linux-kernel - Re: [PATCH 4/5] netdev: implement infrastructure for threadable napi irq

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Date:	Wed, 15 Jun 2016 07:21:53 -0700
From:	Eric Dumazet <edumazet@...gle.com>
To:	Paolo Abeni <pabeni@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"David S. Miller" <davem@...emloft.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev <netdev@...r.kernel.org>,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 4/5] netdev: implement infrastructure for threadable napi irq

On Wed, Jun 15, 2016 at 7:17 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> On Wed, Jun 15, 2016 at 6:42 AM, Paolo Abeni <pabeni@...hat.com> wrote:
>> This commit adds the infrastructure needed for threadable
>> rx interrupt. A reference to the irq thread is used to
>> mark the threaded irq mode.
>> In threaded mode the poll loop is invoked directly from
>> __napi_schedule().
>> napi drivers which want to support threadable irq interrupts
>> must provide an irq mode change handler which actually set
>> napi->thread and register it after requesting the irq.
>>
>> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
>> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
>> ---
>>  include/linux/netdevice.h |  4 ++++
>>  net/core/dev.c            | 59 +++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 63 insertions(+)
>>
>
> I really appreciate the effort, but as I already said this is not going to work.
>
> Many NIC have 2 NAPI contexts per queue, one for TX, one for RX.
>
> Relying on CFS to switch from the two 'threads' you need in the one
> vCPU case will add latencies that your 'pure throughput UDP flood' is
> not able to detect.
>
> I was waiting a fix from Andy Lutomirski to be merged before sending
> my ksoftirqd fix, which will work and wont bring kernel bloat.


Andy's patch was"x86/traps: Don't force in_interrupt() to return true
in IST handlers"