netdev - Re: [Intel-wired-lan] [PATCH 1/1 iwl-net] idpf: disable local BH when scheduling napi for marker packets

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Message-ID: <2e3001f8-a079-4d44-863f-979baca3b38c@intel.com>
Date: Tue, 13 Feb 2024 14:16:47 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Alan Brady <alan.brady@...el.com>
CC: <intel-wired-lan@...ts.osuosl.org>, Przemek Kitszel
	<przemyslaw.kitszel@...el.com>, <netdev@...r.kernel.org>, Emil Tantilov
	<emil.s.tantilov@...el.com>, Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH 1/1 iwl-net] idpf: disable local BH when
 scheduling napi for marker packets

From: Alan Brady <alan.brady@...el.com>
Date: Wed,  7 Feb 2024 16:42:43 -0800

> From: Emil Tantilov <emil.s.tantilov@...el.com>
> 
> Fix softirq's not being handled during napi_schedule() call when
> receiving marker packets for queue disable by disabling local bottom
> half.

BTW, how exactly does this help?

__napi_schedule() already disables interrupts (local_irq_save()).
napi_schedule_prep() only has READ_ONCE() and other atomic read/write
helpers.

It's always been safe to call napi_schedule() with enabled BH, so I
don't really understand how this works.

> 
> The issue can be seen on ifdown:
> NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #08!!!
> 
> Using ftrace to catch the failing scenario:
> ifconfig   [003] d.... 22739.830624: softirq_raise: vec=3 [action=NET_RX]
> <idle>-0   [003] ..s.. 22739.831357: softirq_entry: vec=3 [action=NET_RX]
> 
> No interrupt and CPU is idle.
> 
> After the patch, with BH locks:
> ifconfig   [003] d.... 22993.928336: softirq_raise: vec=3 [action=NET_RX]
> ifconfig   [003] ..s1. 22993.928337: softirq_entry: vec=3 [action=NET_RX]
> 
> Fixes: c2d548cad150 ("idpf: add TX splitq napi poll support")
> Reviewed-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> Signed-off-by: Emil Tantilov <emil.s.tantilov@...el.com>
> Signed-off-by: Alan Brady <alan.brady@...el.com>
> ---
>  drivers/net/ethernet/intel/idpf/idpf_virtchnl.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c b/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c
> index d0cdd63b3d5b..390977a76de2 100644
> --- a/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c
> +++ b/drivers/net/ethernet/intel/idpf/idpf_virtchnl.c
> @@ -2087,8 +2087,10 @@ int idpf_send_disable_queues_msg(struct idpf_vport *vport)
>  		set_bit(__IDPF_Q_POLL_MODE, vport->txqs[i]->flags);
>  
>  	/* schedule the napi to receive all the marker packets */
> +	local_bh_disable();
>  	for (i = 0; i < vport->num_q_vectors; i++)
>  		napi_schedule(&vport->q_vectors[i].napi);
> +	local_bh_enable();
>  
>  	return idpf_wait_for_marker_event(vport);
>  }

Thanks,
Olek