Hello, Rik,
And it turns out that I did not in fact do the recheck at IPI time.
The (untested) patch below is an alleged fix. Thoughts?
Thanx, Paul
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commit e53e0b3e7b3c783962f9461bcb9aa8bc3e3a8688
Author: Paul E. McKenney <[email protected]>
Date: Tue Oct 11 06:09:59 2016 -0700
rcu: Make expedited grace periods recheck dyntick idle state
Expedited grace periods check dyntick-idle state, and avoid sending
IPIs to idle CPUs, including those running guest OSes, and, on NOHZ_FULL
kernels, nohz_full CPUs. However, the kernel has been observed checking
a CPU while it was non-idle, but sending the IPI after it has gone
idle. This commit therefore rechecks idle state immediately before
sending the IPI, refraining from IPIing CPUs that have since gone idle.
Reported-by: Rik van Riel <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e99a5234d9ed..fe98dd24adf8 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -404,6 +404,7 @@ struct rcu_data {
atomic_long_t exp_workdone1; /* # done by others #1. */
atomic_long_t exp_workdone2; /* # done by others #2. */
atomic_long_t exp_workdone3; /* # done by others #3. */
+ int exp_dynticks_snap; /* Double-check need for IPI. */
/* 7) Callback offloading. */
#ifdef CONFIG_RCU_NOCB_CPU
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 24343eb87b58..d3053e99fdb6 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -358,8 +358,10 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+ rdp->exp_dynticks_snap =
+ atomic_add_return(0, &rdtp->dynticks);
if (raw_smp_processor_id() == cpu ||
- !(atomic_add_return(0, &rdtp->dynticks) & 0x1) ||
+ !(rdp->exp_dynticks_snap & 0x1) ||
!(rnp->qsmaskinitnext & rdp->grpmask))
mask_ofl_test |= rdp->grpmask;
}
@@ -377,9 +379,17 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
/* IPI the remaining CPUs for expedited quiescent state. */
for_each_leaf_node_possible_cpu(rnp, cpu) {
unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
+ struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
+ struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+
if (!(mask_ofl_ipi & mask))
continue;
retry_ipi:
+ if (atomic_add_return(0, &rdtp->dynticks) !=
+ rdp->exp_dynticks_snap) {
+ mask_ofl_test |= mask;
+ continue;
+ }
ret = smp_call_function_single(cpu, func, rsp, 0);
if (!ret) {
mask_ofl_ipi &= ~mask;
