Re: question about RCU dynticks_nesting

[Paul E. McKenney]

On Mon, May 04, 2015 at 03:59:02PM -0400, Rik van Riel wrote:
> On 05/04/2015 03:39 PM, Paul E. McKenney wrote:
> > On Mon, May 04, 2015 at 03:00:44PM -0400, Rik van Riel wrote:
> 
> >> In case of the non-preemptible RCU, we could easily also
> >> increase current->rcu_read_lock_nesting at the same time
> >> we increase the preempt counter, and use that as the
> >> indicator to test whether the cpu is in an extended
> >> rcu quiescent state. That way there would be no extra
> >> overhead at syscall entry or exit at all. The trick
> >> would be getting the preempt count and the rcu read
> >> lock nesting count in the same cache line for each task.
> > 
> > But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt
> > counter in production kernels.  Even if there was, we have to sample this
> > on other CPUs, so the overhead of preempt_disable() and preempt_enable()
> > would be where kernel entry/exit is, so I expect that this would be a
> > net loss in overall performance.
> 
> CONFIG_PREEMPT_RCU seems to be independent of CONFIG_PREEMPT.
> Not sure why, but they are :)

Well, they used to be independent.  But the "depends" clauses force
them.  You cannot have TREE_RCU unless !PREEMPT && SMP.

> >> In case of the preemptible RCU scheme, we would have to
> >> examine the per-task state (under the runqueue lock)
> >> to get the current task info of all CPUs, and in
> >> addition wait for the blkd_tasks list to empty out
> >> when doing a synchronize_rcu().
> >>
> >> That does not appear to require special per-cpu
> >> counters; examining the per-cpu rdp and the lists
> >> inside it, with the rnp->lock held if doing any
> >> list manipulation, looks like it would be enough.
> >>
> >> However, the current code is a lot more complicated
> >> than that. Am I overlooking something obvious, Paul?
> >> Maybe something non-obvious? :)
> > 
> > Ummm...  The need to maintain memory ordering when sampling task
> > state from remote CPUs?
> > 
> > Or am I completely confused about what you are suggesting?
> > 
> > That said, are you chasing a real system-visible performance issue
> > that you tracked to RCU's dyntick-idle system?
> 
> The goal is to reduce the syscall overhead of nohz_full.
> 
> Part of the overhead is in the vtime updates, part of it is
> in the way RCU extended quiescent state is tracked.

OK, as long as it is actual measurements rather than guesswork.

								Thanx, Paul

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