linux-kernel - Re: [RFC net-next 0/6] Cleanup IRQ affinity checks in several drivers

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Message-ID: <ZrxZaHGDTO3ohHFH@LQ3V64L9R2.home>
Date: Wed, 14 Aug 2024 08:14:48 +0100
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Harshitha Ramamurthy <hramamurthy@...gle.com>,
	"moderated list:INTEL ETHERNET DRIVERS" <intel-wired-lan@...ts.osuosl.org>,
	Jeroen de Borst <jeroendb@...gle.com>,
	Jiri Pirko <jiri@...nulli.us>, Leon Romanovsky <leon@...nel.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:MELLANOX MLX4 core VPI driver" <linux-rdma@...r.kernel.org>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Praveen Kaligineedi <pkaligineedi@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Saeed Mahameed <saeedm@...dia.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Shailend Chand <shailend@...gle.com>,
	Tariq Toukan <tariqt@...dia.com>,
	Tony Nguyen <anthony.l.nguyen@...el.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Ziwei Xiao <ziweixiao@...gle.com>
Subject: Re: [RFC net-next 0/6] Cleanup IRQ affinity checks in several drivers

On Tue, Aug 13, 2024 at 05:17:10PM -0700, Jakub Kicinski wrote:
> On Mon, 12 Aug 2024 14:56:21 +0000 Joe Damato wrote:
> > Several drivers make a check in their napi poll functions to determine
> > if the CPU affinity of the IRQ has changed. If it has, the napi poll
> > function returns a value less than the budget to force polling mode to
> > be disabled, so that it can be rescheduled on the correct CPU next time
> > the softirq is raised.
> 
> Any reason not to use the irq number already stored in napi_struct ?

Thanks for taking a look.

IIUC, that's possible if i40e, iavf, and gve are updated to call
netif_napi_set_irq first, which I could certainly do.

But as Stanislav points out, I would be adding a call to
irq_get_effective_affinity_mask in the hot path where one did not
exist before for 4 of 5 drivers.

In that case, it might make more sense to introduce:

  bool napi_affinity_no_change(const struct cpumask *aff_mask)

instead and the drivers which have a cached mask can pass it in and
gve can be updated later to cache it.

Not sure how crucial avoiding the irq_get_effective_affinity_mask
call is; I would guess maybe some driver owners would object to
adding a new call in the hot path where one didn't exist before.

What do you think?