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

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commit 2feab2492d upstream.

This reverts commit b0defa7ae0.

b0defa7ae0 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: b0defa7ae0 ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Josh Don 2024-06-20 14:44:50 -07:00 committed by Greg Kroah-Hartman
parent 87f3ceb2b1
commit 1e116c18e3
1 changed files with 3 additions and 9 deletions
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12
kernel/sched/fair.c
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@ -8977,12 +8977,8 @@ 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 */
@ -11259,9 +11255,7 @@ 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;
}
/*