netdev - Re: Performance regression between 4.13 and 4.14

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Message-ID: <17a364e0-89c8-d4f6-3873-353c7dae4fba@candelatech.com>
Date:   Wed, 9 May 2018 11:43:17 -0700
From:   Ben Greear <greearb@...delatech.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: Performance regression between 4.13 and 4.14

On 05/08/2018 10:10 AM, Eric Dumazet wrote:
>
>
> On 05/08/2018 09:44 AM, Ben Greear wrote:
>> Hello,
>>
>> I am trying to track down a performance regression that appears to be between 4.13
>> and 4.14.
>>
>> I first saw the problem with a hacked version of pktgen on some ixgbe NICs.  4.13 can do
>> right at 10G bi-directional on two ports, and 4.14 and later can do only about 6Gbps.
>>
>> I also tried with user-space UDP traffic on a stock kernel, and I can get about 3.2Gbps combined tx+rx
>> on 4.14 and about 4.4Gbps on 4.13.
>>
>> Attempting to bisect seems to be triggering a weirdness in git, and also lots of commits
>> crash or do not bring up networking, which makes the bisect difficult.
>>
>> Looking at perf top, it would appear that some lock is probably to blame.
>
>
> perf record -a -g -e cycles:pp sleep 5
> perf report
>
> Then you'll be able to tell us which lock (or call graph) is killing your perf.
>

I seem to be chasing multiple issues.  For 4.13, at least part of my problem was that LOCKDEP was enabled,
during my bisect, though it does NOT appear enabled in 4.16.  I think maybe CONFIG_LOCKDEP moved to CONFIG_PROVE_LOCKING
in 4.16, or something like that?  My 4.16 .config does have CONFIG_LOCKDEP_SUPPORT enabled, and I see no option to disable it:

[greearb@...-dt3 linux-4.16.x64]$ grep LOCKDEP .config
CONFIG_LOCKDEP_SUPPORT=y


For 4.16, I am disabling RETRAMPOLINE...are there any other such things I need
to disable to keep from getting a performance hit from the spectre-related bug
fixes?  At this point, I do not care about the security implications.

greearb@...-dt3 linux-4.16.x64]$ grep RETPO .config
# CONFIG_RETPOLINE is not set


Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com