netdev - Re: [PATCH net 2/3] i40e: Fix not setting xps

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Date:   Wed, 28 Sep 2022 07:11:10 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Jaron, MichalX" <michalx.jaron@...el.com>
Cc:     "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Maziarz, Kamil" <kamil.maziarz@...el.com>,
        "G, GurucharanX" <gurucharanx.g@...el.com>,
        "Dziedziuch, SylwesterX" <sylwesterx.dziedziuch@...el.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>
Subject: Re: [PATCH net 2/3] i40e: Fix not setting xps_cpus after reset

On Wed, 28 Sep 2022 13:32:41 +0000 Jaron, MichalX wrote:
> > Not sure this is a fix, are there other drivers in the tree which do this? In the
> > drivers I work with IRQ mapping and XPS are just seemingly randomly reset
> > on reconfiguration changes. User space needs to rerun its affinitization script
> > after all changes it makes.
> > 
> > Apart from the fact that I don't think this is a fix, if we were to solve it we
> > should shoot for a more generic solution and not sprinkle all drivers with
> > #ifdef CONFIG_XPS blocks :S  
> 
> XPS to CPUs maps are configured by i40e driver, based on active cpus,
> after initialization or after drivers reset with reinit (i.e. when
> queues count changes). User may want to leave this mapping or set his
> own mapping by writing to xps_cpus file. In case when we do reset on
> our network interface without changing number of queues(when reinit
> is not true), i.e. by calling ethtool -t <interface>, in
> i40e_rebuild() those maps were cleared (set to 0) for every tx by
> netdev_set_num_tc(). After reset those maps were still set to 0
> despite that it was set by driver or by user and user was not
> informed about it.

Set to 0 or reset to default (which I would hope is spread across 
the CPUs in the same fashion as affinity hint)?

> With this fix maps are preserved and restored
> after reset to not surprise user that maps have changed when user
> doesn't want it. Mapping restoration is based on CPUs mapping and is
> done by netif_set_xps_queue() which is XPS function, then I think
> this affinization should be performed well.
> 
> If user doesn't want to change queues then those maps should be
> restored to the way it was.