linux-kernel - Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

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Date:   Thu, 11 Jan 2018 12:53:08 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Safonov <dima@...sta.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Paolo Abeni <pabeni@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Radu Rendec <rrendec@...sta.com>,
        Rik van Riel <riel@...hat.com>,
        Stanislaw Gruszka <sgruszka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

On Thu, Jan 11, 2018 at 12:46 PM, Dmitry Safonov <dima@...sta.com> wrote:
> On Thu, 2018-01-11 at 12:40 -0800, Linus Torvalds wrote:
>> On Thu, Jan 11, 2018 at 12:34 PM, Dmitry Safonov <dima@...sta.com>
>> wrote:
>> >
>> > I could try to write a PoC for that..
>> > What should be the trigger to fall into workqueue?
>> > How to tell if there're too many softirqs of the kind?
>>
>> I suspect it would have to be time-based, probably using the
>> scheduler clock.
>
> I thought about this, but I was a bit afraid of how much pricey it
> would be recalculate it each clock. Well, might just try to write that
> and measure the impact.
>
>> Most softirqs are really really small. So just counting them probably
>> isn't all that meaningful, although the count is good as a fallback
>> (as shown by the jiffy issues).
>>
>> The good news is that we only have a fairly small handful of
>> softirqs,
>> so counting/timing them separately is still mainly a pretty small
>> array (which needs to be percpu, of course).

Note that using (scheduler) clock might also help to break net_rx_action()
not on a stupid netdev_budget, but on a more precise time limit as well.

netdev_budget of 300 packets is quite big :/

(The time_limit based on jiffies + 2 does not work on hosts with one
cpu, since jiffies wont make progress while net_rx_action() is
running)