From mboxrd@z Thu Jan  1 00:00:00 1970
From: Richard Palethorpe <rpalethorpe@suse.de>
Date: Wed, 25 Nov 2020 11:22:57 +0000
Subject: [LTP] [PATCH] fzsync: skip test when avaliable CPUs less than 2
In-Reply-To: <20201125101633.30154-1-liwang@redhat.com>
References: <20201125101633.30154-1-liwang@redhat.com>
Message-ID: <87eekhof3i.fsf@suse.de>
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To: ltp@lists.linux.it

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.

Why not print a warning instead of refusing to run?

-- 
Thank you,
Richard.