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

[Eric Dumazet]

On Wed, Jun 15, 2016 at 7:17 AM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> On Wed, Jun 15, 2016 at 6:42 AM, Paolo Abeni <pabeni@xxxxxxxxxx> 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@xxxxxxxxxx>
>> Signed-off-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
>> ---
>>  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"