Date:   Tue, 21 Nov 2017 21:44:26 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Shuah Khan <shuahkh@....samsung.com>
Cc:     shuah <shuah@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-kselftest <linux-kselftest@...r.kernel.org>
Subject: Re: [RFC PATCH for 4.15 v3 15/22] rseq: selftests: Provide
 self-tests

----- On Nov 21, 2017, at 4:24 PM, Shuah Khan shuahkh@....samsung.com wrote:

> On 11/21/2017 02:22 PM, Mathieu Desnoyers wrote:
>> ----- On Nov 21, 2017, at 12:40 PM, shuah shuah@...nel.org wrote:
>> 
>>> On 11/21/2017 10:05 AM, Mathieu Desnoyers wrote:
>>>> ----- On Nov 21, 2017, at 10:34 AM, shuah shuah@...nel.org wrote:
>>>>
>>>> [...]
>>>>>> ---
>>>>>>  MAINTAINERS                                        |    1 +
>>>>>>  tools/testing/selftests/Makefile                   |    1 +
>>>>>>  tools/testing/selftests/rseq/.gitignore            |    4 +
>>>>>
>>>>> Thanks for the .gitignore files. It is commonly missed change, I end
>>>>> up adding one to clean things up after tests get in.
>>>>
>>>> I'm used to receive patches where contributors forget to add new files
>>>> to gitignore within my own projects, which may contribute to my awareness
>>>> of this pain point. :)
>>>>
>>>> [...]
>>>>
>>>>>> +
>>>>>> +void *test_percpu_inc_thread(void *arg)
>>>>>> +{
>>>>>> +	struct inc_thread_test_data *thread_data = arg;
>>>>>> +	struct inc_test_data *data = thread_data->data;
>>>>>> +	long long i, reps;
>>>>>> +
>>>>>> +	if (!opt_disable_rseq && thread_data->reg
>>>>>> +			&& rseq_register_current_thread())
>>>>>> +		abort();
>>>>>> +	reps = thread_data->reps;
>>>>>> +	for (i = 0; i < reps; i++) {
>>>>>> +		int cpu, ret;
>>>>>> +
>>>>>> +#ifndef SKIP_FASTPATH
>>>>>> +		/* Try fast path. */
>>>>>> +		cpu = rseq_cpu_start();
>>>>>> +		ret = rseq_addv(&data->c[cpu].count, 1, cpu);
>>>>>> +		if (likely(!ret))
>>>>>> +			goto next;
>>>>>> +#endif
>>>>>
>>>>> So the test needs to compiled with this enabled? I think it would be better
>>>>> to make this an argument to be abel to select at test start time as opposed
>>>>> to making this compile time option. Remember that these tests get run in
>>>>> automated test rings. Making this a compile time otpion pertty much ensures
>>>>> that this path will not be tested.
>>>>>
>>>>> So I would reccommend adding a paratemer.
>>>>>
>>>>>> +	slowpath:
>>>>>> +		__attribute__((unused));
>>>>>> +		for (;;) {
>>>>>> +			/* Fallback on cpu_opv system call. */
>>>>>> +			cpu = rseq_current_cpu();
>>>>>> +			ret = cpu_op_addv(&data->c[cpu].count, 1, cpu);
>>>>>> +			if (likely(!ret))
>>>>>> +				break;
>>>>>> +			assert(ret >= 0 || errno == EAGAIN);
>>>>>> +		}
>>>>>> +	next:
>>>>>> +		__attribute__((unused));
>>>>>> +#ifndef BENCHMARK
>>>>>> +		if (i != 0 && !(i % (reps / 10)))
>>>>>> +			printf_verbose("tid %d: count %lld\n", (int) gettid(), i);
>>>>>> +#endif
>>>>>
>>>>> Same comment as before. Avoid compile time options.
>>>>
>>>> The goal of those compiler define are to generate the altered code without
>>>> adding branches into the fast-paths.
>>>
>>> That makes sense. You are looking to not add any overhead.
>>>
>>>>
>>>> Here is an alternative solution that should take care of your concern: I'll
>>>> build multiple targets for param_test.c:
>>>>
>>>> param_test
>>>> param_test_skip_fastpath (built with -DSKIP_FASTPATH)
>>>> param_test_benchmark (build with -DBENCHMARK)
>>>>
>>>> I'll update run_param_test.sh to run both param_test and
>>>> param_test_skip_fastpath.
>>>>
>>>> Note that "param_test_benchmark" is only useful for benchmarking,
>>>> so I don't plan to run it from run_param_test.sh which is meant
>>>> to track regressions.
>>>>
>>>> Is that approach OK with you ?
>>>>
>>>
>>> Yes. This approach addresses my concern about coverage for both paths.
>> 
>> fyi, the updated patches can be found here:
>> 
>> https://git.kernel.org/pub/scm/linux/kernel/git/rseq/linux-rseq.git/commit/?h=rseq/dev&id=a0b8eb0eb5d4d8a280969370aa1dcf51801139c6
>>   "selftests: lib.mk: Introduce OVERRIDE_TARGETS"
>> 
>> https://git.kernel.org/pub/scm/linux/kernel/git/rseq/linux-rseq.git/commit/?h=rseq/dev&id=4ef0214e19bb7415fe7aed6852859b8d66e09a45
>>   "cpu_opv: selftests: Implement selftests (v4)"
>> 
>> https://git.kernel.org/pub/scm/linux/kernel/git/rseq/linux-rseq.git/commit/?h=rseq/dev&id=7d7530b843c7ecb50bea5a136c776cf3e9155d43
>>   "rseq: selftests: Provide self-tests (v4)"
>> 
>> Thanks for the feedback!
>> 
> 
> Are you going to send these to the mailing list? That way I can do a final
> review and give my Ack if they look good.

Sure, I can do one hopefully last round of RFC with those selftests updates.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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