lkml.org 
[lkml]   [2010]   [Nov]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] a local-timer-free version of RCU
On Mon, Nov 08, 2010 at 04:06:47PM +0100, Frederic Weisbecker wrote:
> On Sat, Nov 06, 2010 at 12:28:12PM -0700, Paul E. McKenney wrote:
> > On Fri, Nov 05, 2010 at 05:00:59PM -0400, Joe Korty wrote:
> > > +/**
> > > + * synchronize_sched - block until all CPUs have exited any non-preemptive
> > > + * kernel code sequences.
> > > + *
> > > + * This means that all preempt_disable code sequences, including NMI and
> > > + * hardware-interrupt handlers, in progress on entry will have completed
> > > + * before this primitive returns. However, this does not guarantee that
> > > + * softirq handlers will have completed, since in some kernels
> >
> > OK, so your approach treats preempt_disable code sequences as RCU
> > read-side critical sections by relying on the fact that the per-CPU
> > ->krcud task cannot run until such code sequences complete, correct?
> >
> > This seems to require that each CPU's ->krcud task be awakened at
> > least once per grace period, but I might well be missing something.
>
>
>
> I understood it differently, but I might also be wrong as well. krcud
> executes the callbacks, but it is only woken up for CPUs that want to
> execute callbacks, not for those that only signal a quiescent state,
> which is only determined in two ways through rcu_poll_other_cpus():
>
> - if the CPU is in an rcu_read_lock() critical section, it has the
> IN_RCU_READ_LOCK flag. If so then we set up its DO_RCU_COMPLETION flag so
> that it signals its quiescent state on rcu_read_unlock().
>
> - otherwise it's in a quiescent state.
>
>
> This works for rcu and rcu bh critical sections.

Unfortunately, local_irq_save() is allowed to stand in for
rcu_read_lock_bh(). :-/

> But this works in rcu sched critical sections only if rcu_read_lock_sched() has
> been called explicitly, otherwise that doesn't work (in preempt_disable(),
> local_irq_save(), etc...). I think this is what is not complete when
> Joe said it's not yet a complete rcu implementation.
>
> This is also the part that scaries me most :)

And if we can make all the the preempt_disable(), local_irq_disable(), ...
invoke rcu_read_lock(), then we have some chance of being able to dispense
with the IPIs to CPUs not having callbacks that need to be executed.

Thanx, Paul


\
 
 \ /
  Last update: 2010-11-08 20:51    [from the cache]
©2003-2014 Jasper Spaans. hosted at Digital Ocean