[LTP] [PATCH] fzsync: skip test when avaliable CPUs less than 2

[Richard Palethorpe <rpalethorpe@suse.de>]

Hello,

Martin Doucha <mdoucha@suse.cz> writes:

> On 25. 11. 20 12:22, Richard Palethorpe wrote:
>> Hello Li,
>> 
>> Li Wang <liwang@redhat.com> writes:
>> 
>>> It makes no sense to run parallel thread to simulate race conditions on
>>> system with CPU number less than two, especially for kvm guest, it does
>>> not have any chance to get real parallel running and probably encounter
>>> failure as below:
>> 
>> Most of the tests using FuzzySync do not need true parallism. We were
>> able to reproduce a number of race conditions on a single vCPU. Infact
>> it may actually benefit some races because one thread has to pause to
>> allow the other to run, perhaps creating a huge race window.
>> 
>>>
>>> === 100% reproducible on a 1cpu guest ===
>>>
>>> cmdline="af_alg07"
>>> contacts=""
>>> analysis=exit
>>> <<<test_output>>>
>>> tst_test.c:1248: TINFO: Timeout per run is 0h 05m 00s
>>> ../../../include/tst_fuzzy_sync.h:507: TINFO: Minimum sampling period ended
>>> ../../../include/tst_fuzzy_sync.h:330: TINFO: loop = 1024, delay_bias = 0
>>> ../../../include/tst_fuzzy_sync.h:318: TINFO: start_a - start_b: { avg = -137522ns, avg_dev = 854248ns, dev_ratio = 6.21 }
>>> ../../../include/tst_fuzzy_sync.h:318: TINFO: end_a - start_a  : { avg =  1915ns, avg_dev =   535ns, dev_ratio = 0.28 }
>>> ../../../include/tst_fuzzy_sync.h:318: TINFO: end_b - start_b  : { avg =  1885ns, avg_dev =    42ns, dev_ratio = 0.02 }
>>> ../../../include/tst_fuzzy_sync.h:318: TINFO: end_a - end_b    : { avg = -137492ns, avg_dev = 854818ns, dev_ratio = 6.22 }
>>> ../../../include/tst_fuzzy_sync.h:318: TINFO: spins            : { avg = 554786  , avg_dev =  7355  , dev_ratio = 0.01 }
>>> ../../../include/tst_fuzzy_sync.h:636: TINFO: Exceeded execution time, requesting exit
>>> af_alg07.c:96: TFAIL: fchownat() failed to fail, kernel may be vulnerable
>>>
>>> Signed-off-by: Li Wang <liwang@redhat.com>
>>> CC: Richard Palethorpe <rpalethorpe@suse.de>
>>> ---
>>>  include/tst_fuzzy_sync.h | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/include/tst_fuzzy_sync.h b/include/tst_fuzzy_sync.h
>>> index 4141f5c64..2e864b312 100644
>>> --- a/include/tst_fuzzy_sync.h
>>> +++ b/include/tst_fuzzy_sync.h
>>> @@ -281,6 +281,9 @@ static void tst_init_stat(struct tst_fzsync_stat *s)
>>>  static void tst_fzsync_pair_reset(struct tst_fzsync_pair *pair,
>>>  				  void *(*run_b)(void *))
>>>  {
>>> +	if (get_nprocs() < 2)
>>> +		tst_brk(TCONF, "Fuzzy Sync requires at least two CPUs available");
>>> +
>>>  	tst_fzsync_pair_cleanup(pair);
>>>  
>>>  	tst_init_stat(&pair->diff_ss);
>> 
>> Perhaps this test would pass with more loops and a big enough delay
>> range, but this is also wasting time on a single vCPU. I'm not sure
>> whether we should filter this test at the LTP level; it may trigger the
>> bug on some single CPU configs.
>
> No, af_alg07 requires 2 CPUs, otherwise it'll report false positives.
> The test will pass only if fchownat() hits a half-closed socket and
> returns error. But IIRC the half-closed socket will be destroyed during
> reschedule which means there's no race window to hit anymore. But it
> would be better to put the TCONF condition into the test itself.

Interesting, I wonder if this is also true for the real-time kernel with
the threads set to RT priority?

-- 
Thank you,
Richard.