netdev - Re: [PATCH net-next 5/5] net: improve napi threaded config

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Message-ID: <CAEA6p_AsJuGb3C2MmWNDQYaZQtcCQc2CHdqcSPiH9i9NmPZMdQ@mail.gmail.com>
Date:   Thu, 1 Oct 2020 10:01:06 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Felix Fietkau <nbd@....name>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next 5/5] net: improve napi threaded config

On Thu, Oct 1, 2020 at 3:01 AM Felix Fietkau <nbd@....name> wrote:
>
>
> On 2020-09-30 21:21, Wei Wang wrote:
> > This commit mainly addresses the threaded config to make the switch
> > between softirq based and kthread based NAPI processing not require
> > a device down/up.
> > It also moves the kthread_create() call to the sysfs handler when user
> > tries to enable "threaded" on napi, and properly handles the
> > kthread_create() failure. This is because certain drivers do not have
> > the napi created and linked to the dev when dev_open() is called. So
> > the previous implementation does not work properly there.
> >
> > Signed-off-by: Wei Wang <weiwan@...gle.com>
> > ---
> > Changes since RFC:
> > changed the thread name to napi/<dev>-<napi-id>
> >
> >  net/core/dev.c       | 49 +++++++++++++++++++++++++-------------------
> >  net/core/net-sysfs.c |  9 +++-----
> >  2 files changed, 31 insertions(+), 27 deletions(-)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index b4f33e442b5e..bf878d3a9d89 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -1490,17 +1490,24 @@ EXPORT_SYMBOL(netdev_notify_peers);
> >
> >  static int napi_threaded_poll(void *data);
> >
> > -static void napi_thread_start(struct napi_struct *n)
> > +static int napi_kthread_create(struct napi_struct *n)
> >  {
> > -     if (test_bit(NAPI_STATE_THREADED, &n->state) && !n->thread)
> > -             n->thread = kthread_create(napi_threaded_poll, n, "%s-%d",
> > -                                        n->dev->name, n->napi_id);
> > +     int err = 0;
> > +
> > +     n->thread = kthread_create(napi_threaded_poll, n, "napi/%s-%d",
> > +                                n->dev->name, n->napi_id);
> > +     if (IS_ERR(n->thread)) {
> > +             err = PTR_ERR(n->thread);
> > +             pr_err("kthread_create failed with err %d\n", err);
> > +             n->thread = NULL;
> > +     }
> > +
> > +     return err;
> If I remember correctly, using kthread_create with no explicit first
> wakeup means the task will sit there and contribute to system loadavg
> until it is woken up the first time.
> Shouldn't we use kthread_run here instead?
>

Right. kthread_create() basically creates the thread and leaves it in
sleep mode. I think that is what we want. We rely on the next
___napi_schedule() call to wake up this thread when there is work to
do.

> - Felix