kernel/sched/fair.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK and all
tasks are not movable, detach_tasks() should not iterate more than tasks
available in the busiest rq.
Previously the (env->loop > env->loop_max) condition prevented us from
scanning non-movable tasks more than rq size times, but after we start
checking the LBF_ALL_PINNED flag, the "all tasks are not movable" case
is under threat.
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
kernel/sched/fair.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d7a3c63a2171..faa2a765e899 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11219,7 +11219,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
.dst_rq = this_rq,
.dst_grpmask = group_balance_mask(sd->groups),
.idle = idle,
- .loop_break = SCHED_NR_MIGRATE_BREAK,
.cpus = cpus,
.fbq_type = all,
.tasks = LIST_HEAD_INIT(env.tasks),
@@ -11265,6 +11264,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
* correctly treated as an imbalance.
*/
env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
+ /*
+ * If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK
+ * and all tasks are not movable, detach_tasks() should not
+ * iterate more than tasks available in rq.
+ */
+ env.loop_break = min(SCHED_NR_MIGRATE_BREAK, busiest->nr_running);
more_balance:
rq_lock_irqsave(busiest, &rf);
--
2.39.3On Thu, 14 Dec 2023 at 15:42, Konstantin Khorenko
<khorenko@virtuozzo.com> wrote:
>
> If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK and all
> tasks are not movable, detach_tasks() should not iterate more than tasks
> available in the busiest rq.
>
> Previously the (env->loop > env->loop_max) condition prevented us from
It's usually better to give the commit directly when we know it :
Before commit : b0defa7ae03e ("sched/fair: Make sure to try to detach
at least one movable task"),
the (env->loop > env->loop_max) condition prevented us from ...
> scanning non-movable tasks more than rq size times, but after we start
> checking the LBF_ALL_PINNED flag, the "all tasks are not movable" case
> is under threat.
>
Fixes: b0defa7ae03e ("sched/fair: Make sure to try to detach at least
one movable task")
> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
> ---
> kernel/sched/fair.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d7a3c63a2171..faa2a765e899 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11219,7 +11219,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> .dst_rq = this_rq,
> .dst_grpmask = group_balance_mask(sd->groups),
> .idle = idle,
> - .loop_break = SCHED_NR_MIGRATE_BREAK,
> .cpus = cpus,
> .fbq_type = all,
> .tasks = LIST_HEAD_INIT(env.tasks),
> @@ -11265,6 +11264,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> * correctly treated as an imbalance.
> */
> env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
> + /*
> + * If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK
> + * and all tasks are not movable, detach_tasks() should not
> + * iterate more than tasks available in rq.
> + */
> + env.loop_break = min(SCHED_NR_MIGRATE_BREAK, busiest->nr_running);
Should it be after more_balance: ?
In case we do "more_balance:" on a new_dst_cpu and it ends up that
finally there is no more movable task as we released the lock of
busiest rq in the meantime ?
Also you can remove one more superfluous init of loop_break:
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11361,7 +11361,6 @@ static int load_balance(int this_cpu, struct
rq *this_rq,
*/
if (!cpumask_subset(cpus, env.dst_grpmask)) {
env.loop = 0;
- env.loop_break = SCHED_NR_MIGRATE_BREAK;
goto redo;
}
goto out_all_pinned;
>
> more_balance:
> rq_lock_irqsave(busiest, &rf);
> --
> 2.39.3
>
On 18.12.2023 17:48, Vincent Guittot wrote:
> On Thu, 14 Dec 2023 at 15:42, Konstantin Khorenko
> <khorenko@virtuozzo.com> wrote:
>>
>> If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK and all
>> tasks are not movable, detach_tasks() should not iterate more than tasks
>> available in the busiest rq.
>>
>> Previously the (env->loop > env->loop_max) condition prevented us from
>
> It's usually better to give the commit directly when we know it :
> Before commit : b0defa7ae03e ("sched/fair: Make sure to try to detach
> at least one movable task"),
> the (env->loop > env->loop_max) condition prevented us from ...
You are definitely right, added.
>> scanning non-movable tasks more than rq size times, but after we start
>> checking the LBF_ALL_PINNED flag, the "all tasks are not movable" case
>> is under threat.
>>
>
> Fixes: b0defa7ae03e ("sched/fair: Make sure to try to detach at least
> one movable task")
Added this too, thank you.
>> Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
>> ---
>> kernel/sched/fair.c | 7 ++++++-
>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index d7a3c63a2171..faa2a765e899 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -11219,7 +11219,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>> .dst_rq = this_rq,
>> .dst_grpmask = group_balance_mask(sd->groups),
>> .idle = idle,
>> - .loop_break = SCHED_NR_MIGRATE_BREAK,
>> .cpus = cpus,
>> .fbq_type = all,
>> .tasks = LIST_HEAD_INIT(env.tasks),
>> @@ -11265,6 +11264,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>> * correctly treated as an imbalance.
>> */
>> env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
>> + /*
>> + * If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK
>> + * and all tasks are not movable, detach_tasks() should not
>> + * iterate more than tasks available in rq.
>> + */
>> + env.loop_break = min(SCHED_NR_MIGRATE_BREAK, busiest->nr_running);
>
> Should it be after more_balance: ?
> In case we do "more_balance:" on a new_dst_cpu and it ends up that
> finally there is no more movable task as we released the lock of
> busiest rq in the meantime ?
Well, both yes and no here.
If we simply move the env.loop_break assignment after "more_balance" label,
we will break the handling LBF_NEED_BREAK case.
But you are right, the new_dst_cpu case should also not just reset loop_break to
SCHED_NR_MIGRATE_BREAK, but do the same min() calculation.
So i've added one more label before the env.loop_break = min(...) and used it in
the new_dst_cpu case.
> Also you can remove one more superfluous init of loop_break:
Yep, dropped it, thank you.
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11361,7 +11361,6 @@ static int load_balance(int this_cpu, struct
> rq *this_rq,
> */
> if (!cpumask_subset(cpus, env.dst_grpmask)) {
> env.loop = 0;
> - env.loop_break = SCHED_NR_MIGRATE_BREAK;
> goto redo;
> }
> goto out_all_pinned;
>
>>
>> more_balance:
>> rq_lock_irqsave(busiest, &rf);
>> --
>> 2.39.3
>>
If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK and all
tasks are not movable, detach_tasks() should not iterate more than tasks
available in the busiest rq.
Before commit: b0defa7ae03e ("sched/fair: Make sure to try to detach at
least one movable task"), the (env->loop > env->loop_max) condition
prevented us from scanning non-movable tasks more than rq size times,
but after we start checking the LBF_ALL_PINNED flag, the "all tasks are
not movable" case is under threat.
Note: in case all tasks in the rq could not be moved in detach_tasks()
we always increase loop_break by SCHED_NR_MIGRATE_BREAK, so we can step
over loop_max, but i think it's a rare case and does not worth adding
here extra check for rq->nr_running overlimit.
Fixes: b0defa7ae03e ("sched/fair: Make sure to try to detach at least
one movable task")
Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
Changes:
v1->v2:
* added the exact commit id caused the unefficiency + Fixes: tag
* dropped a couple of extra redundunt env.loop_break assignments in
load_balance()
kernel/sched/fair.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d7a3c63a2171..bd69c33fe9b4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11219,7 +11219,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
.dst_rq = this_rq,
.dst_grpmask = group_balance_mask(sd->groups),
.idle = idle,
- .loop_break = SCHED_NR_MIGRATE_BREAK,
.cpus = cpus,
.fbq_type = all,
.tasks = LIST_HEAD_INIT(env.tasks),
@@ -11266,6 +11265,14 @@ static int load_balance(int this_cpu, struct rq *this_rq,
*/
env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
+more_balance_reset_break:
+ /*
+ * If busiest rq is small, nr_running < SCHED_NR_MIGRATE_BREAK
+ * and all tasks are not movable, detach_tasks() should not
+ * iterate more than tasks available in rq.
+ */
+ env.loop_break = min(SCHED_NR_MIGRATE_BREAK, busiest->nr_running);
+
more_balance:
rq_lock_irqsave(busiest, &rf);
update_rq_clock(busiest);
@@ -11328,13 +11335,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
env.dst_cpu = env.new_dst_cpu;
env.flags &= ~LBF_DST_PINNED;
env.loop = 0;
- env.loop_break = SCHED_NR_MIGRATE_BREAK;
/*
* Go back to "more_balance" rather than "redo" since we
* need to continue with same src_cpu.
*/
- goto more_balance;
+ goto more_balance_reset_break;
}
/*
@@ -11360,7 +11366,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
*/
if (!cpumask_subset(cpus, env.dst_grpmask)) {
env.loop = 0;
- env.loop_break = SCHED_NR_MIGRATE_BREAK;
goto redo;
}
goto out_all_pinned;
--
2.39.3© 2016 - 2025 Red Hat, Inc.