linux-kernel - Re: [PATCH net-next V2 06/11] net/mlx5e: SHAMPO: Separate pool for headers

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Message-ID: <as33bglnquchl7phggfc6pllohkwfcqe7hetfiazt2rtzm3ema@6mx5sipb6afg>
Date: Tue, 27 May 2025 15:53:18 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Jakub Kicinski <kuba@...nel.org>, Saeed Mahameed <saeed@...nel.org>
Cc: Tariq Toukan <tariqt@...dia.com>, 
	"David S. Miller" <davem@...emloft.net>, Paolo Abeni <pabeni@...hat.com>, 
	Eric Dumazet <edumazet@...gle.com>, Andrew Lunn <andrew+netdev@...n.ch>, 
	Saeed Mahameed <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>, 
	Richard Cochran <richardcochran@...il.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, netdev@...r.kernel.org, linux-rdma@...r.kernel.org, 
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org, Moshe Shemesh <moshe@...dia.com>, 
	Mark Bloch <mbloch@...dia.com>, Gal Pressman <gal@...dia.com>, Cosmin Ratiu <cratiu@...dia.com>
Subject: Re: [PATCH net-next V2 06/11] net/mlx5e: SHAMPO: Separate pool for
 headers

On Tue, May 27, 2025 at 08:29:56AM -0700, Jakub Kicinski wrote:
> On Thu, 22 May 2025 16:08:48 -0700 Saeed Mahameed wrote:
> > On 22 May 15:30, Jakub Kicinski wrote:
> > >On Fri, 23 May 2025 00:41:21 +0300 Tariq Toukan wrote:  
> > >> Allocate a separate page pool for headers when SHAMPO is enabled.
> > >> This will be useful for adding support to zc page pool, which has to be
> > >> different from the headers page pool.  
> > >
> > >Could you explain why always allocate a separate pool?  
> > 
> > Better flow management, 0 conditional code on data path to alloc/return
> > header buffers, since in mlx5 we already have separate paths to handle
> > header, we don't have/need bnxt_separate_head_pool() and
> > rxr->need_head_pool spread across the code.. 
> > 
> > Since we alloc and return pages in bulks, it makes more sense to manage
> > headers and data in separate pools if we are going to do it anyway for 
> > "undreadable_pools", and when there's no performance impact.
> 
> I think you need to look closer at the bnxt implementation.
> There is no conditional on the buffer alloc path. If the head and
> payload pools are identical we simply assign the same pointer to 
> (using mlx5 naming) page_pool and hd_page_pool.
> 
> Your arguments are not very convincing, TBH.
> The memory sitting in the recycling rings is very much not free.

I can add 2 more small argumens for always using 2 page pools:

- For large ring size + high MTU the page_pool size will go above the
  internal limit of the page_pool in HW GRO mode.

- Debugability (already mentioned by Saeed in the counters pach): if
  something goes wrong (page leaks for example) we can easily pinpoint
  to where the issue is.

Thanks,
Dragos