Messages in this thread | | | Date | Thu, 29 Sep 2022 09:42:04 -0700 | From | "Paul E. McKenney" <> | Subject | Re: RCU vs NOHZ |
| |
On Thu, Sep 29, 2022 at 06:23:16PM +0200, Peter Zijlstra wrote: > On Thu, Sep 29, 2022 at 08:46:18AM -0700, Paul E. McKenney wrote: > > On Thu, Sep 29, 2022 at 08:20:44AM -0700, Paul E. McKenney wrote: > > > > > > There is a directly invoked RCU hook for any transition that enables or > > > > > disables the tick, namely the ct_*_enter() and ct_*_exit() functions, > > > > > that is, those functions formerly known as rcu_*_enter() and rcu_*_exit(). > > > > > > > > Context tracking doesn't know about NOHZ, therefore RCU can't either. > > > > Context tracking knows about IDLE, but not all IDLE is NOHZ-IDLE. > > > > > > > > Specifically we have: > > > > > > > > ct_{idle,irq,nmi,user,kernel}_enter() > > > > > > > > And none of them are related to NOHZ in the slightest. So no, RCU does > > > > not have a NOHZ callback. > > > > > > > > I'm still thikning you're conflating NOHZ_FULL (stopping the tick when > > > > in userspace) and regular NOHZ (stopping the tick when idle). > > > > Exactly how are ct_user_enter() and ct_user_exit() completely unrelated > > to nohz_full CPUs? > > That's the thing; I'm not talking about nohz_full. I'm talking about > regular nohz. World of difference there.
And indeed, for !nohz_full CPUs, the tick continues throughout userspace execution. But you really did have ct_user_enter() and ct_user_exit() on your list.
And for idle (as opposed to nohz_full userspace execution), there is still ct_{idle,irq,nmi}_enter(). And RCU does pay attention to these.
So exactly what are you trying to tell me here? ;-)
> nohz_full is a gimmick that shouldn't be used outside of very specific > cases. Regular nohz otoh is used by everybody always.
I will let you take that up with the people using it.
Thanx, Paul
| |