netdev - Re: [PATCH] selftests/net: skip msg_zerocopy test if we have less than 4 CPUs

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Date:   Wed, 5 Aug 2020 09:22:26 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Shuah Khan <shuah@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, kernel-janitors@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] selftests/net: skip msg_zerocopy test if we have less
 than 4 CPUs

On 05/08/2020 09:06, Willem de Bruijn wrote:
> On Wed, Aug 5, 2020 at 2:54 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>>
>>
>>
>> On 8/4/20 5:30 AM, Colin King wrote:
>>> From: Colin Ian King <colin.king@...onical.com>
>>>
>>> The current test will exit with a failure if it cannot set affinity on
>>> specific CPUs which is problematic when running this on single CPU
>>> systems. Add a check for the number of CPUs and skip the test if
>>> the CPU requirement is not met.
>>>
>>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>>> ---
>>>  tools/testing/selftests/net/msg_zerocopy.sh | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/tools/testing/selftests/net/msg_zerocopy.sh b/tools/testing/selftests/net/msg_zerocopy.sh
>>> index 825ffec85cea..97bc527e1297 100755
>>> --- a/tools/testing/selftests/net/msg_zerocopy.sh
>>> +++ b/tools/testing/selftests/net/msg_zerocopy.sh
>>> @@ -21,6 +21,11 @@ readonly DADDR6='fd::2'
>>>
>>>  readonly path_sysctl_mem="net.core.optmem_max"
>>>
>>> +if [[ $(nproc) -lt 4 ]]; then
>>> +     echo "SKIP: test requires at least 4 CPUs"
>>> +     exit 4
>>> +fi
>>> +
>>>  # No arguments: automated test
>>>  if [[ "$#" -eq "0" ]]; then
>>>       $0 4 tcp -t 1
>>>
>>
>> Test explicitly uses CPU 2 and 3, right ?
>>
>> nproc could be 500, yet cpu 2 or 3 could be offline
>>
>> # cat /sys/devices/system/cpu/cpu3/online
>> 0
>> # echo $(nproc)
>> 71
> 
> The cpu affinity is only set to bring some stability across runs.
> 
> The test does not actually verify that a run with zerocopy is some
> factor faster than without, as that factor is hard to choose across
> all platforms. As a result the automated run mainly gives code coverage.
> 
> It's preferable to always run. And on sched_setaffinity failure log a
> message about possible jitter and continue. I can send that patch, if
> the approach sounds good.
> 
That's sounds preferable to my bad fix for sure :-)

Colin