Re: [PATCH] Optimize libc_lock_lock for MIPS XLP.

[Tom de Vries <Tom_deVries@mentor.com>]

On 28/06/12 19:30, Chris Metcalf wrote:
> On 6/27/2012 5:45 PM, Maxim Kuvyrkov wrote:
>> On 15/06/2012, at 2:49 PM, Maxim Kuvyrkov wrote:
>>
>>>> On 15/06/2012, at 2:44 PM, Chris Metcalf wrote:
>>>>
>>>>>> On 6/14/2012 9:20 PM, Maxim Kuvyrkov wrote:
>>>> ...
>>>>>>>> As I read it, in case of a contended lock __lll_lock_wait will reset the value of the lock to "2" before calling lll_futex_wait().  I agree that there is a timing window in which the other threads will see a value of the lock greater than "2", but the value will not get as high as hundreds or billions as it will be constantly reset to "2" in atomic_exchange in lll_lock_wait().
>>>>>>>>
>>>>>>>> I do not see how threads will get into a busywait state, though.  Would you please elaborate on that?
>>>>>>
>>>>>> You are correct.  I was thinking the that the while loop had a cmpxchg, but
>>>>>> since it's just a straight-up exchange, the flow will be something like:
>>>>>>
>>>>>> - Fail to initially call lll_futex_wait() if the lock is contended
>>>>>> - Fall through to while loop
>>>>>> - Spin as long as the lock is contended enough that *futex > 2
>>>>>> - Enter futex_wait
>>>>>>
>>>>>> So a little busy under high contention, but probably settles out reasonably
>>>>>> well.
>>>>
>> Attached is an improved patch that also optimizes __libc_lock_trylock using XLP's atomic instructions.
>>
>> The patch also removes unnecessary indirection step represented by new macros lll_add_lock, which is then used to define __libc_lock_lock, and defines __libc_lock_lock and __libc_lock_trylock directly in lowlevellock.h .  This makes changes outside of ports/ trivial.
>>
>> Tested on MIPS XLP with no regressions.  OK to apply for 2.17?
> 
> It looks OK to me.  I would want someone else to sign off on it before
> applying to 2.17.
> 

Chris,

I cannot sign off on this, but I reviewed the current patch as well and it looks
ok to me too.

Thanks,
- Tom