Re: [PATCH RFC 3/3] net: stmmac: Migrate IRQ balancing to cpumask_local_spread()

[Florian Bezdeka <florian.bezdeka@siemens.com>]

Hi Yury,

On Wed, 2026-08-19 at 14:45 -0400, Yury Norov wrote:
> On Wed, Aug 19, 2026 at 04:30:32PM +0200, Florian Bezdeka wrote:
> > The previous balancing mechanism was based on num_online_cpus(), which
> > is a problem for systems cpu-isolating workloads. IRQs were targeting
> > CPUs that were isolated for those sensitive workloads.
> > 
> > With a migration to cpumask_local_spread() we
> >     - get NUMA locality
> >     - honor the default SMP affinity mask, which avoids targeting
> >       isolated CPUs.
> > 
> > This also aligns with the pattern used by most network drivers dealing
> > with IRQ affinities / affinity hints.
> > 
> > Signed-off-by: Florian Bezdeka <florian.bezdeka@siemens.com>
> > ---
> >  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 21 +++++++++++++++++----
> >  1 file changed, 17 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> > index a71f0df263785dd8badc45292ca3067ab33bda05..949ced7e46d2814b57c6bd86b4886ac3bf33996c 100644
> > --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> > +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> > @@ -3826,11 +3826,16 @@ static int stmmac_request_irq_multi_msi(struct net_device *dev)
> >  	struct stmmac_priv *priv = netdev_priv(dev);
> >  	struct stmmac_msi *msi = priv->msi;
> >  	enum request_irq_err irq_err;
> > +	cpumask_var_t affinity;
> >  	int irq_idx = 0;
> >  	char *int_name;
> > +	int node;
> >  	int ret;
> >  	int i;
> >  
> > +	if (!zalloc_cpumask_var(&affinity, GFP_KERNEL))
> > +		return -ENOMEM;
> > +
> >  	/* For common interrupt */
> >  	int_name = msi->int_name_mac;
> >  	sprintf(int_name, "%s:%s", dev->name, "mac");
> > @@ -3916,6 +3921,7 @@ static int stmmac_request_irq_multi_msi(struct net_device *dev)
> >  	}
> >  
> >  	/* Request Rx MSI irq */
> > +	node = dev_to_node(&priv->dev->dev);
> >  	for (i = 0; i < priv->plat->rx_queues_to_use; i++) {
> >  		if (i >= MTL_MAX_RX_QUEUES)
> >  			break;
> > @@ -3935,8 +3941,10 @@ static int stmmac_request_irq_multi_msi(struct net_device *dev)
> >  			irq_idx = i;
> >  			goto irq_error;
> >  		}
> > -		irq_set_affinity_hint(msi->rx_irq[i],
> > -				      cpumask_of(i % num_online_cpus()));
> > +
> > +		cpumask_clear(affinity);
> > +		cpumask_set_cpu(cpumask_local_spread(i, node), affinity);
> > +		irq_set_affinity_and_hint(msi->rx_irq[i], affinity);
> 
> If you want to spread more than one IRQ, you'd better convert your
> loop into for_each_numa_hop_mask(). That way you don't need to
> introduce new function. See the comment on top of
> cpumask_local_spread():
> 

I was knowingly ignoring this "optimization". As already noted the patch
1 thread: I'm currently trying to demonstrate the shortcomings,
optimizations will follow once we agree on a plan / vision.