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

[Simon Horman <simon.horman@corigine.com>]

On Tue, May 16, 2023 at 06:59:35PM -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@nvidia.com>
> Fixes: 60bbf7eeef10 ("mlx5: use page_pool for xdp_return_frame call")
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
> I'm pointing Fixes at where page_pool was added, although that's
> probably not 100% fair.
> 
> v2:
>  - don't call napi_complete_done()
> v1: https://lore.kernel.org/all/20230512025740.1068965-1-kuba@kernel.org/

Reviewed-by: Simon Horman <simon.horman@corigine.com>