[LTP] [PATCH 2/2] syscalls/pselect: Add a zero latency constraint

[Daniel Lezcano]

On 10/08/2017 13:50, Jiri Jaburek wrote:
> On 08/10/17 10:01, Daniel Lezcano wrote:
>> The pselect_01 testcase works well on an x86 as it is a fast platform, with
>> fast exit latency idle routine.
>>
>> However on ARM[64] the idle routine can take much more time, for example
>> 1500us.
>>
>> The pselect fails on the ARM[64] platforms because of these slow exit latencies,
>> the delay between the expected expiration and the observed one is not
>> acceptable.
>>
>> The fix could be to increase the deviation on ARM64 but that wouldn't make
>> sense as some platforms, for the same architecture, can have faster or
>> different delays, hence we can potentially miss a bug.
>>
>> The simplest solution is to set the cpu_dma latency constraint to zero, so the
>> idle driver will always choose the fastest idle state, thus fixing the issue
>> above. The latency constraint will apply only for this test.
> 
> I think a more generic LTP-wide solution could be made; there are more
> tests that fail ie. on virtualized environments because of CPU over-
> provisioning on the host (x86, s390, ppc, etc.)

Not sure a latency constraint will fix the above.

> and I can imagine ie.
> networking tests having similar needs as well (probably currently worked
> around using big timeouts).

Here you can set the network latency.

> Basically some system that would allow a user to set some "latency
> multiplier" based on the system under test, so that ie. dedicated x86
> machines with fairly deterministic CPU/net/etc. scheduling could have it
> set low, and so that other systems could still run tests that are now
> deemed unsuitable for such hardware.

Here the solution is to provide the cpu_dma latency, it will prevent the
cpuidle driver to choose a deeper idle state introducing an excessive
wakeup latency.

The excessive idle exit latency is the root cause of the pselect to
fail, setting the cpu_dma latency is the correct fix to let this test to
do the right thing. It is the purpose of the cpu_dma latency constraint.



>> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
>> ---
>>  testcases/kernel/syscalls/pselect/pselect01.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/testcases/kernel/syscalls/pselect/pselect01.c b/testcases/kernel/syscalls/pselect/pselect01.c
>> index a2b5339..42027b6 100644
>> --- a/testcases/kernel/syscalls/pselect/pselect01.c
>> +++ b/testcases/kernel/syscalls/pselect/pselect01.c
>> @@ -44,6 +44,9 @@ int sample_fn(int clk_id, long long usec)
>>  }
>>  
>>  static struct tst_test test = {
>> +	.needs_root = 1,
>> +	.needs_latency = 1,
>> +	.latency = 0,
>>  	.tid = "pselect()",
>>  	.sample = sample_fn,
>>  };
>>
> 
> 


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