Subject: Re: [PATCH V3] powerpc: Implement {cmp}xchg for u8 and u16

From: Pan Xinhui <[email-masked]>
Date: Thu, 21 Apr 2016 23:35:07 +0800

On 2016年04月20日 22:24, Peter Zijlstra wrote:
> On Wed, Apr 20, 2016 at 09:24:00PM +0800, Pan Xinhui wrote:
> 
>> +#define __XCHG_GEN(cmp, type, sfx, skip, v)				\
>> +static __always_inline unsigned long					\
>> +__cmpxchg_u32##sfx(v unsigned int *p, unsigned long old,		\
>> +			 unsigned long new);				\
>> +static __always_inline u32						\
>> +__##cmp##xchg_##type##sfx(v void *ptr, u32 old, u32 new)		\
>> +{									\
>> +	int size = sizeof (type);					\
>> +	int off = (unsigned long)ptr % sizeof(u32);			\
>> +	volatile u32 *p = ptr - off;					\
>> +	int bitoff = BITOFF_CAL(size, off);				\
>> +	u32 bitmask = ((0x1 << size * BITS_PER_BYTE) - 1) << bitoff;	\
>> +	u32 oldv, newv, tmp;						\
>> +	u32 ret;							\
>> +	oldv = READ_ONCE(*p);						\
>> +	do {								\
>> +		ret = (oldv & bitmask) >> bitoff;			\
>> +		if (skip && ret != old)					\
>> +			break;						\
>> +		newv = (oldv & ~bitmask) | (new << bitoff);		\
>> +		tmp = oldv;						\
>> +		oldv = __cmpxchg_u32##sfx((v u32*)p, oldv, newv);	\
>> +	} while (tmp != oldv);						\
>> +	return ret;							\
>> +}
> 
> So for an LL/SC based arch using cmpxchg() like that is sub-optimal.
> 
> Why did you choose to write it entirely in C?
> 
yes, you are right. more load/store will be done in C code.
However such xchg_u8/u16 is just used by qspinlock now. and I did not see any performance regression.
So just wrote in C, for simple. :)

Of course I have done xchg tests.
we run code just like xchg((u8*)&v, j++); in several threads.
and the result is,
[  768.374264] use time[1550072]ns in xchg_u8_asm
[  768.377102] use time[2826802]ns in xchg_u8_c

I think this is because there is one more load in C.
If possible, we can move such code in asm-generic/.

thanks
xinhui