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Message-ID: <d806769b-c568-fa7c-f7aa-ded9ffea11b4@efficios.com>
Date:   Fri, 26 May 2023 12:27:51 -0400
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Subject: Re: [RFC][PATCH 2/2] sched: Use fancy new guards
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To:     Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     torvalds@linux-foundation.org, keescook@chromium.org,
        pbonzini@redhat.com, linux-kernel@vger.kernel.org,
        ojeda@kernel.org, ndesaulniers@google.com, mingo@redhat.com,
        will@kernel.org, longman@redhat.com, boqun.feng@gmail.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        paulmck@kernel.org, frederic@kernel.org, quic_neeraju@quicinc.com,
        joel@joelfernandes.org, josh@joshtriplett.org,
        jiangshanlai@gmail.com, qiang1.zhang@intel.com,
        rcu@vger.kernel.org, tj@kernel.org, tglx@linutronix.de
References: <20230526150549.250372621@infradead.org>
 <20230526151947.027972233@infradead.org>
 <2023052626-blunderer-delegator-4b82@gregkh>
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
In-Reply-To: <2023052626-blunderer-delegator-4b82@gregkh>
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On 5/26/23 12:25, Greg KH wrote:
> On Fri, May 26, 2023 at 05:05:51PM +0200, Peter Zijlstra wrote:
>> Convert kernel/sched/core.c to use the fancy new guards to simplify
>> the error paths.
> 
> That's slightly crazy...
> 
> I like the idea, but is this really correct:
> 
> 
>>
>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> ---
>>   kernel/sched/core.c  | 1223 +++++++++++++++++++++++----------------------------
>>   kernel/sched/sched.h |   39 +
>>   2 files changed, 595 insertions(+), 667 deletions(-)
>>
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1097,24 +1097,21 @@ int get_nohz_timer_target(void)
>>   
>>   	hk_mask = housekeeping_cpumask(HK_TYPE_TIMER);
>>   
>> -	rcu_read_lock();
>> -	for_each_domain(cpu, sd) {
>> -		for_each_cpu_and(i, sched_domain_span(sd), hk_mask) {
>> -			if (cpu == i)
>> -				continue;
>> +	void_scope(rcu) {
>> +		for_each_domain(cpu, sd) {
>> +			for_each_cpu_and(i, sched_domain_span(sd), hk_mask) {
>> +				if (cpu == i)
>> +					continue;
>>   
>> -			if (!idle_cpu(i)) {
>> -				cpu = i;
>> -				goto unlock;
>> +				if (!idle_cpu(i))
>> +					return i;
> 
> You can call return from within a "scope" and it will clean up properly?
> 
> I tried to read the cpp "mess" but couldn't figure out how to validate
> this at all, have a set of tests for this somewhere?
> 
> Anyway, the naming is whack, but I don't have a proposed better name,
> except you might want to put "scope_" as the prefix not the suffix, but
> then that might look odd to, so who knows.

FWIW C++ has std::scoped_lock. So perhaps using a similar wording may help ?

Thanks,

Mathieu

> 
> But again, the idea is good, it might save us lots of "you forgot to
> clean this up on the error path" mess that we are getting constant churn
> for these days...
> 
> thanks,
> 
> greg k-h

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