On Tue, Jan 22, 2019 at 02:01:15PM +0000, Patrick Bellasi wrote: > On 22-Jan 14:28, Peter Zijlstra wrote: > > On Tue, Jan 22, 2019 at 10:43:05AM +0000, Patrick Bellasi wrote: > > > On 22-Jan 10:37, Peter Zijlstra wrote: > > > > > > Sure, I get that. What I don't get is why you're adding that (2) here. > > > > Like said, __sched_setscheduler() already does a dequeue/enqueue under > > > > rq->lock, which should already take care of that. > > > > > > Oh, ok... got it what you mean now. > > > > > > With: > > > > > > [PATCH v6 01/16] sched/core: Allow sched_setattr() to use the current policy > > > <20190115101513.2822-2-patrick.bellasi@xxxxxxx> > > > > > > we can call __sched_setscheduler() with: > > > > > > attr->sched_flags & SCHED_FLAG_KEEP_POLICY > > > > > > whenever we want just to change the clamp values of a task without > > > changing its class. Thus, we can end up returning from > > > __sched_setscheduler() without doing an actual dequeue/enqueue. > > > > I don't see that happening.. when KEEP_POLICY we set attr.sched_policy = > > SETPARAM_POLICY. That is then checked again in __setscheduler_param(), > > which is in the middle of that dequeue/enqueue. > > Yes, I think I've forgot a check before we actually dequeue the task. > > The current code does: > > ---8<--- > SYSCALL_DEFINE3(sched_setattr) > > // A) request to keep the same policy > if (attr.sched_flags & SCHED_FLAG_KEEP_POLICY) > attr.sched_policy = SETPARAM_POLICY; > > sched_setattr() > // B) actually enforce the same policy > if (policy < 0) > policy = oldpolicy = p->policy; > > // C) tune the clamp values > if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) > retval = __setscheduler_uclamp(p, attr); > > // D) tune attributes if policy is the same > if (unlikely(policy == p->policy)) > if (fair_policy(policy) && attr->sched_nice != task_nice(p)) > goto change; > if (rt_policy(policy) && attr->sched_priority != p->rt_priority) > goto change; > if (dl_policy(policy) && dl_param_changed(p, attr)) > goto change; if (util_changed) goto change; ? > return 0; > change: > > // E) dequeue/enqueue task > ---8<--- > > So, probably in D) I've missed a check on SCHED_FLAG_KEEP_POLICY to > enforce a return in that case... > > > Also, and this might be 'broken', SETPARAM_POLICY _does_ reset all the > > other attributes, it only preserves policy, but it will (re)set nice > > level for example (see that same function). > > Mmm... right... my bad! :/ > > > So maybe we want to introduce another (few?) FLAG_KEEP flag(s) that > > preserve the other bits; I'm thinking at least KEEP_PARAM and KEEP_UTIL > > or something. > > Yes, I would say we have two options: > > 1) SCHED_FLAG_KEEP_POLICY enforces all the scheduling class specific > attributes, but cross class attributes (e.g. uclamp) > > 2) add SCHED_KEEP_NICE, SCHED_KEEP_PRIO, and SCED_KEEP_PARAMS > and use them in the if conditions in D) So the current KEEP_POLICY basically provides sched_setparam(), and given we have that as a syscall, that is supposedly a useful functionality. Also, NICE/PRIO/DL* is all the same thing and depends on the policy, KEEP_PARAM should cover the lot And I suppose the UTIL_CLAMP is !KEEP_UTIL; we could go either way around with that flag. > In both cases the goal should be to return from code block D). I don't think so; we really do want to 'goto change' for util changes too I think. Why duplicate part of that logic?
