On 6/22/2022 4:05 PM, Zqiang wrote:
In CONFIG_PREEMPT=n and CONFIG_PREEMPT_COUNT=y kernel, after a exp grace period begins, if detected current CPU enters idle in rcu_exp_handler() IPI handler, will immediately report the exp QS of the current cpu, at this time, maybe not being in an RCU read-side critical section, but need wait until rcu-softirq or sched-clock irq or sched-switch occurs on current CPU to check and report exp QS. This commit add a exp QS check in rcu_exp_handler(), when not being in an RCU read-side critical section, report exp QS earlier. Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx> ---
I am a bit unsure on this change. For non-preempt kernel, technically (and by design) context switch points are considered RCU read side critical section end? This change, changes that behavior between CONFIG_PREEMPT_COUNT=y and CONFIG_PREEMPT_COUNT=n case, for non-preemptive kernels.
However, I could be missing something here, and would wait for Paul's review.
Thanks Neeraj
kernel/rcu/tree_exp.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index be667583a554..34f08267410f 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -828,11 +828,14 @@ static void rcu_exp_handler(void *unused) { struct rcu_data *rdp = this_cpu_ptr(&rcu_data); struct rcu_node *rnp = rdp->mynode; + bool preempt_bh_disabled = + !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||__this_cpu_read(rcu_data.cpu_no_qs.b.exp)) return; - if (rcu_is_cpu_rrupt_from_idle()) { + if (rcu_is_cpu_rrupt_from_idle() || + (IS_ENABLED(CONFIG_PREEMPT_COUNT) && !preempt_bh_disabled)) {
rcu_report_exp_rdp(this_cpu_ptr(&rcu_data)); return; }
