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For example, the task list > may have four tasks at the beginning, it may becomes to two > during the scanning in detach_tasks(): > Task list at beginning : "ABCD" > Task list in scanning : "CD" > > (ABCD stands for differnt tasks.) > > In this scenario, the env->loop_max is still four, so > detach_tasks() may scan twice for some tasks: > the scanning order maybe : "DCDC" > > In the Specjbb test, this issue can be catched many times. > (Over 3,300,000 times in a 30-min Specjbb test) sorry, I added an extra 0 here, it should be "330,000" times. > The patch introduces "first_back" to record the first task which > is put back to the task list. If we get a task which is equal to > first_back, we break the loop, and avoid to scan twice for it. > > Signed-off-by: Huang Shijie > --- > v1 --> v2: > Add more comment from Valentin Schneider > v1: https://lore.kernel.org/all/20250707083636.38380-1-shijie@os.amperecomputing.com/ > --- > kernel/sched/fair.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7e2963efe800..7cc9d50e3e11 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -9443,6 +9443,7 @@ static int detach_tasks(struct lb_env *env) > { > struct list_head *tasks = &env->src_rq->cfs_tasks; > unsigned long util, load; > + struct task_struct *first_back = NULL; > struct task_struct *p; > int detached = 0; > > @@ -9481,6 +9482,12 @@ static int detach_tasks(struct lb_env *env) > } > > p = list_last_entry(tasks, struct task_struct, se.group_node); > + /* > + * We're back to an already visited task that couldn't be > + * detached, we've seen all there is to see. > + */ > + if (p == first_back) > + break; > > if (!can_migrate_task(p, env)) > goto next; > @@ -9562,6 +9569,8 @@ static int detach_tasks(struct lb_env *env) > schedstat_inc(p->stats.nr_failed_migrations_hot); > > list_move(&p->se.group_node, tasks); > + if (!first_back) > + first_back = p; > } > > /*