Date:   Thu, 1 Oct 2020 12:24:41 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Felix Fietkau <nbd@....name>
Cc:     Eric Dumazet <edumazet@...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 11:38 AM Felix Fietkau <nbd@....name> wrote:
>
>
> On 2020-10-01 20:03, Eric Dumazet wrote:
> > 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 ?

I found this link:
http://www.brendangregg.com/blog/2017-08-08/linux-load-averages.html
It has a section called "Linux Uninterruptible Tasks" which explains
this behavior specifically. But I don't see a good conclusion on why.
Seems to be a convention.
IMHO, this is actually the problem/decision of the loadavg. It should
not impact how the kernel code is implemented. I think it makes more
sense to only wake up the thread when there is work to do.

> I don't have any specific documentation pointers, but this is something
> I observed on several occasions when playing with kthreads.
>
> From what I can find in the loadavg code it seems that tasks in
> TASK_UNINTERRUPTIBLE state are counted for loadavg alongside actually
> runnable tasks. This seems intentional to me, but I don't know why it
> was made like this.
>
> A kthread does not start the thread function until it has been woken up
> at least once, most likely to give the creating code a chance to perform
> some initializations after successfully creating the thread, before the
> thread function starts doing something. Instead, kthread() sets
> TASK_UNINTERRUPTIBLE and calls schedule() once.
>
> > Probably not a big deal, but this seems quite odd to me.
> I've run into enough users that look at loadavg as a measure of system
> load and would likely start reporting bugs if they observe such
> behavior. I'd like to avoid that.
>
> - Felix

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