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From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
To: linux-kernel@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>,
	Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>
Cc: Suleiman Souhlal <suleiman@google.com>,
	Youssef Esmat <youssefesmat@google.com>,
	David Vernet <void@manifault.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	joseph.salisbury@canonical.com,
	Luca Abeni <luca.abeni@santannapisa.it>,
	Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>,
	Vineeth Pillai <vineeth@bitbyteword.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Phil Auld <pauld@redhat.com>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>
Subject: [PATCH 03/10] sched/core: Fix priority checking for DL server picks
Date: Fri, 16 Feb 2024 13:31:01 -0500
Message-Id: <20240216183108.1564958-4-joel@joelfernandes.org>
In-Reply-To: <20240216183108.1564958-1-joel@joelfernandes.org>
References: <20240216183108.1564958-1-joel@joelfernandes.org>
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In core scheduling, a DL server pick (which is CFS task) should be given higher
priority than tasks in other classes.

Not doing so causes CFS starvation. A kselftest is added later to demonstrate
this.  A CFS task that is competing with RT tasks can be completely starved
without this and the DL server's boosting completely ignored.

Fix these problems.

Reviewed-by: Vineeth Pillai <vineeth@bitbyteword.org>
Reported-by: Suleiman Souhlal <suleiman@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/sched/core.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 63f41453b79e..0a05caf9d3d7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -162,6 +162,9 @@ static inline int __task_prio(const struct task_struct *p)
 	if (p->sched_class == &stop_sched_class) /* trumps deadline */
 		return -2;
 
+	if (p->dl_server)
+		return -1; /* deadline */
+
 	if (rt_prio(p->prio)) /* includes deadline */
 		return p->prio; /* [-1, 99] */
 
@@ -191,8 +194,24 @@ static inline bool prio_less(const struct task_struct *a,
 	if (-pb < -pa)
 		return false;
 
-	if (pa == -1) /* dl_prio() doesn't work because of stop_class above */
-		return !dl_time_before(a->dl.deadline, b->dl.deadline);
+	if (pa == -1) { /* dl_prio() doesn't work because of stop_class above */
+		const struct sched_dl_entity *a_dl, *b_dl;
+
+		a_dl = &a->dl;
+		/*
+		 * Since,'a' and 'b' can be CFS tasks served by DL server,
+		 * __task_prio() can return -1 (for DL) even for those. In that
+		 * case, get to the dl_server's DL entity.
+		 */
+		if (a->dl_server)
+			a_dl = a->dl_server;
+
+		b_dl = &b->dl;
+		if (b->dl_server)
+			b_dl = b->dl_server;
+
+		return !dl_time_before(a_dl->deadline, b_dl->deadline);
+	}
 
 	if (pa == MAX_RT_PRIO + MAX_NICE)	/* fair */
 		return cfs_prio_less(a, b, in_fi);
-- 
2.34.1