Re: [PATCH 10/13] srcutree: Skip callback scheduling for atomic SRCU grace periods

[Paul E. McKenney]

On Mon, Sep 07, 2026 at 03:58:26PM +0800, Kunwu Chan wrote:
> From: Kunwu Chan <kunwu.chan@xxxxxxxxx>
> 
> call_srcu() is forbidden on atomic srcu_struct, so srcu_gp_end() never
> has callbacks to invoke for them.  Yet it schedules callback invocation,
> which for atomic SRCU's SRCU_SIZE_SMALL state arms the boot CPU's
> ->delay_work timer every grace period, only for srcu_invoke_callbacks()
> to find nothing to do.
> 
> Skip this for atomic SRCU.
> 
> Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>

Nice optimization, thank you!  This does not matter for correctness,
and thus does not affect bisectability, so I have queued it as is for
testing and further review.

							Thanx, Paul

> ---
>  kernel/rcu/srcutree.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 2d2a12f86115..c19f59725706 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1018,10 +1018,10 @@ static void srcu_gp_end(struct srcu_struct *ssp, bool is_atomic)
>  
>  	/* Initiate callback invocation as needed. */
>  	ss_state = smp_load_acquire(&sup->srcu_size_state);
> -	if (ss_state < SRCU_SIZE_WAIT_BARRIER) {
> +	if (!is_atomic && ss_state < SRCU_SIZE_WAIT_BARRIER) {
>  		srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, get_boot_cpu_id()),
>  					cbdelay);
> -	} else {
> +	} else if (!is_atomic) {
>  		idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
>  		srcu_for_each_node_breadth_first(ssp, snp) {
>  			raw_spin_lock_irq_rcu_node(snp);
> -- 
> 2.43.0
>