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

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Date:   Thu, 11 Jan 2018 21:13:42 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Eric Dumazet <edumazet@...gle.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, 2018-01-11 at 12:53 -0800, Eric Dumazet wrote:
> 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)

Thanks for the details, Eric.
I'll try to come up with poc if no one beats me at it.

-- 
Thanks,
             Dmitry