproduce
> the balance issues.
I can reproduce this reliably with making the locktorture writer realtime.
--%<--
diff --git a/kernel/locking/locktorture.c b/kernel/locking/locktorture.c
index bc3557677eed..23aabb4f34e5 100644
--- a/kernel/locking/locktorture.c
+++ b/kernel/locking/locktorture.c
@@ -676,7 +676,8 @@ static int lock_torture_writer(void *arg)
DEFINE_TORTURE_RANDOM(rand);
VERBOSE_TOROUT_STRING("lock_torture_writer task started");
- set_user_nice(current, MAX_NICE);
+ if (!rt_task(current))
+ set_user_nice(current, MAX_NICE);
do {
if ((torture_random(&rand) & 0xfffff) == 0)
diff --git a/kernel/torture.c b/kernel/torture.c
index 1d0dd88369e3..55d8ac417a4a 100644
--- a/kernel/torture.c
+++ b/kernel/torture.c
@@ -57,6 +57,9 @@ module_param(verbose_sleep_duration, int, 0444);
static int random_shuffle;
module_param(random_shuffle, int, 0444);
+static int lock_torture_writer_fifo;
+module_param(lock_torture_writer_fifo, int, 0444);
+
static char *torture_type;
static int verbose;
@@ -734,7 +737,7 @@ bool stutter_wait(const char *title)
cond_resched_tasks_rcu_qs();
spt = READ_ONCE(stutter_pause_test);
for (; spt; spt = READ_ONCE(stutter_pause_test)) {
- if (!ret) {
+ if (!ret && !rt_task(current)) {
sched_set_normal(current, MAX_NICE);
ret = true;
}
@@ -944,6 +947,11 @@ int _torture_create_kthread(int (*fn)(void *arg), void *arg, char *s, char *m,
*tp = NULL;
return ret;
}
+
+ if (lock_torture_writer_fifo &&
+ !strncmp(s, "lock_torture_writer", strlen(s)))
+ sched_set_fifo(*tp);
+
wake_up_process(*tp); // Process is sleeping, so ordering provided.
torture_shuffle_task_register(*tp);
return ret;
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