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

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Date:   Wed, 5 Aug 2020 10:44:27 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "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 Wed, Aug 5, 2020 at 10:22 AM Colin Ian King <colin.king@...onical.com> wrote:
>
> 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 :-)

Certainly not a bad fix! Thanks for addressing the issue. Alternative
approach at

http://patchwork.ozlabs.org/project/netdev/patch/20200805084045.1549492-1-willemdebruijn.kernel@gmail.com/