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

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Message-ID: <CANn89iLxx-K6XDgU=c+6S1ExtSoCqQyG+7XjzJW+xoL3ACJYuQ@mail.gmail.com>
Date:	Fri, 13 May 2016 10:03:42 -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 9:50 AM, Paolo Abeni <pabeni@...hat.com> wrote:

>> Indeed, and the patch looks quite simple now ;)
>>
>> diff --git a/kernel/softirq.c b/kernel/softirq.c
>> index 17caf4b63342d7839528f367b283a386413b0362..23c364485d03618773c385d943c0ef39f5931d09 100644
>> --- a/kernel/softirq.c
>> +++ b/kernel/softirq.c
>> @@ -57,6 +57,11 @@ static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp
>>
>>  DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
>>
>> +static inline bool ksoftirqd_running(void)
>> +{
>> +     return __this_cpu_read(ksoftirqd)->state == TASK_RUNNING;
>> +}
>> +
>>  const char * const softirq_to_name[NR_SOFTIRQS] = {
>>       "HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "BLOCK_IOPOLL",
>>       "TASKLET", "SCHED", "HRTIMER", "RCU"
>> @@ -313,7 +318,7 @@ asmlinkage __visible void do_softirq(void)
>>
>>       pending = local_softirq_pending();
>>
>> -     if (pending)
>> +     if (pending && !ksoftirqd_running())
>>               do_softirq_own_stack();
>>
>>       local_irq_restore(flags);
>> @@ -340,6 +345,9 @@ void irq_enter(void)
>>
>>  static inline void invoke_softirq(void)
>>  {
>> +     if (ksoftirqd_running())
>> +             return;
>> +
>>       if (!force_irqthreads) {
>>  #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
>>               /*
>
> In this version of the path, the chunk affecting __local_bh_enable_ip()
> has been removed.
>
> I think it is beneficial, because it allows avoiding a
> local_irq_save()/local_irq_restore() pairs per local_bh_enable under heavy load.
>

Interesting, do you have any numbers ?

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.

I noticed that simply doing a "ping -n gateway" while the UDP flood
was occurring, my udp receiver had quite a different efficiency.

Thanks.