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

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Date:   Thu, 1 Oct 2020 19:11:54 +0200
From:   Felix Fietkau <nbd@....name>
To:     Wei Wang <weiwan@...gle.com>
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 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.

- Felix