linux-kernel - Re: [PATCH RESEND] sched/preempt: fix cond_resched_lock() and cond_resched

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Message-ID: <55658C21.7040102@yandex-team.ru>
Date:	Wed, 27 May 2015 12:19:29 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RESEND] sched/preempt: fix cond_resched_lock() and cond_resched_softirq()

On 27.05.2015 12:09, Peter Zijlstra wrote:
> On Thu, May 14, 2015 at 07:23:27PM +0300, Konstantin Khlebnikov wrote:
>> These functions check should_resched() before unlocking spinlock/bh-enable:
>> preempt_count always non-zero => should_resched() always returns false.
>> cond_resched_lock() works iff spin_needbreak is set.
>>
>> This patch adds argument "preempt_offset" to should_resched() add
>> rearranges preempt_count offset constants for that:
>>
>> PREEMPT_OFFSET - offset after preempt_disable() (0 if CONFIG_PREEMPT_COUNT=n)
>> PREEMPT_LOCK_OFFSET - offset after spin_lock() (alias for PREEMPT_OFFSET)
>> SOFTIRQ_DISABLE_OFFSET - offset after local_bh_distable()
>> SOFTIRQ_LOCK_OFFSET - offset after spin_lock_bh()
>>
>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
>
> Sorry, but it doesn't apply anymore because of that whole
> pagefault_disable() muck we merged.

No problem. I'll dig into this stuff again later.
This bug is harmless and it's here for ages.

>
>> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
>> index 48d3c5d2ecc9..6e73b74c0c60 100644
>> --- a/arch/powerpc/kvm/book3s_hv.c
>> +++ b/arch/powerpc/kvm/book3s_hv.c
>> @@ -2177,7 +2177,7 @@ static int kvmppc_run_vcpu(struct kvm_run *kvm_run, struct kvm_vcpu *vcpu)
>>   		vc->runner = vcpu;
>>   		if (n_ceded == vc->n_runnable) {
>>   			kvmppc_vcore_blocked(vc);
>> -		} else if (should_resched()) {
>> +		} else if (should_resched(PREEMPT_LOCK_OFFSET)) {
>
> I'm thinking this wants to be: need_resched() ?
>
>>   			vc->vcore_state = VCORE_PREEMPT;
>>   			/* Let something else run */
>>   			cond_resched_lock(&vc->lock);
>
>> diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
>> index eb6f9e6c3075..e91fb799a6da 100644
>> --- a/include/asm-generic/preempt.h
>> +++ b/include/asm-generic/preempt.h
>> @@ -71,9 +71,9 @@ static __always_inline bool __preempt_count_dec_and_test(void)
>>   /*
>>    * Returns true when we need to resched and can (barring IRQ state).
>>    */
>> -static __always_inline bool should_resched(void)
>> +static __always_inline bool should_resched(int offset)
>>   {
>> -	return unlikely(!preempt_count() && tif_need_resched());
>> +	return unlikely(preempt_count() == offset && tif_need_resched());
>>   }
>>
>>   #ifdef CONFIG_PREEMPT
>
> So the reason I held off on this patch for a wee bit is because I don't
> like the should_resched() change you did; although I fully understand
> why you did it.
>
> That said, I could not come up with anything better either and I suppose
> that once we fix that ppc-kvm user, there really isn't a user left
> outside of core code and thus we can deal with a slightly dangerous
> function.
>
> I did not really look, but it would be good if we could also get rid of
> the Xen usage.
>

-- 
Konstantin
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