Re: question about RCU dynticks_nesting

[Rik van Riel <riel@redhat.com>]

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 :)

>> 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.

-- 
All rights reversed