Re: [PATCH tip/core/rcu 03/12] rcu: Force on tick when invoking lots of callbacks

Frederic Weisbecker
2019-10-03T14:10:57Z

On Wed, Oct 02, 2019 at 06:38:54PM -0700, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@linux.ibm.com>
>
> Callback invocation can run for a significant time period, and within
> CONFIG_NO_HZ_FULL=y kernels, this period will be devoid of scheduler-clock
> interrupts. In-kernel execution without such interrupts can cause all
> manner of malfunction, with RCU CPU stall warnings being but one result.
>
> This commit therefore forces scheduling-clock interrupts on whenever more
> than a few RCU callbacks are invoked. Because offloaded callback invocation
> can be preempted, this forcing is withdrawn on each context switch. This
> in turn requires that the loop invoking RCU callbacks reiterate the forcing
> periodically.
>
> [ paulmck: Apply Joel Fernandes TICK_DEP_MASK_RCU->TICK_DEP_BIT_RCU fix. ]
> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> ---
> kernel/rcu/tree.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8110514..db673ae 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2151,6 +2151,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> rcu_nocb_unlock_irqrestore(rdp, flags);
>
> /* Invoke callbacks. */
> + if (IS_ENABLED(CONFIG_NO_HZ_FULL))

No need for the IS_ENABLED(), the API takes care of that.

> + tick_dep_set_task(current, TICK_DEP_BIT_RCU);
> rhp = rcu_cblist_dequeue(&rcl);
> for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
> debug_rcu_head_unqueue(rhp);
> @@ -2217,6 +2219,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> /* Re-invoke RCU core processing if there are callbacks remaining. */
> if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist))
> invoke_rcu_core();
> + if (IS_ENABLED(CONFIG_NO_HZ_FULL))

Same here.

Thanks.

> + tick_dep_clear_task(current, TICK_DEP_BIT_RCU);
> }
>
> /*
> --
> 2.9.5
>