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 <l...@amacapital.net> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Comments below. With those fixed: Reviewed-by: Josh Triplett <j...@joshtriplett.org> > --- 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