netdev - Re: [PATCH net-next v7 1/2] Extend napi threaded polling to allow kthread based busy polling

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Message-ID: <CAAywjhRXXjJA1irEUtU4XM0KGtMMKAcyDtZz9WX8OzH42sehjw@mail.gmail.com>
Date: Mon, 25 Aug 2025 16:11:12 -0700
From: Samiullah Khawaja <skhawaja@...gle.com>
To: Stanislav Fomichev <stfomichev@...il.com>
Cc: Jakub Kicinski <kuba@...nel.org>, "David S . Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, almasrymina@...gle.com, 
	willemb@...gle.com, mkarsten@...terloo.ca, Joe Damato <joe@...a.to>, 
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next v7 1/2] Extend napi threaded polling to allow
 kthread based busy polling

On Mon, Aug 25, 2025 at 12:47 PM Stanislav Fomichev
<stfomichev@...il.com> wrote:
>
> On 08/24, Samiullah Khawaja wrote:
> > Add a new state to napi state enum:
> >
> > - NAPI_STATE_THREADED_BUSY_POLL
> >   Threaded busy poll is enabled/running for this napi.
> >
> > Following changes are introduced in the napi scheduling and state logic:
> >
> > - When threaded busy poll is enabled through sysfs or netlink it also
> >   enables NAPI_STATE_THREADED so a kthread is created per napi. It also
> >   sets NAPI_STATE_THREADED_BUSY_POLL bit on each napi to indicate that
> >   it is going to busy poll the napi.
> >
> > - When napi is scheduled with NAPI_STATE_SCHED_THREADED and associated
> >   kthread is woken up, the kthread owns the context. If
> >   NAPI_STATE_THREADED_BUSY_POLL and NAPI_STATE_SCHED_THREADED both are
> >   set then it means that kthread can busy poll.
> >
> > - To keep busy polling and to avoid scheduling of the interrupts, the
> >   napi_complete_done returns false when both NAPI_STATE_SCHED_THREADED
> >   and NAPI_STATE_THREADED_BUSY_POLL flags are set. Also
> >   napi_complete_done returns early to avoid the
> >   NAPI_STATE_SCHED_THREADED being unset.
> >
> > - If at any point NAPI_STATE_THREADED_BUSY_POLL is unset, the
> >   napi_complete_done will run and unset the NAPI_STATE_SCHED_THREADED
> >   bit also. This will make the associated kthread go to sleep as per
> >   existing logic.
> >
> > Signed-off-by: Samiullah Khawaja <skhawaja@...gle.com>
> > Reviewed-by: Willem de Bruijn <willemb@...gle.com>
> >
> > ---
> >  Documentation/ABI/testing/sysfs-class-net |  3 +-
> >  Documentation/netlink/specs/netdev.yaml   |  5 +-
> >  Documentation/networking/napi.rst         | 63 +++++++++++++++++++++-
> >  include/linux/netdevice.h                 | 11 +++-
> >  include/uapi/linux/netdev.h               |  1 +
> >  net/core/dev.c                            | 66 +++++++++++++++++++----
> >  net/core/dev.h                            |  3 ++
> >  net/core/net-sysfs.c                      |  2 +-
> >  net/core/netdev-genl-gen.c                |  2 +-
> >  tools/include/uapi/linux/netdev.h         |  1 +
> >  10 files changed, 142 insertions(+), 15 deletions(-)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-class-net b/Documentation/ABI/testing/sysfs-class-net
> > index ebf21beba846..15d7d36a8294 100644
> > --- a/Documentation/ABI/testing/sysfs-class-net
> > +++ b/Documentation/ABI/testing/sysfs-class-net
> > @@ -343,7 +343,7 @@ Date:             Jan 2021
> >  KernelVersion:       5.12
> >  Contact:     netdev@...r.kernel.org
> >  Description:
> > -             Boolean value to control the threaded mode per device. User could
> > +             Integer value to control the threaded mode per device. User could
> >               set this value to enable/disable threaded mode for all napi
> >               belonging to this device, without the need to do device up/down.
> >
> > @@ -351,4 +351,5 @@ Description:
> >               == ==================================
> >               0  threaded mode disabled for this dev
> >               1  threaded mode enabled for this dev
> > +             2  threaded mode enabled, and busy polling enabled.
>
> I might have asked already but forgot the answer: any reason we keep
> extending sysfs? With a proper ynl control over per-queue settings,
> why do we want an option to enable busy-polling threaded mode for the
> whole device?
That's great. One would enable threaded napi busy poll only for
certain napis and not for the whole device, so this makes perfect
sense. I am open to doing it.