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

Martin Doucha
2020-11-25T11:56:43Z

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.

--
Martin Doucha mdoucha@suse.cz
QA Engineer for Software Maintenance
SUSE LINUX, s.r.o.
CORSO IIa
Krizikova 148/34
186 00 Prague 8
Czech Republic