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SubjectRe: [PATCH V3] powerpc: Implement {cmp}xchg for u8 and u16
On 2016年04月21日 23:52, Boqun Feng wrote:
> On Thu, Apr 21, 2016 at 11:35:07PM +0800, Pan Xinhui wrote:
>> 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
>
> How was xchg_u8_asm() implemented, using lbarx or using a 32bit ll/sc
> loop with shifting and masking in it?
>
yes, using 32bit ll/sc loops.

looks like:
__asm__ __volatile__(
"1: lwarx %0,0,%3\n"
" and %1,%0,%5\n"
" or %1,%1,%4\n"
PPC405_ERR77(0,%2)
" stwcx. %1,0,%3\n"
" bne- 1b"
: "=&r" (_oldv), "=&r" (tmp), "+m" (*(volatile unsigned int *)_p)
: "r" (_p), "r" (_newv), "r" (_oldv_mask)
: "cc", "memory");


> Regards,
> Boqun
>
>> [ 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
>>

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