Messages in this thread | | | Date | Tue, 26 May 2020 17:58:06 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 01/10] rcu: Directly lock rdp->nocb_lock on nocb code entrypoints |
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On Tue, May 26, 2020 at 08:45:42PM -0400, Joel Fernandes wrote: > Hi Paul, > > On Tue, May 26, 2020 at 6:29 PM Paul E. McKenney <paulmck@kernel.org> wrote: > > > > On Tue, May 26, 2020 at 05:27:56PM -0400, Joel Fernandes wrote: > > > On Tue, May 26, 2020 at 02:09:47PM -0700, Paul E. McKenney wrote: > > > [...] > > > > > > > BTW, I'm really itching to give it a try to make the scheduler more deadlock > > > > > > > resilient (that is, if the scheduler wake up path detects a deadlock, then it > > > > > > > defers the wake up using timers, or irq_work on its own instead of passing > > > > > > > the burden of doing so to the callers). Thoughts? > > > > > > > > > > > > I have used similar approaches within RCU, but on the other hand the > > > > > > scheduler often has tighter latency constraints than RCU does. So I > > > > > > think that is a better question for the scheduler maintainers than it > > > > > > is for me. ;-) > > > > > > > > > > Ok, it definitely keeps coming up in my radar first with the > > > > > rcu_read_unlock_special() stuff, and now the nocb ;-). Perhaps it could also > > > > > be good for a conference discussion! > > > > > > > > Again, please understand that RCU has way looser latency constraints > > > > than the scheduler does. Adding half a jiffy to wakeup latency might > > > > not go over well, especially in the real-time application area. > > > > > > Yeah, agreed that the "deadlock detection" code should be pretty light weight > > > if/when it is written. > > > > In addition, to even stand a chance, you would need to use hrtimers. > > The half-jiffy (at a minimum) delay from any other deferral mechanism > > that I know of would be the kiss of death, especially from the viewpoint > > of the real-time guys. > > Just to make sure we are talking about the same kind of overhead - the > deferring is only needed if the rq lock is already held (detected by > trylocking). So there's no overhead in the common case other than the > trylock possibly being slightly more expensive than the regular > locking. Also, once the scheduler defers it, it uses the same kind of > mechanism that other deferral mechanisms use to overcome this deadlock > (timers, irq_work etc), so the overhead then would be no different > than what he have now - the RT users would already have the wake up > latency in current kernels without this idea implemented. Did I miss > something?
Aggressive real-time applications care deeply about the uncommon case.
Thanx, Paul
> > > > But what did the scheduler maintainers say about this idea? > > > > > > Last I remember when it came up during the rcu_read_unlock_special() deadlock > > > discussions, there's no way to know for infra like RCU to know that it was > > > invoked from the scheduler. > > > > > > The idea I am bringing up now (about the scheduler itself detecting a > > > recursion) was never brought up (not yet) with the sched maintainers (at > > > least not by me). > > > > It might be good to bounce if off of them sooner rather than later. > > Ok, I did that now over IRC. Thank you! > > - Joel
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