Re: [LTP] [PATCH v1] mem: disable KSM smart scan for ksm tests

[Stefan Roesch <shr@devkernel.io>]

Petr Vorel <pvorel@suse.cz> writes:

>> On Wed, Nov 29, 2023 at 12:51 AM Petr Vorel <pvorel@suse.cz> wrote:
>
>> > Hi Stefan, Li,
>
>> > > Hi Stefan, Petr,
>
>> > > On Tue, Nov 28, 2023 at 3:46 PM Petr Vorel <pvorel@suse.cz> wrote:
>
>> > > > Hi Stefan,
>
>> > > > > This disables the "smart scan" KSM feature to make sure that the
>> > volatile
>> > > > > count remains at 0.
>
>> > > > > Signed-off-by: Stefan Roesch <devkernel.io>
>> > > > nit: you forgot 'shr@'
>> > > > Signed-off-by: Stefan Roesch <shr@devkernel.io>
>
>> > > > > Reported-by: kernel test robot <oliver.sang@intel.com>
>> > > > > Closes:
>
>> > https://lore.kernel.org/oe-lkp/202311161132.13d8ce5a-oliver.sang@intel.com
>> > > > > ---
>> > > > >  testcases/kernel/mem/lib/mem.c | 4 ++++
>> > > > >  1 file changed, 4 insertions(+)
>
>> > > > > diff --git a/testcases/kernel/mem/lib/mem.c
>> > > > b/testcases/kernel/mem/lib/mem.c
>> > > > > index fbfeef026..ef274a3ac 100644
>> > > > > --- a/testcases/kernel/mem/lib/mem.c
>> > > > > +++ b/testcases/kernel/mem/lib/mem.c
>> > > > > @@ -454,6 +454,9 @@ void create_same_memory(int size, int num, int
>> > unit)
>> > > > >              {'a', size*MB}, {'a', size*MB}, {'d', size*MB}, {'d',
>> > > > size*MB},
>> > > > >       };
>
>> > > > > +  /* Disable smart scan for correct volatile counts. */
>> > > > > +  SAFE_FILE_PRINTF(PATH_KSM "smart_scan", "0");
>> > > > NOTE, this fails on the systems without /sys/kernel/mm/ksm/smart_scan:
>
>> > > > mem.c:458: TBROK: Failed to open FILE '/sys/kernel/mm/ksm/smart_scan'
>> > for
>> > > > writing: EACCES (13)
>
>> > > > NOTE, we normally handle the setup like this in test setup function.
>
>> > > > But new API has .save_restore which is more robust for tasks  like
>> > this.
>> > > > It's already used in ksm01.c, you need just to add this line:
>> > > >         {"/sys/kernel/mm/ksm/smart_scan", "0", TST_SR_SKIP},
>
>
>> > > I guess we need to set 'TST_SR_SKIP_MISSING | TST_SR_TBROK_RO'
>> > > as the last field. Because TST_SR_SKIP will continue the test without
>> > > writing '0' to the smart_scan file, that's not correct if the file
>> > exists.
>> > > It will
>> > > ignore a kernel bug (smart_scan can't be written) by that config.
>
>> > > Per the Doc Petr pointed below:
>> > >   TST_SR_SKIP_MISSING – Continue without saving the file if it does not
>> > > exist
>> > >   TST_SR_TBROK_RO – End test with TBROK if the file is read-only
>> > >   TST_SR_SKIP_RO – Continue without saving the file if it is read-only
>> > >   TST_SR_SKIP – Equivalent to 'TST_SR_SKIP_MISSING | TST_SR_SKIP_RO'
>
>
>
>> > > > (instead of both SAFE_FILE_PRINTF)
>
>> > > > See:
>
>
>> > https://github.com/linux-test-project/ltp/wiki/C-Test-API#127-saving--restoring-procsys-values
>
>> > > > I wonder if ksm01.c is the only ksm test which needs to disable this.
>
>
>> > > I think all of the ksm0*.c tests should disable it by the config. The
>> > > smart_scan
>> > > will impact all the tests with invoke key function create_same_memory().
>
>> > ksm05.c and ksm06.c does not use create_same_memory(). Or did I overlook
>> > something?
>
>
>> Good catch, I looked into these tests, seems only ksm05 is debatable
>> for disabling smart_scan, as a simple regression, it suggests disabling
>> ksm daemon to avoid disturb according to some workload.
>> https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/mem/ksm/ksm05.c#L30
>
>> ksm06 is definitely need disable smart_scan, it tests KSM in different
>> 'run' state for merge_accros_nodes.
>
> Thanks for having a look.
>
>> To be on the safe side, I would suggest applying the patch to all ksm*
>> tests,
>> and write a new single for smart_scan if needed.
>
> Agree. I vote for new single for smart_scan related test.
>

I'll add a new test.

> Kind regards,
> Petr

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