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Date: Tue, 29 Jul 2014 09:22:36 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Peter Zijlstra <peterz@...radead.org> Cc: linux-kernel@...r.kernel.org, mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com Subject: Re: [PATCH RFC tip/core/rcu 2/9] rcu: Provide cond_resched_rcu_qs() to force quiescent states in long loops On Tue, Jul 29, 2014 at 09:55:36AM +0200, Peter Zijlstra wrote: > On Mon, Jul 28, 2014 at 03:56:13PM -0700, Paul E. McKenney wrote: > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> > > > > RCU-tasks requires the occasional voluntary context switch > > from CPU-bound in-kernel tasks. In some cases, this requires > > instrumenting cond_resched(). However, there is some reluctance > > to countenance unconditionally instrumenting cond_resched() (see > > http://lwn.net/Articles/603252/), > > No, if its a good reason mention it, if not ignore it. Fair enough. ;-) > > so this commit creates a separate > > cond_resched_rcu_qs() that may be used in place of cond_resched() in > > locations prone to long-duration in-kernel looping. > > Sounds like a pain and a recipe for mistakes. How is joe kernel hacker > supposed to 1) know about this new api, and 2) decide which to use? > > Heck, even I wouldn't know, and I just read the damn patch. When Joe Hacker gets stall warning messages due to loops in the kernel that contain cond_resched(), that is a hint that cond_resched_rcu_qs() is required. These stall warnings can occur when using RCU-tasks and when using normal RCU in NO_HZ_FULL kernels in cases where the scheduling-clock interrupt is left off while executing a long code path in the kernel. (Of course, in both cases, another eminently reasonable fix is to shorten the offending code path in the kernel.) I should add words to that effect to Documentation/RCU/stallwarn.txt, shouldn't I? Done. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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