netdev - Re: [PATCH] net: Performance fix for process

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Message-ID: <CA+mtBx-yVFHQ7LNuE_S2Ot9EB_rWK_PyOGuRNo8RmYDEVg8gLQ@mail.gmail.com>
Date:	Mon, 30 Jun 2014 09:42:27 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: Performance fix for process_backlog

On Sun, Jun 29, 2014 at 11:11 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Sun, 2014-06-29 at 19:21 -0700, Tom Herbert wrote:
>> In process_backlog the input_pkt_queue is only checked once for new
>> packets and quota is artificially reduced to reflect precisely the
>> number of packets on the input_pkt_queue so that the loop exits
>> appropriately.
>>
>> This patch changes the behavior to be more straightforward and
>> less convoluted. Packets are processed until either the quota
>> is met or there are no more packets to process.
>
> This is the same condition than before. You only changed how its done.
>
>>
>> This patch seems to provide a small, but noticeable performance
>> improvement.
>
> I would prefer you explain why this gets an improvement, because few
> people really understands this.
>
> The reason you might get an improvement here is because one test is
> removed, and this test was fooling cpu branch predictor.
>
Eliminating a couple of conditionals is a nice logic improvement, but
is not material in the performance improvement. The improvement comes
for the fact that we are staying in the the process_backlog loop and
not clearing napi_state as long as there is still work to do. This can
result in fewer IPIs which is shown in number of interrupts/sec.
(1145382 vs. 1021674 in my test).

The cost of the change is one additional lock and check on qlen when
the queue is empty, however in this case they should still be in the
local cache so that's not overly expensive (far cheaper than more
IPIs).

> perf stat ./super_netperf ... would tell us this better.
>
>>
>> Performance data using super_netperf TCP_RR with 200 flows:
>>
>> Before fix:
>>
>> 88.06% CPU utilization
>> 125/190/309 90/95/99% latencies
>> 1.46808e+06 tps
>>
>> With fix:
>>
>> 87.73% CPU utilization
>> 122/183/296 90/95/99% latencies
>> 1.4921e+06 tps
>>
>> Signed-off-by: Tom Herbert <therbert@...gle.com>
>> ---
>>  net/core/dev.c | 23 +++++++++++------------
>>  1 file changed, 11 insertions(+), 12 deletions(-)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index a04b12f..136ce3e 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -4227,9 +4227,8 @@ static int process_backlog(struct napi_struct *napi, int quota)
>>  #endif
>>       napi->weight = weight_p;
>>       local_irq_disable();
>> -     while (work < quota) {
>> +     while (1) {
>>               struct sk_buff *skb;
>> -             unsigned int qlen;
>>
>>               while ((skb = __skb_dequeue(&sd->process_queue))) {
>>                       local_irq_enable();
>> @@ -4243,24 +4242,24 @@ static int process_backlog(struct napi_struct *napi, int quota)
>>               }
>>
>>               rps_lock(sd);
>> -             qlen = skb_queue_len(&sd->input_pkt_queue);
>> -             if (qlen)
>> -                     skb_queue_splice_tail_init(&sd->input_pkt_queue,
>> -                                                &sd->process_queue);
>> -
>> -             if (qlen < quota - work) {
>> +             if (skb_queue_empty(&sd->input_pkt_queue)) {
>>                       /*
>>                        * Inline a custom version of __napi_complete().
>>                        * only current cpu owns and manipulates this napi,
>> -                      * and NAPI_STATE_SCHED is the only possible flag set on backlog.
>> -                      * we can use a plain write instead of clear_bit(),
>> +                      * and NAPI_STATE_SCHED is the only possible flag set
>> +                      * on backlog.
>> +                      * We can use a plain write instead of clear_bit(),
>>                        * and we dont need an smp_mb() memory barrier.
>>                        */
>>                       list_del(&napi->poll_list);
>>                       napi->state = 0;
>> +                     rps_unlock(sd);
>> +
>> +                     break;
>> +             }
>
> Since your first block ends with a break; you do not need an 'else'
>
>>  else
>
> Then you can remove one tab on this,
>
>> +                     skb_queue_splice_tail_init(&sd->input_pkt_queue,
>> +                                                &sd->process_queue);
>
> Alternatively you should have :
>  if (A) {
>         ...
>  } else {
>         ...
>  }
>
>
>>
>> -                     quota = work + qlen;
>> -             }
>>               rps_unlock(sd);
>>       }
>>       local_irq_enable();
>
>
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