netdev - Re: [PATCH v2 net-next 6/8] net: rps: change input_queue_tail_incr

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Date: Sun, 31 Mar 2024 00:07:37 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH v2 net-next 6/8] net: rps: change input_queue_tail_incr_save()

On Sun, Mar 31, 2024 at 12:01 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Sat, Mar 30, 2024 at 3:47 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > Hello Eric,
> >
> > On Fri, Mar 29, 2024 at 11:43 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > input_queue_tail_incr_save() is incrementing the sd queue_tail
> > > and save it in the flow last_qtail.
> > >
> > > Two issues here :
> > >
> > > - no lock protects the write on last_qtail, we should use appropriate
> > >   annotations.
> > >
> > > - We can perform this write after releasing the per-cpu backlog lock,
> > >   to decrease this lock hold duration (move away the cache line miss)
> > >
> > > Also move input_queue_head_incr() and rps helpers to include/net/rps.h,
> > > while adding rps_ prefix to better reflect their role.
> > >
> > > v2: Fixed a build issue (Jakub and kernel build bots)
> > >
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > ---
> > >  include/linux/netdevice.h | 15 ---------------
> > >  include/net/rps.h         | 23 +++++++++++++++++++++++
> > >  net/core/dev.c            | 20 ++++++++++++--------
> > >  3 files changed, 35 insertions(+), 23 deletions(-)
> > >
> > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > index 1c31cd2691d32064613836141fbdeeebc831b21f..14f19cc2616452d7e6afbbaa52f8ad3e61a419e9 100644
> > > --- a/include/linux/netdevice.h
> > > +++ b/include/linux/netdevice.h
> > > @@ -3249,21 +3249,6 @@ struct softnet_data {
> > >         call_single_data_t      defer_csd;
> > >  };
> > >
> > > -static inline void input_queue_head_incr(struct softnet_data *sd)
> > > -{
> > > -#ifdef CONFIG_RPS
> > > -       sd->input_queue_head++;
> > > -#endif
> > > -}
> > > -
> > > -static inline void input_queue_tail_incr_save(struct softnet_data *sd,
> > > -                                             unsigned int *qtail)
> > > -{
> > > -#ifdef CONFIG_RPS
> > > -       *qtail = ++sd->input_queue_tail;
> > > -#endif
> > > -}
> > > -
> > >  DECLARE_PER_CPU_ALIGNED(struct softnet_data, softnet_data);
> > >
> > >  static inline int dev_recursion_level(void)
> > > diff --git a/include/net/rps.h b/include/net/rps.h
> > > index 7660243e905b92651a41292e04caf72c5f12f26e..10ca25731c1ef766715fe7ee415ad0b71ec643a8 100644
> > > --- a/include/net/rps.h
> > > +++ b/include/net/rps.h
> > > @@ -122,4 +122,27 @@ static inline void sock_rps_record_flow(const struct sock *sk)
> > >  #endif
> > >  }
> > >
> > > +static inline u32 rps_input_queue_tail_incr(struct softnet_data *sd)
> > > +{
> > > +#ifdef CONFIG_RPS
> > > +       return ++sd->input_queue_tail;
> > > +#else
> > > +       return 0;
> > > +#endif
> > > +}
> > > +
> > > +static inline void rps_input_queue_tail_save(u32 *dest, u32 tail)
> > > +{
> > > +#ifdef CONFIG_RPS
> > > +       WRITE_ONCE(*dest, tail);
> > > +#endif
> > > +}
> >
> > I wonder if we should also call this new helper to WRITE_ONCE
> > last_qtail in the set_rps_cpu()?
> >
>
> Absolutely, I have another patch series to address remaining races
> (rflow->cpu, rflow->filter ...)
>
> I chose to make a small one, to ease reviews.

Great. Now I have no more questions :)

Thanks,
Jason