Re: [PATCH] percpu_counter: return precise count from __percpu_counter_compare()

Waiman Long
2015-10-08T16:01:24Z

On 10/07/2015 07:04 PM, Dave Chinner wrote:
> On Wed, Oct 07, 2015 at 04:00:42PM -0400, Waiman Long wrote:
>> On 10/06/2015 05:30 PM, Dave Chinner wrote:
>>>>> /*
>>>>> * Aggregate the per-cpu counter magazines back into the global
>>>>> * counter. This avoids the need for repeated compare operations to
>>>>> * run the slow path when the majority of the counter value is held
>>>>> * in the per-cpu magazines. Folding them back into the global
>>>>> * counter means we will continue to hit the fast
>>>>> * percpu_counter_read() path until the counter value falls
>>>>> * completely within the comparison limit passed to
>>>>> * __percpu_counter_compare().
>>>>> */
>>>>> static s64 percpu_counter_aggregate(struct percpu_counter *fbc)
>>>>> {
>>>>> s64 ret;
>>>>> int cpu;
>>>>> unsigned long flags;
>>>>>
>>>>> raw_spin_lock_irqsave(&fbc->lock, flags);
>>>>> ret = fbc->count;
>>>>> for_each_online_cpu(cpu) {
>>>>> s32 count = __this_cpu_read(*fbc->counters);
>>>>> ret += count;
>>>>> __this_cpu_sub(*fbc->counters, count)
>>>>> }
>>>>> fbc->count = ret;
>>>>> raw_spin_unlock_irqrestore(&fbc->lock, flags);
>>>>> return ret;
>>>>> }
>>>> I don't think that will work as some other CPUs may change the
>>>> percpu counters values between percpu_counter_aggregate() and
>>>> __percpu_counter_compare(). To be safe, the precise counter has to
>>>> be compted whenever the comparison value difference is less than
>>>> nr_cpus * batch size.
>>> Well, yes. Why do you think the above function does the same
>>> function as percpu_counter_sum()? So that the percpu_counter_sum()
>>> call *inside* __percpu_counter_compare() can be replaced by this
>>> call. i.e.
>>>
>>> return -1;
>>> }
>>> /* Need to use precise count */
>>> - count = percpu_counter_sum(fbc);
>>> + count = percpu_counter_aggregate(fbc);
>>> if (count> rhs)
>>> return 1;
>>> else if (count< rhs)
>>>
>>> Please think about what I'm saying rather than dismissing it without
>>> first understanding my suggestions.
>> I understood what you were saying. However, the per-cpu counter
>> isn't protected by the spinlock. Reading it is OK, but writing may
>> cause race if that counter is modified by a CPU other than its
>> owning CPU.
> <sigh>
>
> You're still trying to pick apart the code without considering what
> we need to acheive. We don't need to the code to be bullet proof to
> test whether this hypothesis is correct or not - we just need
> something that is "near-enough" to give us the data point to tell us
> where we should focus our efforts. If optimising the counter like
> above does not reduce the overhead, then we may have to change XFS.
> If it does reduce the overhead, then the XFS code remains unchanged
> and we focus on optimising the counter code.

What determine if a precise sum is to be computed is the following code:

if (abs(count - rhs) > (batch * num_online_cpus())) {

So even if we make the global count more accurate using
percpu_counter_aggregate(), it won't have too much effect in reducing
the chance where the precise count needs to be calculated. That is why I
don't bother testing it with the modified code.

Cheers,
Longman