Messages in this thread Patch in this message |  | | Subject | Re: [PATCH v4 2/2] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS | From | Dietmar Eggemann <> | Date | Thu, 22 Jul 2021 10:47:33 +0200 |
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On 19/07/2021 18:16, Quentin Perret wrote: > SCHED_FLAG_KEEP_PARAMS can be passed to sched_setattr to specify that > the call must not touch scheduling parameters (nice or priority). This
What about DL params (runtime, deadline, period)?
Uclamp is not for DL but we could (*) still set uclamp values on a DL task. Obviously they would only be used when the task switches policy.
On tip/sched/core:
root@juno:~# chrt -d -P 1000000000 -T 100000000 -p 0 1671
root@juno:~# uclampset -m200 -M400 -p 1671
root@juno:~# cat /proc/1671/sched | grep uclamp uclamp.min : 200 uclamp.max : 400 effective uclamp.min : 200 effective uclamp.max : 400
root@juno:~# chrt -o -p 0 pid 1702's current scheduling policy: SCHED_OTHER pid 1702's current scheduling priority: 0
root@juno:~# cat /proc/1671/sched | grep uclamp uclamp.min : 200 uclamp.max : 400 effective uclamp.min : 200 effective uclamp.max : 400
> is particularly handy for uclamp when used in conjunction with > SCHED_FLAG_KEEP_POLICY as that allows to issue a syscall that only > impacts uclamp values. > > However, sched_setattr always checks whether the priorities and nice > values passed in sched_attr are valid first, even if those never get
+ DL params (__checkparam_dl())
> used down the line. This is useless at best since userspace can > trivially bypass this check to set the uclamp values by specifying low > priorities. However, it is cumbersome to do so as there is no single > expression of this that skips both RT and CFS checks at once. As such, > userspace needs to query the task policy first with e.g. sched_getattr > and then set sched_attr.sched_priority accordingly. This is racy and > slower than a single call. > > As the priority and nice checks are useless when SCHED_FLAG_KEEP_PARAMS > is specified, simply inherit them in this case to match the policy > inheritance of SCHED_FLAG_KEEP_POLICY. > > Reported-by: Wei Wang <wvw@google.com> > Reviewed-by: Qais Yousef <qais.yousef@arm.com> > Signed-off-by: Quentin Perret <qperret@google.com> > --- > kernel/sched/core.c | 19 +++++++++++++------ > 1 file changed, 13 insertions(+), 6 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index e801d2c3077b..914076eab242 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -7332,6 +7332,16 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a > return -E2BIG; > } > > +static void get_params(struct task_struct *p, struct sched_attr *attr) > +{ > + if (task_has_dl_policy(p)) > + __getparam_dl(p, attr);
(*) This changes the behaviour when setting uclamp values on a DL task.
Before uclamp values could be set but now, because of
void __getparam_dl(struct task_struct *p, struct sched_attr *attr) .. attr->sched_flags = dl_se->flags SCHED_FLAG_UTIL_CLAMP gets overwritten and __sched_setscheduler() bails in:
if (unlikely(policy == p->policy)) { ... retval = 0; goto unlock; } change: I.e. the:
if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) goto change; can't trigger anymore.
> + else if (task_has_rt_policy(p)) > + attr->sched_priority = p->rt_priority; > + else > + attr->sched_nice = task_nice(p); > +} > + > /** > * sys_sched_setscheduler - set/change the scheduler policy and RT priority > * @pid: the pid in question. > @@ -7393,6 +7403,8 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr, > rcu_read_unlock(); > > if (likely(p)) { > + if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS) > + get_params(p, &attr);
SCHED_FLAG_KEEP_PARAMS is handled here but SCHED_FLAG_KEEP_POLICY outside (before) the `if (likely(p))`? > retval = sched_setattr(p, &attr); > put_task_struct(p); > }
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