Re: [PATCH] percpu_counter: Fix __percpu_counter_sum()

Andrew Morton
2008-12-10T05:50:18Z

On Wed, 10 Dec 2008 06:09:08 +0100 Eric Dumazet <dada1@cosmosbay.com> wrote:

> Now percpu_counter_sum() is 'fixed', what about "percpu_counter_add()" ?
>
> void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
> {
> s64 count;
> s32 *pcount;
> int cpu = get_cpu();
>
> pcount = per_cpu_ptr(fbc->counters, cpu);
> count = *pcount + amount;
> if (count >= batch || count <= -batch) {
> spin_lock(&fbc->lock);
> fbc->count += count;
> *pcount = 0;
> spin_unlock(&fbc->lock);
> } else {
> *pcount = count;
> }
> put_cpu();
> }
>
>
> If I read this well, this is not IRQ safe.

Sure. It's racy against interrupts on this cpu, it'll deadlock over
the non-irq-safe spinlock and lockdep will have a coronary over it.

> get_cpu() only disables preemption IMHO

yes

> For nr_files, nr_dentry, nr_inodes, it should not be a problem.

yes

> But for network counters (only in net-next-2.6)
> and lib/proportions.c, we have a problem ?

yes

> Using local_t instead of s32 for cpu
> local counter here is possible, so that fast path doesnt have
> to disable interrupts
>
> (use a local_t instead of s32 for fbc->counters)
>
> void __percpu_counter_add_irqsafe(struct percpu_counter *fbc, s64 amount, s32 batch)
> {
> long count;
> local_t *pcount;
>
> /* following code only matters on 32bit arches */
> if (sizeof(amount) != sizeof(local_t)) {
> if (unlikely(amount >= batch || amount <= -batch))) {
> spin_lock_irqsave(&fbc->lock, flags);
> fbc->count += amount;
> spin_unlock_irqrestore(&fbc->lock, flags);
> return;
> }
> }
> pcount = per_cpu_ptr(fbc->counters, get_cpu());
> count = local_add_return((long)amount, pcount);
> if (unlikely(count >= batch || count <= -batch)) {
> unsigned long flags;
>
> local_sub(count, pcount);
> spin_lock_irqsave(&fbc->lock, flags);
> fbc->count += count;
> spin_unlock_irqrestore(&fbc->lock, flags);
> }
> put_cpu();
> }

I think it's reasonable. If the batching is working as intended, the
increased cost of s/spin_lock/spin_lock_irqsave/ should be
insignificant.

In fact, if *at all* possible it would be best to make percpu_counters
irq-safe under all circumstances and avoid fattening and complicating the
interface.

But before adding more dependencies on local_t I do think we should
refresh ourselves on Christoph's objections to them - I remember
finding them fairly convincing at the time, but I don't recall the
details.

<searches for a long time>

Here, I think:
http://lkml.indiana.edu/hypermail/linux/kernel/0805.3/2482.html

Rusty, Christoph: talk to me. If we add a new user of local_t in core
kernel, will we regret it?

Thanks.