linux-kernel - Re: [PATCH 2/3] powerpc/spinlock: support vcpu preempted check

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Message-Id: <5771F166.5060604@linux.vnet.ibm.com>
Date:	Tue, 28 Jun 2016 11:39:18 +0800
From:	xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	Boqun Feng <boqun.feng@...il.com>
CC:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, peterz@...radead.org, mingo@...hat.com,
	mpe@...erman.id.au, paulus@...ba.org, benh@...nel.crashing.org,
	Waiman.Long@....com, will.deacon@....com, dave@...olabs.net
Subject: Re: [PATCH 2/3] powerpc/spinlock: support vcpu preempted check



On 2016年06月27日 22:58, Boqun Feng wrote:
> Hi Xinhui,
>
> On Mon, Jun 27, 2016 at 01:41:29PM -0400, Pan Xinhui wrote:
>> This is to fix some holder preemption issues. Spinning at one
>> vcpu which is preempted is meaningless.
>>
>> Kernel need such interfaces, So lets support it.
>>
>> We also should suooprt both the shared and dedicated mode.
>> So add lppaca_dedicated_proc macro in lppaca.h
>>
>> Suggested-by: Boqun Feng <boqun.feng@...il.com>
>> Signed-off-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
>> ---
>>   arch/powerpc/include/asm/lppaca.h   |  6 ++++++
>>   arch/powerpc/include/asm/spinlock.h | 15 +++++++++++++++
>>   2 files changed, 21 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/lppaca.h b/arch/powerpc/include/asm/lppaca.h
>> index d0a2a2f..0a263d3 100644
>> --- a/arch/powerpc/include/asm/lppaca.h
>> +++ b/arch/powerpc/include/asm/lppaca.h
>> @@ -111,12 +111,18 @@ extern struct lppaca lppaca[];
>>    * we will have to transition to something better.
>>    */
>>   #define LPPACA_OLD_SHARED_PROC		2
>> +#define LPPACA_OLD_DEDICATED_PROC      (1 << 6)
>>
>
> I think you should describe a little bit about the magic number here,
right.

> i.e. what document/specification says this should work, and how this
> works.
>
yep, I need add some comments here. for example, this bit is firmware reserved...
thanks, will do that.

>>   static inline bool lppaca_shared_proc(struct lppaca *l)
>>   {
>>   	return !!(l->__old_status & LPPACA_OLD_SHARED_PROC);
>>   }
>>
>> +static inline bool lppaca_dedicated_proc(struct lppaca *l)
>> +{
>> +	return !!(l->__old_status & LPPACA_OLD_DEDICATED_PROC);
>> +}
>> +
>>   /*
>>    * SLB shadow buffer structure as defined in the PAPR.  The save_area
>>    * contains adjacent ESID and VSID pairs for each shadowed SLB.  The
>> diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
>> index 523673d..ae938ee 100644
>> --- a/arch/powerpc/include/asm/spinlock.h
>> +++ b/arch/powerpc/include/asm/spinlock.h
>> @@ -52,6 +52,21 @@
>>   #define SYNC_IO
>>   #endif
>>
>> +/* For fixing some spinning issues in a guest.
>> + * kernel would check if vcpu is preempted during a spin loop.
>> + * we support that.
>> + */
>> +#define arch_vcpu_is_preempted arch_vcpu_is_preempted
>> +static inline bool arch_vcpu_is_preempted(int cpu)
>
> This function should be guarded by #ifdef PPC_PSERIES .. #endif, right?
> Because if the kernel is not compiled with guest support,
> vcpu_is_preempted() should always be false, right?
>
oh, I forgot that. thanks for pointing it out.

>> +{
>> +	struct lppaca *lp = &lppaca_of(cpu);
>> +
>> +	if (unlikely(!(lppaca_shared_proc(lp) ||
>> +			lppaca_dedicated_proc(lp))))
>
> Do you want to detect whether we are running in a guest(ie. pseries
> kernel) here? Then I wonder whether "machine_is(pseries)" works here.
>
I tried as you said yesterday. but .h file has dependencies.
As you said, if we add #ifdef PPC_PSERIES, this is not a big problem. only powernv will be affected as they are built into same kernel img.

> Regards,
> Boqun
>
>> +		return false;
>> +	return !!(be32_to_cpu(lp->yield_count) & 1);
>> +}
>> +
>>   static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
>>   {
>>   	return lock.slock == 0;
>> --
>> 2.4.11
>>