netdev - RE: [Intel-wired-lan] [PATCH iwl-next] ice: use irq_update_affinity

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Date: Tue, 28 May 2024 16:00:29 +0000
From: "Pucha, HimasekharX Reddy" <himasekharx.reddy.pucha@...el.com>
To: mschmidt <mschmidt@...hat.com>, "Brandeburg, Jesse"
	<jesse.brandeburg@...el.com>, "Nguyen, Anthony L"
	<anthony.l.nguyen@...el.com>, "intel-wired-lan@...ts.osuosl.org"
	<intel-wired-lan@...ts.osuosl.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, Thomas Gleixner
	<tglx@...utronix.de>, Nitesh Narayan Lal <nitesh@...hat.com>
Subject: RE: [Intel-wired-lan] [PATCH iwl-next] ice: use
 irq_update_affinity_hint()

> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@...osl.org> On Behalf Of Michal Schmidt
> Sent: Thursday, May 23, 2024 4:43 AM
> To: Brandeburg, Jesse <jesse.brandeburg@...el.com>; Nguyen, Anthony L <anthony.l.nguyen@...el.com>; intel-wired-lan@...ts.osuosl.org
> Cc: netdev@...r.kernel.org; Thomas Gleixner <tglx@...utronix.de>; Nitesh Narayan Lal <nitesh@...hat.com>
> Subject: [Intel-wired-lan] [PATCH iwl-next] ice: use irq_update_affinity_hint()
>
> irq_set_affinity_hint() is deprecated. Use irq_update_affinity_hint() instead. This removes the side-effect of actually applying the affinity.
>
> The driver does not really need to worry about spreading its IRQs across CPUs. The core code already takes care of that.
> On the contrary, when the driver applies affinities by itself, it breaks the users' expectations:
> 1. The user configures irqbalance with IRQBALANCE_BANNED_CPULIST in
>     order to prevent IRQs from being moved to certain CPUs that run a
>     real-time workload.
>  2. ice reconfigures VSIs at runtime due to a MIB change
>     (ice_dcb_process_lldp_set_mib_change). Reopening a VSI resets the
>     affinity in ice_vsi_req_irq_msix().
>  3. ice has no idea about irqbalance's config, so it may move an IRQ to
>     a banned CPU. The real-time workload suffers unacceptable latency.
>
> I am not sure if updating the affinity hints is at all useful, because irqbalance ignores them since 2016 ([1]), but at least it's harmless.
>
> This ice change is similar to i40e commit d34c54d1739c ("i40e: Use irq_update_affinity_hint()").
>
> [1] https://github.com/Irqbalance/irqbalance/commit/dcc411e7bfdd
>
> Signed-off-by: Michal Schmidt <mschmidt@...hat.com>
> ---
>  drivers/net/ethernet/intel/ice/ice_lib.c  | 4 ++--  drivers/net/ethernet/intel/ice/ice_main.c | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
>

Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@...el.com> (A Contingent worker at Intel)