[tip: sched/urgent] Revert "sched/fair: Make sure to try to detach at least one movable task"

["tip-bot2 for Josh Don" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     2feab2492deb2f14f9675dd6388e9e2bf669c27a
Gitweb:        https://git.kernel.org/tip/2feab2492deb2f14f9675dd6388e9e2bf669c27a
Author:        Josh Don <joshdon@google.com>
AuthorDate:    Thu, 20 Jun 2024 14:44:50 -07:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 01 Jul 2024 13:01:43 +02:00

Revert "sched/fair: Make sure to try to detach at least one movable task"

This reverts commit b0defa7ae03ecf91b8bfd10ede430cff12fcbd06.

b0defa7ae03ec changed the load balancing logic to ignore env.max_loop if
all tasks examined to that point were pinned. The goal of the patch was
to make it more likely to be able to detach a task buried in a long list
of pinned tasks. However, this has the unfortunate side effect of
creating an O(n) iteration in detach_tasks(), as we now must fully
iterate every task on a cpu if all or most are pinned. Since this load
balance code is done with rq lock held, and often in softirq context, it
is very easy to trigger hard lockups. We observed such hard lockups with
a user who affined O(10k) threads to a single cpu.

When I discussed this with Vincent he initially suggested that we keep
the limit on the number of tasks to detach, but increase the number of
tasks we can search. However, after some back and forth on the mailing
list, he recommended we instead revert the original patch, as it seems
likely no one was actually getting hit by the original issue.

Fixes: b0defa7ae03e ("sched/fair: Make sure to try to detach at least one movable task")
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20240620214450.316280-1-joshdon@google.com
---
 kernel/sched/fair.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8a5b1ae..24dda70 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9149,12 +9149,8 @@ static int detach_tasks(struct lb_env *env)
 			break;
 
 		env->loop++;
-		/*
-		 * We've more or less seen every task there is, call it quits
-		 * unless we haven't found any movable task yet.
-		 */
-		if (env->loop > env->loop_max &&
-		    !(env->flags & LBF_ALL_PINNED))
+		/* We've more or less seen every task there is, call it quits */
+		if (env->loop > env->loop_max)
 			break;
 
 		/* take a breather every nr_migrate tasks */
@@ -11393,9 +11389,7 @@ more_balance:
 
 		if (env.flags & LBF_NEED_BREAK) {
 			env.flags &= ~LBF_NEED_BREAK;
-			/* Stop if we tried all running tasks */
-			if (env.loop < busiest->nr_running)
-				goto more_balance;
+			goto more_balance;
 		}
 
 		/*