Re: Improve spinlock performance by moving work to one core

[Ling Ma]

Run thread.c with clean kernel  4.3.0-rc4, perf top -G also indicates
cache_flusharray and cache_alloc_refill functions spend 25.6% time
on queued_spin_lock_slowpath totally. it means the compared data
from our spinlock-test.patch is reliable.

Thanks
Ling

2015-11-26 11:49 GMT+08:00 Ling Ma <ling.ma.program@xxxxxxxxx>:
> Hi Longman,
>
> All compared data is from the below operation in spinlock-test.patch:
>
> +#if ORG_QUEUED_SPINLOCK
> +       org_queued_spin_lock((struct qspinlock *)&pa.n->list_lock);
> +       refill_fn(&pa);
> +       org_queued_spin_unlock((struct qspinlock *)&pa.n->list_lock);
> +#else
> +       new_spin_lock((struct nspinlock *)&pa.n->list_lock, refill_fn, &pa);
> +#endif
>
> and
>
> +#if ORG_QUEUED_SPINLOCK
> +       org_queued_spin_lock((struct qspinlock *)&pa.n->list_lock);
> +       flusharray_fn(&pa);
> +       org_queued_spin_unlock((struct qspinlock *)&pa.n->list_lock);
> +#else
> +       new_spin_lock((struct nspinlock *)&pa.n->list_lock, flusharray_fn, &pa);
> +#endif
>
> So the result is correct and fair.
>
> Yes, we updated the code in include/asm-generic/qspinlock.h to
> simplified modification and avoid kernel crash,
> for example there are 10 lock scenarios to use new spin lock,
> because bottle-neck is only from one or two scenarios, we only modify them,
> other lock scenarios will continue to use the lock in qspinlock.h, we
> must modify the code,
> otherwise the operation will be hooked in the queued and never be waken up.
>
> Thanks
> Ling
>
>
>
> 2015-11-26 3:05 GMT+08:00 Waiman Long <waiman.long@xxxxxxx>:
>> On 11/23/2015 04:41 AM, Ling Ma wrote:
>>> Hi Longman,
>>>
>>> Attachments include user space application thread.c and kernel patch
>>> spinlock-test.patch based on kernel 4.3.0-rc4
>>>
>>> we run thread.c with kernel patch, test original and new spinlock respectively,
>>> perf top -G indicates thread.c cause cache_alloc_refill and
>>> cache_flusharray functions to spend ~25% time on original spinlock,
>>> after introducing new spinlock in two functions, the cost time become ~22%.
>>>
>>> The printed data  also tell us the new spinlock improves performance
>>> by about 15%( 93841765576 / 81036259588) on E5-2699V3
>>>
>>> Appreciate your comments.
>>>
>>>
>>
>> I saw that you make the following changes in the code:
>>
>> static __always_inline void queued_spin_lock(struct qspinlock *lock)
>> {
>> u32 val;
>> -
>> +repeat:
>> val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
>> if (likely(val == 0))
>> return;
>> - queued_spin_lock_slowpath(lock, val);
>> + goto repeat;
>> + //queued_spin_lock_slowpath(lock, val);
>> }
>>
>>
>> This effectively changes the queued spinlock into an unfair byte lock.
>> Without a pause to moderate the cmpxchg() call, that is especially bad
>> for performance. Is the performance data above refers to the unfair byte
>> lock versus your new spinlock?
>>
>> Cheers,
>> Longman
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