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

[Frederic Weisbecker]

On Thu, Jan 11, 2018 at 09:13:42PM +0000, Dmitry Safonov wrote:
> On Thu, 2018-01-11 at 12:53 -0800, Eric Dumazet wrote:
> > On Thu, Jan 11, 2018 at 12:46 PM, Dmitry Safonov <dima@xxxxxxxxxx>
> > 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@xxxxxxxxxx
> > > > >
> > > > 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.

I just gave it a try. Sorry I couldn't resist :-s