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    [W:0.022 / U:62.700 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site