On Tue, Apr 26, 2022 at 03:00:31PM +0800, Zqiang wrote:
> At present, there are two situations which the rcu callback function
> be exectued in the kthreads, one is if the use_softirq is set to zero,
> the RCU_SOFTIRQ processing is carried out by the per-CPU rcuc
> kthreads, for non-offload rdp, the rdp's rcu callback function be
> exectued in rcuc kthreads. another one is if the rdp is set to
> offloaded, the rdp's rcu callback function be exectued in the rcuop kthreads.
>
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
>Good catch!
>
>From what I can see, this affects only tracing. Or did I miss a use case?
Yes it is affects tracing, it was add make tracing info more accurate.
>
>Also, should the two definitions of rcu_is_callbacks_kthread() be merged?
>It looks like the current situation dates back to when the only time rcuc kthreads were created was in kernels built with CONFIG_RCU_BOOST=y.
>If so, the definition of this function should move from tree_plugin.h to tree.c.
Yes it should not depend on CONFIG_RCU_BOOST option.
>
> Thanx, Paul
> ---
> v1->v2:
> fix compilation error when CONFIG_RCU_NOCB_CPU is no define
>
> kernel/rcu/tree.c | 4 ++--
> kernel/rcu/tree.h | 2 +-
> kernel/rcu/tree_plugin.h | 12 +++++++++---
> 3 files changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index
> 5c587e00811c..9dc4c4e82db6 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2610,7 +2610,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
> trace_rcu_batch_end(rcu_state.name, 0,
> !rcu_segcblist_empty(&rdp->cblist),
> need_resched(), is_idle_task(current),
> - rcu_is_callbacks_kthread());
> + rcu_is_callbacks_kthread(rdp));
> return;
> }
>
> @@ -2688,7 +2688,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
> rcu_nocb_lock_irqsave(rdp, flags);
> rdp->n_cbs_invoked += count;
> trace_rcu_batch_end(rcu_state.name, count, !!rcl.head, need_resched(),
> - is_idle_task(current), rcu_is_callbacks_kthread());
> + is_idle_task(current), rcu_is_callbacks_kthread(rdp));
>
> /* Update counts and requeue any remaining callbacks. */
> rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl); diff --git
> a/kernel/rcu/tree.h b/kernel/rcu/tree.h index
> 996387962de3..3cdc18997a38 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -433,7 +433,7 @@ static void rcu_flavor_sched_clock_irq(int user);
> static void dump_blkd_tasks(struct rcu_node *rnp, int ncheck); static
> void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);
> static void rcu_preempt_boost_start_gp(struct rcu_node *rnp); -static
> bool rcu_is_callbacks_kthread(void);
> +static bool rcu_is_callbacks_kthread(struct rcu_data *rdp);
> static void rcu_cpu_kthread_setup(unsigned int cpu); static void
> rcu_spawn_one_boost_kthread(struct rcu_node *rnp); static bool
> rcu_preempt_has_tasks(struct rcu_node *rnp); diff --git
> a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index
> 971bb6a00ede..120bade40e02 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1155,9 +1155,15 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
> * Is the current CPU running the RCU-callbacks kthread?
> * Caller must have preemption disabled.
> */
> -static bool rcu_is_callbacks_kthread(void)
> +static bool rcu_is_callbacks_kthread(struct rcu_data *rdp)
> {
> - return __this_cpu_read(rcu_data.rcu_cpu_kthread_task) == current;
> + bool ret;
> +#ifdef CONFIG_RCU_NOCB_CPU
> + ret = rdp->rcu_cpu_kthread_task == current || rdp->nocb_cb_kthread
> +== current; #else
> + ret = rdp->rcu_cpu_kthread_task == current; #endif
>
>The problem here is that the first part of that condition is duplicated.
>This is an accident waiting to happen when someone fixes one side of that #ifdef without also adjusting the other side. One approach is to define a function that tests "nocb_cb_kthread == current" in CONFIG_RCU_NOCB_CPU=y kernels and just returns "false" otherwise.
Thanks for your suggestion, I will resend.
>
>Alternatively, you could make ->nocb_cb_kthread defined unconditionally, but left always NULL in CONFIG_RCU_NOCB_CPU=n kernels.
> + return ret;
> }
>
> #define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY *
> HZ, 1000) @@ -1242,7 +1248,7 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags); }
>
> -static bool rcu_is_callbacks_kthread(void)
> +static bool rcu_is_callbacks_kthread(struct rcu_data *rdp)
> {
> return false;
> }
> --
> 2.25.1
>
