On Thu, 19 Jan 2017 18:44:08 +0100 Peter Zijlstra <[email protected]> wrote: - if (likely(prev->sched_class == class && > > + if (likely((prev->sched_class == class || > > + prev->sched_class == idle_class) && > > rq->nr_running == rq->cfs.h_nr_running)) { > > OK, so I hate this patch because it makes the condition more complex, > and while staring at what it does for code generation I couldn't for the > life of me figure out why we care about prev->sched_class to begin with.
I was thinking it would save on checking the rq at all, but rq is used by pick_next_task_*() anyway, so I doubt it's much savings. > > (we used to, but the current code not so much) > > So I simply removed that entire clause, like below, and lo and behold, > the system booted... I thought about doing this too, but decided against it because I was thinking that since class (and now idle_class) are constants, it would help the non cfs case. Checking prev->sched_class against a constant I thought would be quick. But yeah, I doubt it matters much in the grand scale of things. > > Could you give it a spin to see if anything comes apart? Yeah this works. You can add: Reported-by: Steven Rostedt (VMware) <[email protected]> Tested-by: Steven Rostedt (VMware) <[email protected]> Thanks, -- Steve > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 49ce1cb..51ca21e 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3321,15 +3321,14 @@ static inline void schedule_debug(struct task_struct > *prev) > static inline struct task_struct * > pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) > { > - const struct sched_class *class = &fair_sched_class; > + const struct sched_class *class; > struct task_struct *p; > > /* > * Optimization: we know that if all tasks are in > * the fair class we can call that function directly: > */ > - if (likely(prev->sched_class == class && > - rq->nr_running == rq->cfs.h_nr_running)) { > + if (likely(rq->nr_running == rq->cfs.h_nr_running)) { > p = fair_sched_class.pick_next_task(rq, prev, rf); > if (unlikely(p == RETRY_TASK)) > goto again;
