lkml.org 
[lkml]   [2011]   [Sep]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: linux-next-20110923: warning kernel/rcutree.c:1833
    On Sun, Sep 25, 2011 at 06:41:18PM -0700, Paul E. McKenney wrote:
    > On Sun, Sep 25, 2011 at 06:26:11PM -0700, Paul E. McKenney wrote:
    > > On Mon, Sep 26, 2011 at 03:10:33AM +0200, Frederic Weisbecker wrote:
    > > > 2011/9/26 Frederic Weisbecker <fweisbec@gmail.com>:
    > > > > On Sun, Sep 25, 2011 at 09:48:04AM -0700, Paul E. McKenney wrote:
    > > > >> This is required for RCU_FAST_NO_HZ, which checks to see whether the
    > > > >> current CPU can accelerate the current grace period so as to enter
    > > > >> dyntick-idle mode sooner than it would otherwise.  This takes effect
    > > > >> in the situation where rcu_needs_cpu() sees that there are callbacks.
    > > > >> It then notes a quiescent state (which is illegal in an RCU read-side
    > > > >> critical section), calls force_quiescent_state(), and so on.  For this
    > > > >> to work, the current CPU must be in an RCU read-side critical section.
    > > > >
    > > > > You mean it must *not* be in an RCU read-side critical section (ie: in a
    > > > > quiescent state)?
    > > > >
    > > > > That assumption at least fails anytime in idle for the RCU
    > > > > sched flavour given that preemption is disabled in the idle loop.
    > > > >
    > > > >> If this cannot be made to work, another option is to call a new RCU
    > > > >> function in the case where rcu_needs_cpu() returned false, but after
    > > > >> the RCU read-side critical section has exited.
    > > > >
    > > > > You mean when rcu_needs_cpu() returns true (when we have callbacks
    > > > > enqueued)?
    > > > >
    > > > >> This new RCU function
    > > > >> could then attempt to rearrange RCU so as to allow the CPU to enter
    > > > >> dyntick-idle mode more quickly.  It is more important for this to
    > > > >> happen when the CPU is going idle than when it is executing a user
    > > > >> process.
    > > > >>
    > > > >> So, is this doable?
    > > > >
    > > > > At least not when we have RCU sched callbacks enqueued, given preemption
    > > > > is disabled. But that sounds plausible in order to accelerate the switch
    > > > > to dyntick-idle mode when we only have rcu and/or rcu bh callbacks.
    > > >
    > > > But the RCU sched case could be dealt with if we embrace every use of
    > > > it with rcu_read_lock_sched() and rcu_read_unlock_sched(), or some light
    > > > version that just increases a local counter that rcu_needs_cpu() could check.
    > > >
    > > > It's an easy thing to add: we can ensure preempt is disabled when we call it
    > > > and we can force rcu_dereference_sched() to depend on it.
    > >
    > > Or just check to see if this is the first level of interrupt from the
    > > idle task after the scheduler is up.
    >
    > Hmmm... Is it the case that rcu_needs_cpu() gets called from within an
    > RCU read-side critical section only when called from an interrupt that
    > interrupted an RCU read-side critical section (keeping in mind that the
    > idle loop is a quiescent state regardless of preemption)?
    >
    > If so, I should be able to do the appropriate checks within
    > rcu_needs_cpu().

    It sounds better to me if you can do all the checks from rcu_needs_cpu()
    so that all you need is to wait for another jiffy to escape the read side
    critical section.

    Doing something from the read side exit path would require some weird
    trickiness.
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2011-09-26 11:45    [W:0.026 / U:91.940 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site