linux-kernel - Re: [PATCH] percpu_counter: return precise count from __percpu_counter

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Date:	Thu, 08 Oct 2015 12:01:16 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Chinner <david@...morbit.com>
CC:	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH] percpu_counter: return precise count from __percpu_counter_compare()

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

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