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. - 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/
