netdev - Re: [RFC PATCH 0/2] net: threadable napi poll loop

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Date:	Fri, 13 May 2016 10:36:34 -0700
From:	Eric Dumazet <edumazet@...gle.com>
To:	Paolo Abeni <pabeni@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jiri@...lanox.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Alexander Duyck <aduyck@...antis.com>,
	Tom Herbert <tom@...bertland.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/2] net: threadable napi poll loop

On Fri, May 13, 2016 at 10:19 AM, Paolo Abeni <pabeni@...hat.com> wrote:

> The difference is small, in the noise range:
>
> [with this patch applied]
> super_netperf 100 -H 192.168.122.1 -t UDP_STREAM -l 60 -- -m 1
> 9.00
>
> [adding the test into __local_bh_enable_ip(), too]
> super_netperf 100 -H 192.168.122.1 -t UDP_STREAM -l 60 -- -m 1
> 9.14
>
> but reproducible, in my experiments.
> I have similar data for different number of flows.
>
>> I believe I did this so that we factorize the logic in do_softirq()
>> and keep the code local to kernel/softirq.c
>>
>> Otherwise, netif_rx_ni() could also process softirq while ksoftirqd
>> was scheduled,
>> so I would have to  'export' the ksoftirqd_running(void) helper in an
>> include file.
>
> The idea could be to add the test in __local_bh_enable_ip(), maintaining
> the test also in do_softirq() (as currently done, i.e for
> local_softirq_pending())
>

Then I guess even the !in_interrupt() test we do is expensive and
could be avoided,
since do_softirq() is doing it again in the unlikely case it really is needed.

@@ -162,7 +170,8 @@ void __local_bh_enable_ip(unsigned long ip,
unsigned int cnt)
         */
        preempt_count_sub(cnt - 1);

-       if (unlikely(!in_interrupt() && local_softirq_pending())) {
+       if (unlikely(local_softirq_pending()) &&
+                    !ksoftirqd_running()) {
                /*
                 * Run softirq if any pending. And do it in its own stack
                 * as we may be calling this deep in a task call stack already.