netdev - Re: Re: dvb usb issues since kernel 4.9

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Message-ID: <CA+55aFwq42Nzq47csw=ME8zbHYiw2rPN_Zp+=+Bu+Ruq9XquhQ@mail.gmail.com>
Date:   Tue, 9 Jan 2018 10:58:30 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Josef Griebichler <griebichler.josef@....at>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>,
        Rik van Riel <riel@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        LMML <linux-media@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: Re: dvb usb issues since kernel 4.9

On Tue, Jan 9, 2018 at 9:57 AM, Eric Dumazet <edumazet@...gle.com> wrote:
>
> Your patch considers TASKLET_SOFTIRQ being a candidate for 'immediate
> handling', but TCP Small queues heavily use TASKLET,
> so as far as I am concerned a revert would have the same effect.

Does it actually?

TCP ends up dropping packets outside of the window etc, so flooding a
machine with TCP packets and causing some further processing up the
stack sounds very different from the basic packet flooding thing that
happens with NET_RX_SOFTIRQ.

Also, honestly, the kinds of people who really worry about flooding
tend to have packet filtering in the receive path etc.

So I really think "you can use up 90% of CPU time with a UDP packet
flood from the same network" is very very very different - and
honestly not at all as important - as "you want to be able to use a
USB DVB receiver and watch/record TV".

Because that whole "UDP packet flood from the same network" really is
something you _fundamentally_ have other mitigations for.

I bet that whole commit was introduced because of a benchmark test,
rather than real life. No?

In contrast, now people are complaining about real loads not working.

             Linus