Re: [PATCH v3] x86/hpet: Reduce HPET counter read contention

[Waiman Long <waiman.long@hpe.com>]

On 04/12/2016 12:37 PM, Waiman Long wrote:
> On 04/11/2016 04:21 PM, Andy Lutomirski wrote:
>>
>>> +
>>> +                       /* Unlock */
>>> +                       smp_store_release(&hpet_save.seq, new + 1);
>>> +                       local_irq_restore(flags);
>>> +                       return (cycle_t)time;
>>> +               }
>>> +               local_irq_restore(flags);
>>> +               seq = new;
>>> +       }
>>> +
>>> +       /*
>>> +        * Wait until the locked sequence number changes which 
>>> indicates
>>> +        * that the saved HPET value is up-to-date.
>>> +        */
>>> +       while (READ_ONCE(hpet_save.seq) == seq) {
>>> +               /*
>>> +                * Since reading the HPET is much slower than a single
>>> +                * cpu_relax() instruction, we use two here in an 
>>> attempt
>>> +                * to reduce the amount of cacheline contention in the
>>> +                * hpet_save.seq cacheline.
>>> +                */
>>> +               cpu_relax();
>>> +               cpu_relax();
>>> +       }
>>> +
>>> +       return (cycle_t)READ_ONCE(hpet_save.hpet);
>>> +}
>> I wonder if this could be simplified.  Pseudocode:
>>
>> u32 time;
>> unsigned long flags;
>>
>> local_irq_save(flags);
>>
>> if (spin_trylock(&hpet_lock)) {
>>    time = hpet_readl(HPET_COUNTER);
>>    WRITE_ONCE(last_hpet_counter, time);
>
> You will need a spin_unlock(&hpet_lock) here.
>
>> } else {
>>    spin_unlock_wait(&hpet_lock);
>>    /* When this function started, hpet_lock was locked.  Now it's
>> unlocked, which means that time is at least as new as whatever the
>> lock holder returned. */
>>   time = READ_ONCE(last_hpet_counter);
>> }
>>
>> local_irq_restore(flags);
>> return time;
>>
>> Should be fasterunder heavy contention, too: spinlocks are very nicely
>> optimized.
>
> I don't think it will be faster. The current spinlock code isn't more 
> optimized than what you can do with a cmpxchg and smp_store_release. 
> In fact, it is what the spinlock code is actually doing. Other 
> differences includes:
>
> 1) A CPU will not do local_irq_save/local_irq_restore when the lock is 
> not free.
> 2) My patch also use a change a sequence number to indicate an updated 
> time stamp is available. So there will be cases where CPUs running 
> your code will have to wait while the those running my code can grab 
> the time stamp and return immediately.
>

Moreover, if the timing is such that right after one CPU release the 
lock, the next one get it immediately and there is a continuous stream 
of incoming CPUs. It is possible that the ones that are waiting for the 
lock to be free will see the lock being acquired for an indefinite 
period of time. This is certainly not something we want to have and this 
is what the sequence number is for.

Cheers,
Longman