Re: dyntick-idle CPU and node's qsmask

[Joel Fernandes <joel@joelfernandes.org>]

On Sat, Nov 10, 2018 at 03:04:36PM -0800, Paul E. McKenney wrote:
> On Sat, Nov 10, 2018 at 01:46:59PM -0800, Joel Fernandes wrote:
> > Hi Paul and everyone,
> > 
> > I was tracing/studying the RCU code today in paul/dev branch and noticed that
> > for dyntick-idle CPUs, the RCU GP thread is clearing the rnp->qsmask
> > corresponding to the leaf node for the idle CPU, and reporting a QS on their
> > behalf.
> > 
> > rcu_sched-10    [003]    40.008039: rcu_fqs:              rcu_sched 792 0 dti
> > rcu_sched-10    [003]    40.008039: rcu_fqs:              rcu_sched 801 2 dti
> > rcu_sched-10    [003]    40.008041: rcu_quiescent_state_report: rcu_sched 805 5>0 0 0 3 0
> > 
> > That's all good but I was wondering if we can do better for the idle CPUs if
> > we can some how not set the qsmask of the node in the first place. Then no
> > reporting would be needed of quiescent state is needed for idle CPUs right?
> > And we would also not need to acquire the rnp lock I think.
> > 
> > At least for a single node tree RCU system, it seems that would avoid needing
> > to acquire the lock without complications. Anyway let me know your thoughts
> > and happy to discuss this at the hallways of the LPC as well for folks
> > attending :)
> 
> We could, but that would require consulting the rcu_data structure for
> each CPU while initializing the grace period, thus increasing the number
> of cache misses during grace-period initialization and also shortly after
> for any non-idle CPUs.  This seems backwards on busy systems where each

When I traced, it appears to me that rcu_data structure of a remote CPU was
being consulted anyway by the rcu_sched thread. So it seems like such cache
miss would happen anyway whether it is during grace-period initialization or
during the fqs stage? I guess I'm trying to say, the consultation of remote
CPU's rcu_data happens anyway.

> CPU will with high probability report its own quiescent state before three
> jiffies pass, in which case the cache misses on the rcu_data structures
> would be wasted motion.

If all the CPUs are busy and reporting their QS themselves, then I think the
qsmask is likely 0 so then rcu_implicit_dynticks_qs (called from
force_qs_rnp) wouldn't be called and so there would no cache misses on
rcu_data right?

> Now, this does increase overhead on mostly idle systems, but the theory
> is that mostly idle systems are most able to absorb this extra overhead.

Yes. Could we use rcuperf to check the impact of such change?

Anyway it was just an idea that popped up when I was going through traces :)
Thanks for the discussion and happy to discuss further or try out anything.

- Joel