On Fri, Jun 20, 2014 at 05:14:18PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 20, 2014 at 04:52:15PM -0700, j...@joshtriplett.org wrote:
> > 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.
>
> Fair enough, but we still need to support !PREEMPT kernels.
>
> > *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.
>
> OK. Exactly which thread are you suggesting should cause the preemption?
>
> > 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.
>
> That does sound intuitive, but I am not yet prepared to believe that
> the scheduler guys missed this trick. There might well be some good
> reason for cond_resched() doing something, though I cannot think what it
> might be (something to do with preempt_enable_no_resched(), perhaps?).
> We should at least ask them, although if you want to do some testing
> before asking them, I of course have no objection to your doing so.
Oh, and it turns out to be possible to drive RCU's need-a-qs check much
farther down the cond_resched() rabbit hole than I expected. Looks like
it can be driven all the way down to rcu_note_context_switch().
Thanx, Paul
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