linux-kernel - Re: [PATCH] s390x/spinlock: Provide vcpu_is

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Message-Id: <20160929151116.3b5e8f07@mschwide>
Date:   Thu, 29 Sep 2016 15:11:16 +0200
From:   Martin Schwidefsky <schwidefsky@...ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel-request@...ts.xenproject.org, benh@...nel.crashing.org,
        paulus@...ba.org, mpe@...erman.id.au, mingo@...hat.com,
        paulmck@...ux.vnet.ibm.com, will.deacon@....com,
        kernellwp@...il.com, jgross@...e.com, pbonzini@...hat.com,
        bsingharora@...il.com
Subject: Re: [PATCH] s390x/spinlock: Provide vcpu_is_preempted globally

On Thu, 29 Sep 2016 13:54:16 +0200
Christian Borntraeger <borntraeger@...ibm.com> wrote:

> this implements the s390 backend for commit
> "kernel/sched: introduce vcpu preempted check interface"
> by simply reusing the existing cpu_is_preempted function.
> 
> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
> ---
> Martin, Heiko,
> 
> this patch is a minimal change by not touching all existing users of
> cpu_is_preempted in spinlock.c. If you want it differently, let me
> know.
> 
> 
>  arch/s390/include/asm/spinlock.h | 7 +++++++
>  arch/s390/lib/spinlock.c         | 3 ++-
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/include/asm/spinlock.h b/arch/s390/include/asm/spinlock.h
> index 63ebf37..6e82986 100644
> --- a/arch/s390/include/asm/spinlock.h
> +++ b/arch/s390/include/asm/spinlock.h
> @@ -21,6 +21,13 @@ _raw_compare_and_swap(unsigned int *lock, unsigned int old, unsigned int new)
>  	return __sync_bool_compare_and_swap(lock, old, new);
>  }
> 
> +int arch_vcpu_is_preempted(int cpu);
> +#define vcpu_is_preempted cpu_is_preempted
> +static inline bool cpu_is_preempted(int cpu)
> +{
> +	return arch_vcpu_is_preempted(cpu);
> +}
> +
>  /*
>   * Simple spin lock operations.  There are two variants, one clears IRQ's
>   * on the local processor, one does not.
> diff --git a/arch/s390/lib/spinlock.c b/arch/s390/lib/spinlock.c
> index e5f50a7..9f473c8 100644
> --- a/arch/s390/lib/spinlock.c
> +++ b/arch/s390/lib/spinlock.c
> @@ -37,7 +37,7 @@ static inline void _raw_compare_and_delay(unsigned int *lock, unsigned int old)
>  	asm(".insn rsy,0xeb0000000022,%0,0,%1" : : "d" (old), "Q" (*lock));
>  }
> 
> -static inline int cpu_is_preempted(int cpu)
> +int arch_vcpu_is_preempted(int cpu)
>  {
>  	if (test_cpu_flag_of(CIF_ENABLED_WAIT, cpu))
>  		return 0;
> @@ -45,6 +45,7 @@ static inline int cpu_is_preempted(int cpu)
>  		return 0;
>  	return 1;
>  }
> +EXPORT_SYMBOL(arch_vcpu_is_preempted);
> 
>  void arch_spin_lock_wait(arch_spinlock_t *lp)
>  {

Hmm, if I look at the code we now have an additional function for
the spinlock loops. The call arch_vcpu_is_preempted which test
CIF_ENABLED_WAIT and then calls smp_vcpu_scheduled(). The test
used to be inline.

A better solution would be to move the CIF_ENABLED_WAIT test to the
smp_vcpu_scheduled() function, rename it to arch_vcpu_is_preempted()
and then export that function. The cpu_is_preempted() function is
replaced by arch_vcpu_is_preempted() which does make a lot of sense,
no?

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.