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

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Date:   Thu, 1 Oct 2020 20:03:22 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Felix Fietkau <nbd@....name>
Cc:     Wei Wang <weiwan@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        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 7:12 PM Felix Fietkau <nbd@....name> wrote:
>
> On 2020-10-01 19:01, Wei Wang wrote:
> > 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.
> But what if you have a device that's basically idle and napi isn't
> scheduled until much later? It will get a confusing loadavg until then.
> I'd prefer waking up the thread immediately and filtering going back to
> sleep once in the thread function before running the loop if
> NAPI_STATE_SCHED wasn't set.
>

I was not aware of this kthread_create() impact on loadavg.
This seems like a bug to me. (although I do not care about loadavg)

Do you have pointers on some documentation ?

Probably not a big deal, but this seems quite odd to me.