linux-kernel - Re: [PATCH net] xsk: Bring back busy polling support

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Message-ID: <Z4AIGWGVrEfk1yvE@LQ3V64L9R2>
Date: Thu, 9 Jan 2025 09:32:09 -0800
From: Joe Damato <jdamato@...tly.com>
To: Stanislav Fomichev <sdf@...ichev.me>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org, horms@...nel.org, ast@...nel.org,
	daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com,
	bjorn@...nel.org, magnus.karlsson@...el.com,
	maciej.fijalkowski@...el.com, jonathan.lemon@...il.com,
	mkarsten@...terloo.ca, alazar@...dia.com
Subject: Re: [PATCH net] xsk: Bring back busy polling support

On Wed, Jan 08, 2025 at 04:34:36PM -0800, Stanislav Fomichev wrote:
> Commit 86e25f40aa1e ("net: napi: Add napi_config") moved napi->napi_id
> assignment to a later point in time (napi_hash_add_with_id). This breaks
> __xdp_rxq_info_reg which copies napi_id at an earlier time and now
> stores 0 napi_id. It also makes sk_mark_napi_id_once_xdp and
> __sk_mark_napi_id_once useless because they now work against 0 napi_id.
> Since sk_busy_loop requires valid napi_id to busy-poll on, there is no way
> to busy-poll AF_XDP sockets anymore.
> 
> Bring back the ability to busy-poll on XSK by resolving socket's napi_id
> at bind time. This relies on relatively recent netif_queue_set_napi,
> but (assume) at this point most popular drivers should have been converted.
> This also removes per-tx/rx cycles which used to check and/or set
> the napi_id value.
> 
> Confirmed by running a busy-polling AF_XDP socket
> (github.com/fomichev/xskrtt) on mlx5 and looking at BusyPollRxPackets
> from /proc/net/netstat.

Thanks Stanislav for finding and fixing this.

I've CC'd Alex who reported a bug a couple weeks ago that might be
fixed by this change.

Alex: would you mind applying this patch to your tree to see if this
solves the issue you reported [1] ?

[1]: https://lore.kernel.org/netdev/DM8PR12MB5447837576EA58F490D6D4BFAD052@DM8PR12MB5447.namprd12.prod.outlook.com/