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    SubjectRe: [PATCH v2 tip/core/rcu 01/39] rcu: Maintain special bits at bottom of ->dynticks counter
    On Mon, Apr 17, 2017 at 05:07:55PM -0700, Josh Triplett wrote:
    > On Mon, Apr 17, 2017 at 04:44:48PM -0700, Paul E. McKenney wrote:
    > > Currently, IPIs are used to force other CPUs to invalidate their TLBs
    > > in response to a kernel virtual-memory mapping change. This works, but
    > > degrades both battery lifetime (for idle CPUs) and real-time response
    > > (for nohz_full CPUs), and in addition results in unnecessary IPIs due to
    > > the fact that CPUs executing in usermode are unaffected by stale kernel
    > > mappings. It would be better to cause a CPU executing in usermode to
    > > wait until it is entering kernel mode to do the flush, first to avoid
    > > interrupting usemode tasks and second to handle multiple flush requests
    > > with a single flush in the case of a long-running user task.
    > >
    > > This commit therefore reserves a bit at the bottom of the ->dynticks
    > > counter, which is checked upon exit from extended quiescent states.
    > > If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
    > > invoked, which, if not supplied, is an empty single-pass do-while loop.
    > > If this bottom bit is set on -entry- to an extended quiescent state,
    > > then a WARN_ON_ONCE() triggers.
    > >
    > > This bottom bit may be set using a new rcu_eqs_special_set() function,
    > > which returns true if the bit was set, or false if the CPU turned
    > > out to not be in an extended quiescent state. Please note that this
    > > function refuses to set the bit for a non-nohz_full CPU when that CPU
    > > is executing in usermode because usermode execution is tracked by RCU
    > > as a dyntick-idle extended quiescent state only for nohz_full CPUs.
    > >
    > > Reported-by: Andy Lutomirski <luto@amacapital.net>
    > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    >
    > Comments below. With those fixed:
    > Reviewed-by: Josh Triplett <josh@joshtriplett.org>

    Good eyes, fixed!

    Thanx, Paul

    > > --- a/kernel/rcu/tree.c
    > > +++ b/kernel/rcu/tree.c
    > > @@ -290,15 +300,20 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
    > > static void rcu_dynticks_eqs_enter(void)
    > > {
    > > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
    > > - int special;
    > > + int seq;
    > >
    > > /*
    > > * CPUs seeing atomic_inc_return() must see prior RCU read-side
    > > * critical sections, and we also must force ordering with the
    > > * next idle sojourn.
    > > */
    > > - special = atomic_inc_return(&rdtp->dynticks);
    > > - WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && special & 0x1);
    > > + seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
    >
    > You changed atomic_inc_return to atomic_add_return here, but the comment
    > above still says atomic_inc_return.
    >
    > > @@ -308,15 +323,22 @@ static void rcu_dynticks_eqs_enter(void)
    > > static void rcu_dynticks_eqs_exit(void)
    > > {
    > > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
    > > - int special;
    > > + int seq;
    > >
    > > /*
    > > * CPUs seeing atomic_inc_return() must see prior idle sojourns,
    > > * and we also must force ordering with the next RCU read-side
    > > * critical section.
    > > */
    > > - special = atomic_inc_return(&rdtp->dynticks);
    > > - WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(special & 0x1));
    > > + seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
    >
    > Likewise.
    >
    > - Josh Triplett
    >

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