linux-kernel - Re: [RFC PATCH for 4.15 v3 15/22] rseq: selftests: Provide self-tests

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Date:   Tue, 21 Nov 2017 10:40:34 -0700
From:   Shuah Khan <shuah@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     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>,
        Shuah Khan <shuahkh@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [RFC PATCH for 4.15 v3 15/22] rseq: selftests: Provide self-tests

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.

thanks,
-- Shuah