Commit 4b26cfdd authored by Joel Fernandes (Google)'s avatar Joel Fernandes (Google) Committed by Peter Zijlstra
Browse files

sched/core: Fix priority checking for DL server picks 



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.

Reported-by: default avatarSuleiman Souhlal <suleiman@google.com>
Signed-off-by: default avatar"Joel Fernandes (Google)" <joel@joelfernandes.org>
Signed-off-by: default avatarDaniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarVineeth Pillai <vineeth@bitbyteword.org>
Tested-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/48b78521d86f3b33c24994d843c1aad6b987dda9.1716811044.git.bristot@kernel.org
parent d741f297

kernel/sched/core.c

+21 −2
Original line number Diff line number Diff line
@@ -163,6 +163,9 @@ 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] */
@@ -192,8 +195,24 @@ 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);