On Wed, Sep 05, 2012 at 11:48:40PM +0200, Peter Zijlstra wrote:
> On Wed, 2012-09-05 at 14:39 -0700, Paul E. McKenney wrote:
> > RCU callback execution can add significant OS jitter and also can degrade
> > scheduling latency. This commit therefore adds the ability for selected
> > CPUs ("rcu_nocbs=" boot parameter) to have their callbacks offloaded to
> > kthreads. If the "rcu_nocb_poll" boot parameter is also specified, these
> > kthreads will do polling, removing the need for the offloaded CPUs to do
> > wakeups. At least one CPU must be doing normal callback processing:
> > currently CPU 0 cannot be selected as a no-CBs CPU. In addition, attempts
> > to offline the last normal-CBs CPU will fail.
> >
> > This is an experimental patch, so just FYI for the moment. Known
> > shortcomings include:
> >
> > o The counters should be atomic_long_t rather than atomic_t.
> >
> > o No-CBs CPUs can be configured only at boot time.
> >
> > o Only a modest number of CPUs can be configured as no-CBs CPUs.
> > Definitely a few tens, perhaps a few hundred, but no way thousands.
> >
> > o At least one CPU must remain a normal-CBs CPU.
> >
> > o Not much in the way of energy-efficiency features, though there
> > are some natural energy savings inherent in the implementation
> >
> > o The per-no-CBs-CPU kthreads are not subject to RCU priority
> > boosting.
> >
> > o Care is required when setting the kthreads to RT priority.
> >
> > Later versions will address some of them, but others are likely to remain.
>
> My LPC feedback in writing...
>
> So I see RCU as consisting of two parts:
> A) Grace period tracking,
> 2) Running the callbacks.
>
> This series seems to conflate the two, it talks of doing the callbacks
> elsewhere (kthread), but it also moves the grace period detectoring into
> the same kthread.
>
> The latter part is what complicates the thing. I'd suggest doing the
> very simple callbacks only implementation first and leaving the grace
> period machinery in the tick.
>
> Its typically the callbacks that consume most CPU time, whereas the
> grace period computations, while tricky and subtle, are relatively
> cheap.
>
> In particular, it solves the need to wait for grace periods from the
> kthread (and bounce that no-nocb cpu to make progress), and it makes the
> atomic list operations stuff a lot easier.
I was excited by this possibility when you first mentioned it, but
the low-OS-jitter fans are going to need the grace-period computation
to be offloaded as well. So if I use your (admittedly much simpler)
approach, I get to rewrite it when Frederic's adaptive-ticks work goes
in. Given that this is probably happening relatively soon, it would be
better if I just did the implementation that will be needed long-term,
rather than rewriting.
Though I am sure that people will be sad about fewer RCU patches. ;-)
Thanx, Paul
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