On Fri, Jun 20, 2014 at 04:30:33PM -0700, Paul E. McKenney wrote: > On Fri, Jun 20, 2014 at 03:39:51PM -0700, j...@joshtriplett.org wrote: > > On Fri, Jun 20, 2014 at 03:11:20PM -0700, Paul E. McKenney wrote: > > > On Fri, Jun 20, 2014 at 02:24:23PM -0700, j...@joshtriplett.org wrote: > > > > On Fri, Jun 20, 2014 at 12:12:36PM -0700, Paul E. McKenney wrote: > > > > > o Make cond_resched() a no-op for PREEMPT=y. This might well turn > > > > > out to be a good thing, but it doesn't help give RCU the > > > > > quiescent > > > > > states that it needs. > > > > > > > > What about doing this, together with letting the fqs logic poke > > > > un-quiesced kernel code as needed? That way, rather than having > > > > cond_resched do any work, you have the fqs logic recognize that a > > > > particular CPU has gone too long without quiescing, without disturbing > > > > that CPU at all if it hasn't gone too long. > > > > > > My next stop is to post the previous series, but with a couple of > > > exports and one bug fix uncovered by testing thus far, but after > > > another round of testing. Then I am going to take a close look at > > > this one: > > > > > > o Push the checks further into cond_resched(), so that the > > > fastpath does the same sequence of instructions that the original > > > did. This might work well, but requires IPIs, which are not so > > > good for latencies on the remote CPU. It nevertheless might be a > > > decent long-term solution given that if your CPU is spending many > > > jiffies looping in the kernel, you aren't getting good latencies > > > anyway. It also has the benefit of allowing RCU to take advantage > > > of the implicit quiescent states of all cond_resched() calls, > > > and of eliminating the need for a separate cond_resched_rcu_qs() > > > and for RCU_COND_RESCHED_QS. > > > > > > The one you call out is of course interesting as well. But there are > > > a couple of questions: > > > > > > 1. Why wasn't cond_resched() a no-op in CONFIG_PREEMPT to start > > > with? It just seems to obvious a thing to do for it to possibly > > > be an oversight. (What, me paranoid?) > > > > > > 2. When RCU recognizes that a particular CPU has gone too long, > > > exactly what are you suggesting that RCU do about it? When > > > formulating your answer, please give due consideration to the > > > implications of that CPU being a NO_HZ_FULL CPU. ;-) > > > > Send it an IPI that either causes it to flag a quiescent state > > immediately if currently quiesced or causes it to quiesce at the next > > opportunity if not. > > OK. But if we are in a !PREEMPT kernel,
That's not the case I was suggesting. *If* the kernel is fully preemptible, then it makes little sense to put any code in cond_resched, when instead another thread can simply cause a preemption if it needs a quiescent state. That has the advantage of not imposing any unnecessary polling on code running in the kernel. In a !PREEMPT kernel, it makes a bit more sense to have cond_resched as a voluntary preemption point. But voluntary preemption points don't make as much sense in a kernel prepared to preempt a thread anywhere. - Josh Triplett -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
