netdev - Re: [RFC net-next 1/6] netdevice: Add napi_affinity_no

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Message-ID: <Zrp50DnNfbOJoKr7@LQ3V64L9R2.home>
Date: Mon, 12 Aug 2024 22:08:32 +0100
From: Joe Damato <jdamato@...tly.com>
To: Stanislav Fomichev <sdf@...ichev.me>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Lorenzo Bianconi <lorenzo@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC net-next 1/6] netdevice: Add napi_affinity_no_change

On Mon, Aug 12, 2024 at 01:23:27PM -0700, Stanislav Fomichev wrote:
> On 08/12, Joe Damato wrote:
> > Several drivers have their own, very similar, implementations of
> > determining if IRQ affinity has changed. Create napi_affinity_no_change
> > to centralize this logic in the core.
> > 
> > This will be used in following commits for various drivers to eliminate
> > duplicated code.
> > 
> > Signed-off-by: Joe Damato <jdamato@...tly.com>
> > ---
> >  include/linux/netdevice.h |  8 ++++++++
> >  net/core/dev.c            | 14 ++++++++++++++
> >  2 files changed, 22 insertions(+)
> > 
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index 0ef3eaa23f4b..dc714a04b90a 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -464,6 +464,14 @@ enum rx_handler_result {
> >  typedef enum rx_handler_result rx_handler_result_t;
> >  typedef rx_handler_result_t rx_handler_func_t(struct sk_buff **pskb);
> >  
> > +/**
> > + * napi_affinity_no_change - determine if CPU affinity changed
> > + * @irq: the IRQ whose affinity may have changed
> > + *
> > + * Return true if the CPU affinity has NOT changed, false otherwise.
> > + */
> > +bool napi_affinity_no_change(unsigned int irq);
> > +
> >  void __napi_schedule(struct napi_struct *n);
> >  void __napi_schedule_irqoff(struct napi_struct *n);
> >  
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 751d9b70e6ad..9c56ad49490c 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -89,6 +89,7 @@
> >  #include <linux/errno.h>
> >  #include <linux/interrupt.h>
> >  #include <linux/if_ether.h>
> > +#include <linux/irq.h>
> >  #include <linux/netdevice.h>
> >  #include <linux/etherdevice.h>
> >  #include <linux/ethtool.h>
> > @@ -6210,6 +6211,19 @@ void __napi_schedule_irqoff(struct napi_struct *n)
> >  }
> >  EXPORT_SYMBOL(__napi_schedule_irqoff);
> >  
> > +bool napi_affinity_no_change(unsigned int irq)
> > +{
> > +	int cpu_curr = smp_processor_id();
> > +	const struct cpumask *aff_mask;
> > +
> 
> [..]
> 
> > +	aff_mask = irq_get_effective_affinity_mask(irq);
> 
> Most drivers don't seem to call this on every napi_poll (and
> cache the aff_mask somewhere instead). Should we try to keep this
> out of the past path as well?

Hm, I see what you mean. It looks like only gve calls it on every
poll, while the others use a cached value.

Maybe a better solution is to:
  1. Have the helper take the cached affinity mask from the driver
     and return true/false.
  2. Update gve to cache the mask (like the other 4 are doing).

FWIW, it seems i40e added this code to solve a specific bug [1] and
I would assume other drivers either hit the same issue (or were
inspired by i40e).

In general: I think the logic is here to stay and other drivers may
do something similar in the future.

It'd be nice to have one helper instead of several different
copies/implementations.

[1]: https://patchwork.ozlabs.org/project/intel-wired-lan/patch/1473895479-23035-9-git-send-email-bimmy.pujari@intel.com/