On Fri, May 24, 2024 at 11:04 PM Jacob Keller <[email protected]> wrote:
> On 5/22/2024 4:12 PM, Michal Schmidt wrote:
> > 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
> >
>
> This is sent tagged for next, but the commit description reads more like
> it fixes deployments running irqbalance. Would it make sense to mark
> this with Fixes and target net instead?
I chose to not add a Fixes tag here. The similar commits for i40e and
other drivers did not have them either. If I had to add Fixes, the
referenced commit could be cdedef59deb0 ("ice: Configure VSIs for
Tx/Rx"), from 2018. But that's not good, because
irq_update_affinity_hint was added in 2021 with commit 65c7cdedeb30
("genirq: Provide new interfaces for affinity hints"). I'm OK with
next.
Michal
