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

[Waiman Long]

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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