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    SubjectRe: [PATCH 1/4] nohz: Split extended quiescent state handling from nohz switch
    On Wed, Sep 07, 2011 at 07:22:33AM -0700, Paul E. McKenney wrote:
    > On Sun, Sep 04, 2011 at 04:36:43PM -0700, Paul E. McKenney wrote:
    > > On Sat, Aug 20, 2011 at 07:30:49PM +0200, Frederic Weisbecker wrote:
    > > > It is assumed that rcu won't be used once we switch to tickless
    > > > mode and until we restart the tick. However this is not always
    > > > true, as in x86-64 where we dereference the idle notifiers after
    > > > the tick is stopped.
    > > >
    > > > To prepare for fixing this, split the tickless mode switching and
    > > > RCU extended quiescent state logics.
    > > > Make tick_nohz_stop/restart_sched_tick() RCU agnostic but provide
    > > > a new pair of APIs tick_nohz_enter/exit_idle() that keep the
    > > > old behaviour by handling both the nohz mode and RCU extended
    > > > quiescent states, then convert every archs to use these.
    > > >
    > > > Archs that want to switch to RCU extended QS to some custom points
    > > > can do it later by changing the parameter in tick_nohz_enter,exit_idle()
    > > > to false and call rcu_enter,exit() separately.
    > >
    > > This approach looks quite good to me! A few comments below.
    >
    > But I get RCU stall warnings when running it on powerpc on top of
    > the patch set at:
    >
    > https://lkml.org/lkml/2011/9/7/64
    >
    > At first glance, it appears that CPUs are entering dyntick-idle
    > mode without RCU being informed. Any thoughts on diagnostics?
    >
    > Thanx, Paul

    I've just tested with your rcu/next branch from github and applied
    my patches on top of it but did not experience a stall in x86.

    That may be related to powerpc more precisely, or may be any arch
    but on some particular condition.


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