2021-06-11 12:24:32

by Qais Yousef

[permalink] [raw]
Subject: [PATCH] sched/uclamp: Fix uclamp_tg_restrict()

Now cpu.uclamp.min acts as a protection, we need to make sure that the
uclamp request of the task is within the allowed range of the cgroup,
that is it is clamp()'ed correctly by tg->uclamp[UCLAMP_MIN] and
tg->uclamp[UCLAMP_MAX].

As reported by Xuewen [1] we can have some corner cases where there's
inverstion between uclamp requested by task (p) and the uclamp values of
the taskgroup it's attached to (tg). Following table demonstrates
2 corner cases:

| p | tg | effective
-----------+-----+------+-----------
CASE 1
-----------+-----+------+-----------
uclamp_min | 60% | 0% | 60%
-----------+-----+------+-----------
uclamp_max | 80% | 50% | 50%
-----------+-----+------+-----------
CASE 2
-----------+-----+------+-----------
uclamp_min | 0% | 30% | 30%
-----------+-----+------+-----------
uclamp_max | 20% | 50% | 20%
-----------+-----+------+-----------

With this fix we get:

| p | tg | effective
-----------+-----+------+-----------
CASE 1
-----------+-----+------+-----------
uclamp_min | 60% | 0% | 50%
-----------+-----+------+-----------
uclamp_max | 80% | 50% | 50%
-----------+-----+------+-----------
CASE 2
-----------+-----+------+-----------
uclamp_min | 0% | 30% | 30%
-----------+-----+------+-----------
uclamp_max | 20% | 50% | 30%
-----------+-----+------+-----------

Additionally uclamp_update_active_tasks() must now unconditionally
update both UCLAMP_MIN/MAX because changing the tg's UCLAMP_MAX for
instance could have an impact on the effective UCLAMP_MIN of the tasks.

| p | tg | effective
-----------+-----+------+-----------
old
-----------+-----+------+-----------
uclamp_min | 60% | 0% | 50%
-----------+-----+------+-----------
uclamp_max | 80% | 50% | 50%
-----------+-----+------+-----------
*new*
-----------+-----+------+-----------
uclamp_min | 60% | 0% | *60%*
-----------+-----+------+-----------
uclamp_max | 80% |*70%* | *70%*
-----------+-----+------+-----------

[1] https://lore.kernel.org/lkml/[email protected]om/

Reported-by: Xuewen Yan <[email protected]>
Fixes: 0c18f2ecfcc2 ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min")
Signed-off-by: Qais Yousef <[email protected]>
---

Xuewen, Yun, Wei

If you can give this a spin and provide Tested-by that would be much
appreciated.

Thanks!


kernel/sched/core.c | 43 +++++++++++++++----------------------------
1 file changed, 15 insertions(+), 28 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9e9a5be35cde..0318b00baa97 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1403,38 +1403,28 @@ static void uclamp_sync_util_min_rt_default(void)
static inline struct uclamp_se
uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
{
- struct uclamp_se uc_req = p->uclamp_req[clamp_id];
+ /* Copy by value as we could modify it */
+ struct uclamp_se uc_eff = p->uclamp_req[clamp_id];
#ifdef CONFIG_UCLAMP_TASK_GROUP
+ unsigned int tg_min, tg_max, value;

/*
* Tasks in autogroups or root task group will be
* restricted by system defaults.
*/
if (task_group_is_autogroup(task_group(p)))
- return uc_req;
+ return uc_eff;
if (task_group(p) == &root_task_group)
- return uc_req;
+ return uc_eff;

- switch (clamp_id) {
- case UCLAMP_MIN: {
- struct uclamp_se uc_min = task_group(p)->uclamp[clamp_id];
- if (uc_req.value < uc_min.value)
- return uc_min;
- break;
- }
- case UCLAMP_MAX: {
- struct uclamp_se uc_max = task_group(p)->uclamp[clamp_id];
- if (uc_req.value > uc_max.value)
- return uc_max;
- break;
- }
- default:
- WARN_ON_ONCE(1);
- break;
- }
+ tg_min = task_group(p)->uclamp[UCLAMP_MIN].value;
+ tg_max = task_group(p)->uclamp[UCLAMP_MAX].value;
+ value = uc_eff.value;
+ value = clamp(value, tg_min, tg_max);
+ uclamp_se_set(&uc_eff, value, false);
#endif

- return uc_req;
+ return uc_eff;
}

/*
@@ -1661,8 +1651,7 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)

#ifdef CONFIG_UCLAMP_TASK_GROUP
static inline void
-uclamp_update_active_tasks(struct cgroup_subsys_state *css,
- unsigned int clamps)
+uclamp_update_active_tasks(struct cgroup_subsys_state *css)
{
enum uclamp_id clamp_id;
struct css_task_iter it;
@@ -1670,10 +1659,8 @@ uclamp_update_active_tasks(struct cgroup_subsys_state *css,

css_task_iter_start(css, 0, &it);
while ((p = css_task_iter_next(&it))) {
- for_each_clamp_id(clamp_id) {
- if ((0x1 << clamp_id) & clamps)
- uclamp_update_active(p, clamp_id);
- }
+ for_each_clamp_id(clamp_id)
+ uclamp_update_active(p, clamp_id);
}
css_task_iter_end(&it);
}
@@ -9626,7 +9613,7 @@ static void cpu_util_update_eff(struct cgroup_subsys_state *css)
}

/* Immediately update descendants RUNNABLE tasks */
- uclamp_update_active_tasks(css, clamps);
+ uclamp_update_active_tasks(css);
}
}

--
2.25.1