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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
inversion 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/CAB8ipk_a6VFNjiEnHRHkUMBKbA+qzPQvhtNjJ_YNzQhqV_o8Zw@mail.gmail.com/

Fixes: 0c18f2ecfc ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min")
Reported-by: Xuewen Yan <xuewen.yan94@gmail.com>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210617165155.3774110-1-qais.yousef@arm.com
This commit is contained in:
Qais Yousef 2021-06-17 17:51:55 +01:00 committed by Peter Zijlstra
parent d7d607096a
commit 0213b7083e
1 changed files with 18 additions and 31 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

49
kernel/sched/core.c
View File

@ -1403,8 +1403,10 @@ 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)
{
/* Copy by value as we could modify it */
struct uclamp_se uc_req = 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
@ -1415,23 +1417,11 @@ uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
if (task_group(p) == &root_task_group)
return uc_req;
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_req.value;
value = clamp(value, tg_min, tg_max);
uclamp_se_set(&uc_req, value, false);
#endif
return uc_req;
@ -1630,8 +1620,9 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
}
static inline void
uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
uclamp_update_active(struct task_struct *p)
{
enum uclamp_id clamp_id;
struct rq_flags rf;
struct rq *rq;
@ -1651,9 +1642,11 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
* affecting a valid clamp bucket, the next time it's enqueued,
* it will already see the updated clamp bucket value.
*/
if (p->uclamp[clamp_id].active) {
uclamp_rq_dec_id(rq, p, clamp_id);
uclamp_rq_inc_id(rq, p, clamp_id);
for_each_clamp_id(clamp_id) {
if (p->uclamp[clamp_id].active) {
uclamp_rq_dec_id(rq, p, clamp_id);
uclamp_rq_inc_id(rq, p, clamp_id);
}
}
task_rq_unlock(rq, p, &rf);
@ -1661,20 +1654,14 @@ 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;
struct task_struct *p;
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);
}
}
while ((p = css_task_iter_next(&it)))
uclamp_update_active(p);
css_task_iter_end(&it);
}
@ -9634,7 +9621,7 @@ 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);
}
}