linux-kernel - Re: [PATCH v2 2/4] powerpc/spinlock: support vcpu preempted check

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Date:	Fri, 15 Jul 2016 23:35:14 +0800
From:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>
To:	Balbir Singh <bsingharora@...il.com>,
	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	virtualization@...ts.linux-foundation.org,
	linux-s390@...r.kernel.org
Cc:	dave@...olabs.net, peterz@...radead.org, mpe@...erman.id.au,
	boqun.feng@...il.com, will.deacon@....com, waiman.long@....com,
	mingo@...hat.com, paulus@...ba.org, benh@...nel.crashing.org,
	schwidefsky@...ibm.com, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v2 2/4] powerpc/spinlock: support vcpu preempted check

Hi, Baibir
	sorry for late responce, I missed reading your mail.

在 16/7/6 18:54, Balbir Singh 写道:
> On Tue, 2016-06-28 at 10:43 -0400, Pan Xinhui wrote:
>> This is to fix some lock holder preemption issues. Some other locks
>> implementation do a spin loop before acquiring the lock itself. Currently
>> kernel has an interface of bool vcpu_is_preempted(int cpu). It take the cpu
> 								^^ takes
>> as parameter and return true if the cpu is preempted. Then kernel can break
>> the spin loops upon on the retval of vcpu_is_preempted.
>>
>> As kernel has used this interface, So lets support it.
>>
>> Only pSeries need supoort it. And the fact is powerNV are built into same
> 		   ^^ support
>> kernel image with pSeries. So we need return false if we are runnig as
>> powerNV. The another fact is that lppaca->yiled_count keeps zero on
> 					  ^^ yield
>> powerNV. So we can just skip the machine type.
>>

Blame on me, I indeed need avoid such typo..
thanks for pointing it out.

>> Suggested-by: Boqun Feng <boqun.feng@...il.com>
>> Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>> Signed-off-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
>> ---
>>  arch/powerpc/include/asm/spinlock.h | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>>
>> diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
>> index 523673d..3ac9fcb 100644
>> --- a/arch/powerpc/include/asm/spinlock.h
>> +++ b/arch/powerpc/include/asm/spinlock.h
>> @@ -52,6 +52,24 @@
>>  #define SYNC_IO
>>  #endif
>>
>> +/*
>> + * This support kernel to check if one cpu is preempted or not.
>> + * Then we can fix some lock holder preemption issue.
>> + */
>> +#ifdef CONFIG_PPC_PSERIES
>> +#define vcpu_is_preempted vcpu_is_preempted
>> +static inline bool vcpu_is_preempted(int cpu)
>> +{
>> +	/*
>> +	 * pSeries and powerNV can be built into same kernel image. In
>> +	 * principle we need return false directly if we are running as
>> +	 * powerNV. However the yield_count is always zero on powerNV, So
>> +	 * skip such machine type check
>
> Or you could use the ppc_md interface callbacks if required, but your
> solution works as well
>

thanks, So I can keep my code as is.

thanks
xinhui

>> +	 */
>> +	return !!(be32_to_cpu(lppaca_of(cpu).yield_count) & 1);
>> +}
>> +#endif
>> +
>>  static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
>>  {
>>  	return lock.slock == 0;
>
>
> Balbir Singh.
>