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SubjectRe: [PATCH] rcu: Add exp QS check in rcu_exp_handler() for no-preemptible expedited RCU
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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@intel.com>
> ---

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;
> }

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