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    SubjectRe: [PATCH 11/41] nohz/cpuset: Don't turn off the tick if rcu needs it
    On Thu, May 31, 2012 at 06:01:21PM +0200, Frederic Weisbecker wrote:
    > On Wed, May 23, 2012 at 09:27:39AM -0700, Paul E. McKenney wrote:
    > > On Wed, May 23, 2012 at 06:06:33PM +0200, Frederic Weisbecker wrote:
    > > > On Wed, May 23, 2012 at 08:15:42AM -0700, Paul E. McKenney wrote:
    > > > > On Wed, May 23, 2012 at 03:52:09PM +0200, Frederic Weisbecker wrote:
    > > > > > > > +#ifdef CONFIG_CPUSETS_NO_HZ
    > > > > > > > +static bool can_stop_adaptive_tick(void)
    > > > > > > > +{
    > > > > > > > + if (!sched_can_stop_tick())
    > > > > > > > + return false;
    > > > > > > > +
    > > > > > > > + /* Is there a grace period to complete ? */
    > > > > > > > + if (rcu_pending(smp_processor_id()))
    > > > > > >
    > > > > > > You lost me on this one. Why can't this be rcu_needs_cpu()?
    > > > > >
    > > > > > We already have an rcu_needs_cpu() check in tick_nohz_stop_sched_tick()
    > > > > > that prevents the tick to shut down if the CPU has local callbacks to handle.
    > > > > >
    > > > > > The rcu_pending() check is there in case some other CPU is waiting for the
    > > > > > current one to help completing a grace period, by reporting a quiescent state
    > > > > > for example. This happens because we may stop the tick in the kernel, not only
    > > > > > userspace. And if we are in the kernel, we still need to be part of the global
    > > > > > state machine.
    > > > >
    > > > > Ah! But RCU will notice that the CPU is in dyntick-idle mode, and will
    > > > > therefore take any needed quiescent-state action on that CPU's behalf.
    > > > > So there should be no need to call rcu_pending() anywhere outside of the
    > > > > RCU core code.
    > > >
    > > > No. If the tick is stopped and we are in the kernel, we may be using RCU
    > > > anytime, so we need to be part of the RCU core.
    > >
    > > OK, so the only problem is if we spend a long time CPU-bound in the kernel,
    > > where "long" is milliseconds or tens of milliseconds. In that case, the
    > > RCU core will notice that the CPU has not responded but is not idle, for
    > > example, in rcu_implicit_dynticks_qs(). It can take action at this point
    > > to get the offending CPU to pay attention to RCU.
    > >
    > > Does this make sense, or am I still missing something?
    >
    > Yeah that's exactly the purpose of the rcu_pending() check before shutting down
    > the tick and the IPI to wake it up.

    Hmmm... We appear to be talking past each other.

    If you use rcu_pending(), you defeat CONFIG_RCU_FAST_NO_HZ and thus fail
    to shut of the tick in situations where the application does a system
    call involving an RCU update every few tens of milliseconds. This is not
    good.

    What we should do instead is to call rcu_needs_cpu() instead of rcu_pending().
    In the common case of short system calls, this will allow the tick to be
    turned off a higher fraction of the time with no penalty. In the very
    unusual case where a system call runs CPU-bound for tens of milliseconds,
    RCU's existing force_quiescent_state() machinery can easily be used to
    force the CPU to pay attention to RCU.

    Make sense, or am I missing something?

    (And yes, the CONFIG_RCU_FAST_NO_HZ heuristics likely need to be adjusted
    to better support adaptive ticks -- try less hard to retire callbacks,
    for example.)

    Thanx, Paul



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