[PATCH] sched/pi: Reweight fair_policy() tasks when inheriting prio

[Qais Yousef <qyousef@layalina.io>]

For fair tasks inheriting the priority (nice) without reweighting is
a NOP as the task's share won't change.

This is visible when running with PTHREAD_PRIO_INHERIT where fair tasks
with low priority values are susceptible to starvation leading to PI
like impact on lock contention.

The logic in rt_mutex will reset these low priority fair tasks into nice
0, but without the additional reweight operation to actually update the
weights, it doesn't have the desired impact of boosting them to allow
them to run sooner/longer to release the lock.

Apply the reweight for fair_policy() tasks to achieve the desired boost
for those low nice values tasks. Note that boost here means resetting
their nice to 0; as this is what the current logic does for fair tasks.

Handling of idle_policy() requires more code refactoring and is not
handled yet. idle_policy() are treated specially and only run when the
CPU is idle and get a hardcoded low weight value. Changing weights won't
be enough without a promotion first to SCHED_OTHER.

Tested with a test program that creates three threads.

	1. main thread that spanws high prio and low prio task and busy
	   loops

	2. low priority thread that holds a pthread_mutex() with
	   PTHREAD_PRIO_INHERIT protocol. Runs at nice +10. Busy loops
	   after holding the lock.

	3. high priority thread that holds a pthread_mutex() with
	   PTHREADPTHREAD_PRIO_INHERIT, but made to start after the low
	   priority thread. Runs at nice 0. Should remain blocked by the
	   low priority thread.

All tasks are pinned to CPU0.

Without the patch I can see the low priority thread running only for
~10% of the time which is what expected without it being boosted.

With the patch the low priority thread runs for ~50% which is what
expected if it gets boosted to nice 0.

I modified the test program logic afterwards to ensure that after
releasing the lock the low priority thread goes back to running for 10%
of the time, and it does.

Reported-by: Yabin Cui <yabinc@google.com>
Signed-off-by: Qais Yousef <qyousef@layalina.io>
---
 kernel/sched/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0621e4ee31de..b90a541810da 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7242,8 +7242,10 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 	} else {
 		if (dl_prio(oldprio))
 			p->dl.pi_se = &p->dl;
-		if (rt_prio(oldprio))
+		else if (rt_prio(oldprio))
 			p->rt.timeout = 0;
+		else if (!task_has_idle_policy(p))
+			reweight_task(p, prio - MAX_RT_PRIO);
 	}
 
 	__setscheduler_prio(p, prio);
-- 
2.34.1