netdev - Re: [RFC PATCH] net/sched: add indirect call wrapper hint.

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Date:   Fri, 19 Jun 2020 11:24:29 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC PATCH] net/sched: add indirect call wrapper hint.

On Thu, 2020-06-18 at 12:00 -0700, Eric Dumazet wrote:
> 
> On 6/18/20 10:31 AM, Paolo Abeni wrote:
> > The sched layer can use several indirect calls per
> > packet, with not work-conservative qdisc being
> > more affected due to the lack of the BYPASS path.
> > 
> > This change tries to improve the situation using
> > the indirect call wrappers infrastructure for the
> > qdisc enqueue end dequeue indirect calls.
> > 
> > To cope with non-trivial scenarios, a compile-time know is
> > introduced, so that the qdisc used by ICW can be different
> > from the default one.
> > 
> > Tested with pktgen over qdisc, with CONFIG_HINT_FQ_CODEL=y:
> > 
> > qdisc		threads vanilla	patched delta
> > 		nr	Kpps	Kpps	%
> > pfifo_fast	1	3300	3700	12
> > pfifo_fast	2	3940	4070	3
> > fq_codel	1	3840	4110	7
> > fq_codel	2	1920	2260	17
> > fq		1	2230	2210	-1
> > fq		2	1530	1540	1
> 
> Hi Paolo
> 
> This test is a bit misleading, pktgen has a way to bypass the qdisc.

The above figures were collected using the 
pktgen_bench_xmit_mode_queue_xmit.sh script, which in turn uses
'xmit_mode queue_xmit': packets traverse the qdisc layer via the usual
dev_queue_xmit()

> Real numbers for more typical workloads would be more appealing,
> before we consider a quite invasive patch ?

I'll add figures for netperf UDP single threaded/many threads...

> What is the status of static_call infrastructure ?

... unless you prefer waiting for the above. AFAICS, that is under
discussion. v4 was posted in May[1] and collected quite a bit of
feedback.

> >  
> > +#ifndef CODEL_SCOPE
> > +#define CODEL_SCOPE static
> > +#endif
> 
> This looks additional burden, just remove the static attribute,
> if a function might be called directly.

Yep, that will slim down the patch a bit, I will do. 

Thanks for the feedback,

Paolo

[1] https://lwn.net/ml/linux-kernel/20200501202849.647891881@infradead.org/