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    SubjectRe: task isolation discussion at Linux Plumbers
    On Tue, Nov 8, 2016 at 5:40 PM, Paul E. McKenney
    <paulmck@linux.vnet.ibm.com> wrote:

    > commit 49961e272333ac720ac4ccbaba45521bfea259ae
    > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Date: Tue Nov 8 14:25:21 2016 -0800
    >
    > rcu: Maintain special bits at bottom of ->dynticks counter
    >
    > 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

    missing words here?

    >
    > 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_dynticks_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_dynticks_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.

    I'm inclined to suggest s/dynticks/eqs/ in the public API. To me,
    "dynticks" is a feature, whereas "eqs" means "extended quiescent
    state" and means something concrete about the CPU state

    >
    > Reported-by: Andy Lutomirski <luto@amacapital.net>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    >
    > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
    > index 4f9b2fa2173d..130d911e4ba1 100644
    > --- a/include/linux/rcutiny.h
    > +++ b/include/linux/rcutiny.h
    > @@ -33,6 +33,11 @@ static inline int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
    > return 0;
    > }
    >
    > +static inline bool rcu_dynticks_special_set(int cpu)
    > +{
    > + return false; /* Never flag non-existent other CPUs! */
    > +}
    > +
    > static inline unsigned long get_state_synchronize_rcu(void)
    > {
    > return 0;
    > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    > index dbf20b058f48..8de83830e86b 100644
    > --- a/kernel/rcu/tree.c
    > +++ b/kernel/rcu/tree.c
    > @@ -279,23 +279,36 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
    > };
    >
    > /*
    > + * Steal a bit from the bottom of ->dynticks for idle entry/exit
    > + * control. Initially this is for TLB flushing.
    > + */
    > +#define RCU_DYNTICK_CTRL_MASK 0x1
    > +#define RCU_DYNTICK_CTRL_CTR (RCU_DYNTICK_CTRL_MASK + 1)
    > +#ifndef rcu_dynticks_special_exit
    > +#define rcu_dynticks_special_exit() do { } while (0)
    > +#endif
    > +

    > /*
    > @@ -305,17 +318,21 @@ static void rcu_dynticks_eqs_enter(void)
    > static void rcu_dynticks_eqs_exit(void)
    > {
    > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
    > + int seq;
    >
    > /*
    > - * CPUs seeing atomic_inc() must see prior idle sojourns,
    > + * CPUs seeing atomic_inc_return() must see prior idle sojourns,
    > * and we also must force ordering with the next RCU read-side
    > * critical section.
    > */
    > - smp_mb__before_atomic(); /* See above. */
    > - atomic_inc(&rdtp->dynticks);
    > - smp_mb__after_atomic(); /* See above. */
    > + seq = atomic_inc_return(&rdtp->dynticks);
    > WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
    > - !(atomic_read(&rdtp->dynticks) & 0x1));
    > + !(seq & RCU_DYNTICK_CTRL_CTR));
    > + if (seq & RCU_DYNTICK_CTRL_MASK) {
    > + atomic_and(~RCU_DYNTICK_CTRL_MASK, &rdtp->dynticks);
    > + smp_mb__after_atomic(); /* Clear bits before acting on them */
    > + rcu_dynticks_special_exit();

    I think this needs to be reversed for NMI safety: do the callback and
    then clear the bits.

    > +/*
    > + * Set the special (bottom) bit of the specified CPU so that it
    > + * will take special action (such as flushing its TLB) on the
    > + * next exit from an extended quiescent state. Returns true if
    > + * the bit was successfully set, or false if the CPU was not in
    > + * an extended quiescent state.
    > + */
    > +bool rcu_dynticks_special_set(int cpu)
    > +{
    > + int old;
    > + int new;
    > + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
    > +
    > + do {
    > + old = atomic_read(&rdtp->dynticks);
    > + if (old & RCU_DYNTICK_CTRL_CTR)
    > + return false;
    > + new = old | ~RCU_DYNTICK_CTRL_MASK;

    Shouldn't this be old | RCU_DYNTICK_CTRL_MASK?

    > + } while (atomic_cmpxchg(&rdtp->dynticks, old, new) != old);
    > + return true;
    > }

    --Andy

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