Re: [PATCH v2 2/2] rcu: Don't dump the stalled CPU on where RCU GP kthread last ran twice

[Paul E. McKenney]

On Wed, Jul 12, 2023 at 11:15:57PM +0800, Zhen Lei wrote:
> The stacks of all stalled CPUs will be dumped in rcu_dump_cpu_stacks().
> If the CPU on where RCU GP kthread last ran is stalled, its stack does
> not need to be dumped again. We can search the corresponding backtrace
> based on the printed CPU ID.
> 
> For example:
> [   87.328275] rcu: rcu_sched kthread starved for ... ->cpu=3  <--------|
> ... ...                                                                 |
> [   89.385007] NMI backtrace for cpu 3                         <--------|
> [   89.385179] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 5.10.0+ #22 <--|
> [   89.385188] Hardware name: linux,dummy-virt (DT)
> [   89.385196] pstate: 60000005 (nZCv daif -PAN -UAO -TCO BTYPE=--)
> [   89.385204] pc : arch_cpu_idle+0x40/0xc0
> [   89.385211] lr : arch_cpu_idle+0x2c/0xc0
> ... ...
> [   89.385566] Call trace:
> [   89.385574]  arch_cpu_idle+0x40/0xc0
> [   89.385581]  default_idle_call+0x100/0x450
> [   89.385589]  cpuidle_idle_call+0x2f8/0x460
> [   89.385596]  do_idle+0x1dc/0x3d0
> [   89.385604]  cpu_startup_entry+0x5c/0xb0
> [   89.385613]  secondary_start_kernel+0x35c/0x520
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

I queued both patches, thank you both!

							Thanx, Paul

> ---
>  kernel/rcu/tree_stall.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index dcfaa3d5db2cbc7..cc884cd49e026a3 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -534,12 +534,14 @@ static void rcu_check_gp_kthread_starvation(void)
>  		       data_race(READ_ONCE(rcu_state.gp_state)),
>  		       gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
>  		if (gpk) {
> +			struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> +
>  			pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
>  			pr_err("RCU grace-period kthread stack dump:\n");
>  			sched_show_task(gpk);
>  			if (cpu_is_offline(cpu)) {
>  				pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
> -			} else  {
> +			} else if (!(data_race(READ_ONCE(rdp->mynode->qsmask)) & rdp->grpmask)) {
>  				pr_err("Stack dump where RCU GP kthread last ran:\n");
>  				dump_cpu_task(cpu);
>  			}
> -- 
> 2.25.1
>