netdev - Re: [PATCH net v2] net/mlx5e: do as little as possible in napi poll when budget is 0

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Date: Tue, 16 May 2023 19:04:52 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: davem@...emloft.net
Cc: netdev@...r.kernel.org, edumazet@...gle.com, pabeni@...hat.com, Tariq
 Toukan <tariqt@...dia.com>, saeedm@...dia.com, leon@...nel.org,
 brouer@...hat.com
Subject: Re: [PATCH net v2] net/mlx5e: do as little as possible in napi poll
 when budget is 0

On Tue, 16 May 2023 18:59:35 -0700 Jakub Kicinski wrote:
> NAPI gets called with budget of 0 from netpoll, which has interrupts
> disabled. We should try to free some space on Tx rings and nothing
> else.
> 
> Specifically do not try to handle XDP TX or try to refill Rx buffers -
> we can't use the page pool from IRQ context. Don't check if IRQs moved,
> either, that makes no sense in netpoll. Netpoll calls _all_ the rings
> from whatever CPU it happens to be invoked on.
> 
> In general do as little as possible, the work quickly adds up when
> there's tens of rings to poll.
> 
> The immediate stack trace I was seeing is:
> 
>     __do_softirq+0xd1/0x2c0
>     __local_bh_enable_ip+0xc7/0x120
>     </IRQ>
>     <TASK>
>     page_pool_put_defragged_page+0x267/0x320
>     mlx5e_free_xdpsq_desc+0x99/0xd0
>     mlx5e_poll_xdpsq_cq+0x138/0x3b0
>     mlx5e_napi_poll+0xc3/0x8b0
>     netpoll_poll_dev+0xce/0x150
> 
> AFAIU page pool takes a BH lock, releases it and since BH is now
> enabled tries to run softirqs.
> 
> Reviewed-by: Tariq Toukan <tariqt@...dia.com>
> Fixes: 60bbf7eeef10 ("mlx5: use page_pool for xdp_return_frame call")
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>

There's a condition which should be simplified later on:

		if (budget && work_done == budget)
			work_done--;

but I think we may be better off doing that as a follow up,
to keep this patch short and readable.