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[Xen-devel] [PATCH 0/5] xen: RCU: x86/ARM: Add support of rcu_idle_{enter, exit}

Hello everyone,

It took some time, and, honestly, it was a bit of a nightmare, but I think I
did manage to find a way to make our RCU implementation in a status where it
does not only work "by chance" (as it does right now! :-O).

A bit of background in this post:


About the actual patch series, apart from the first two preparatory patches
(patch 1 was part of another series, but it's more convenient for me to send it
in here), this is all about the following:

*) making sure that CPUs that are idle when an RCU grace period is recorded to
   be starting, don't contribute to make such grace period longer than it needs
   to be. In fact, if they're idle, they can't hold references, and can safely
   be ignored. We do this in patch 3, using a cpumask, as Linux was doing on
   s390 (which was tickless already, back when we imported the RCU code), and
   on the early days of dynticks support on x86;

*) making sure that CPUs that have RCU related stuff to do, don't just go idle
   without actually doing those. This is rather simple, we just have to actually
   start using a function that we did import, but isn't used or called right 
   rcu_needs_cpu(). However, given that Xen is tickless (i.e., we stop all the
   timers when a CPU becomes idle), there's the problem of how to deal with the
   CPUs that have RCU callbacks queued. In fact, just using rcu_needs_cpu(), as
   we in fact do, means they'll spin, until the end of the grace period. 
   it's correct, we clearly don't want this, and yet, in patch 4, we let that

*) find a solution for the problem that using rcu_needs_cpu() (which is
   _correct_ and _necessary_) poses, for CPUs with queued callbacks. This is 
   patch 5 does. Basically, we introduce some kind of periodic tick. This timer,
   however, is only started on those CPUs, and only when they enter an idle 
   The idea is to make sure that, if they won't wake up soon enough (well, not
   that soon, period is 10 msec) by themselves, we'll poke them and give them a
   chance to check whether the grace period has actually ended, and invoke the

Without this series applied, this is the situation on x86, looking at when
complete_domain_destroy() is invoked via call_rcu, and at when it is actually
executed (printk added by me, for the sake of this experiment, they're not in
the patches):

(XEN) [  238.758551] XXX calling complete_domain_destroy(d5), from CPU 8, at 
238758535948 ns
(XEN) [  239.338958] XXX executing complete_domain_destroy(d5), from CPU 8, at 
239338943633 ns
With CPU load:
(XEN) [  322.740427] XXX calling complete_domain_destroy(d7), from CPU 13, at 
322740410755 ns
(XEN) [  322.742598] XXX executing complete_domain_destroy(d7), from CPU 13, at 
322742595289 ns

When the system is idle, the delay between the call and the execution is
significant (which is counterintuitive, as the system is idle!!). Moreover,
there's a big difference in the behavior of the system, between the idle and
the loaded case.

With this series applied, here's what happen:

(XEN) [  106.341590] XXX calling complete_domain_destroy(d1), from CPU 9, at 
106341573553 ns
(XEN) [  106.344391] XXX executing complete_domain_destroy(d1), from CPU 9, at 
106344387574 ns
With CPU load:
(XEN) [  176.166842] XXX calling complete_domain_destroy(d2), from CPU 13, at 
176166826183 ns
(XEN) [  176.167571] XXX executing complete_domain_destroy(d2), from CPU 13, at 
176167568269 ns

Which I call much better! :-)

This patch series addresses the XEN-27 issue, which I think Julien wants to
consider a blocker for 4.10:


There is a git branch available, with the series in, here:

git://xenbits.xen.org/people/dariof/xen.git rel/rcu/introduce-idle-enter-exit

And, finally, Stefano and Julien, I've compile tested, but have not runtime
tested the patches on ARM.

Thanks and Regards,
Dario Faggioli (5):
      xen: in do_softirq() sample smp_processor_id() once and for all.
      xen: ARM: suspend the tick (if in use) when going idle.
      xen: RCU/x86/ARM: discount CPUs that were idle when grace period started.
      xen: RCU: don't let a CPU with a callback go idle.
      xen: RCU: avoid busy waiting until the end of grace period.

 xen/arch/arm/domain.c         |   33 ++++++++++++-----
 xen/arch/x86/acpi/cpu_idle.c  |   31 +++++++++++-----
 xen/arch/x86/cpu/mwait-idle.c |   15 ++++++--
 xen/arch/x86/domain.c         |    8 ++++
 xen/common/rcupdate.c         |   80 +++++++++++++++++++++++++++++++++++++++--
 xen/common/softirq.c          |    8 +---
 xen/include/xen/rcupdate.h    |    6 +++
 xen/include/xen/sched.h       |    6 ++-
 8 files changed, 153 insertions(+), 34 deletions(-)
<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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