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

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Date:   Thu, 1 Oct 2020 22:48:11 +0200
From:   Felix Fietkau <nbd@....name>
To:     Wei Wang <weiwan@...gle.com>
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 2020-10-01 21:24, Wei Wang wrote:
> 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.
There were other users of kthread where the same issue was fixed.
With a quick search, I found these commits:
e890591413819eeb604207ad3261ba617b2ec0bb
3f776e8a25a9d281125490562e1cc5bd7c14cf7c

Please note that one of these describes that a kthread that was created
but not woken was triggering a blocked task warning - so it's not just
the loadavg that matters here.

All the other users of kthread that I looked at also do an initial
wakeup of the thread. Not doing it seems like wrong use of the API to me.

- Felix