Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups

Patrick Bellasi
2018-09-06T13:48:54Z

Hi Juri!

On 05-Sep 12:45, Juri Lelli wrote:
> Hi,
>
> On 28/08/18 14:53, Patrick Bellasi wrote:
>
> [...]
>
> > static inline int __setscheduler_uclamp(struct task_struct *p,
> > const struct sched_attr *attr)
> > {
> > - if (attr->sched_util_min > attr->sched_util_max)
> > - return -EINVAL;
> > - if (attr->sched_util_max > SCHED_CAPACITY_SCALE)
> > - return -EINVAL;
> > + int group_id[UCLAMP_CNT] = { UCLAMP_NOT_VALID };
> > + int lower_bound, upper_bound;
> > + struct uclamp_se *uc_se;
> > + int result = 0;
> >
> > - p->uclamp[UCLAMP_MIN] = attr->sched_util_min;
> > - p->uclamp[UCLAMP_MAX] = attr->sched_util_max;
> > + mutex_lock(&uclamp_mutex);
>
> This is going to get called from an rcu_read_lock() section, which is a
> no-go for using mutexes:
>
> sys_sched_setattr ->
> rcu_read_lock()
> ...
> sched_setattr() ->
> __sched_setscheduler() ->
> ...
> __setscheduler_uclamp() ->
> ...
> mutex_lock()

Rightm, great catch, thanks!

> Guess you could fix the issue by getting the task struct after find_
> process_by_pid() in sys_sched_attr() and then calling sched_setattr()
> after rcu_read_lock() (putting the task struct at the end). Peter
> actually suggested this mod to solve a different issue.

I guess you mean something like this ?

---8<---
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5792,10 +5792,15 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
rcu_read_lock();
retval = -ESRCH;
p = find_process_by_pid(pid);
- if (p != NULL)
- retval = sched_setattr(p, &attr);
+ if (likely(p))
+ get_task_struct(p);
rcu_read_unlock();

+ if (likely(p)) {
+ retval = sched_setattr(p, &attr);
+ put_task_struct(p);
+ }
+
return retval;
}
---8<---

Cheers,
Patrick

--
#include <best/regards.h>

Patrick Bellasi