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sched/core: Fix priority checking for DL server picks

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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.

Reported-by: Suleiman Souhlal <suleiman@google.com>
Signed-off-by: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vineeth Pillai <vineeth@bitbyteword.org>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/48b78521d86f3b33c24994d843c1aad6b987dda9.1716811044.git.bristot@kernel.org
This commit is contained in:
Joel Fernandes (Google) 2024-05-27 14:06:53 +02:00 committed by Peter Zijlstra
parent d741f297bc
commit 4b26cfdd39
1 changed files with 21 additions and 2 deletions
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23
kernel/sched/core.c
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@ -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);