Subject: Re: [PATCH net-next 3/3] packet: use percpu mmap tx frame pending refcount

From: Daniel Borkmann <[email-masked]>
Date: Mon, 13 Jan 2014 12:19:01 +0100

On 01/13/2014 06:51 AM, Cong Wang wrote:
> On Sun, Jan 12, 2014 at 8:22 AM, Daniel Borkmann <[email-masked]> wrote:
>> +static void packet_inc_pending(struct packet_ring_buffer *rb)
>> +{
>> +       this_cpu_inc(*rb->pending_refcnt);
>> +}
>> +
>> +static void packet_dec_pending(struct packet_ring_buffer *rb)
>> +{
>> +       this_cpu_dec(*rb->pending_refcnt);
>> +}
>> +
>> +static int packet_read_pending(const struct packet_ring_buffer *rb)
>> +{
>> +       int i, refcnt = 0;
>> +
>> +       /* We don't use pending refcount in rx_ring. */
>> +       if (rb->pending_refcnt == NULL)
>> +               return 0;
>> +
>> +       for_each_possible_cpu(i)
>> +               refcnt += *per_cpu_ptr(rb->pending_refcnt, i);
>> +
>> +       return refcnt;
>> +}
>
> How is this supposed to work? Since there is no lock,
> you can't read accurate refcnt. Take a look at lib/percpu_counter.c.
>
> I guess for some reason you don't care the accuracy?

Yep, not per se. Look at how we do net device reference counting.
The reason is that we call packet_read_pending() *only* after we
finished processing all frames in TX_RING and we wait for
completion in case MSG_DONTWAIT is *not set*, when that happens
we're back to 0.

But I think I found a different problem with this idea. It could
happen with net devices as well, but probably less likely as there
might be a better distribution of hold/puts among CPUs. However,
for TX_RING, if we pin the process to a particular CPU, and since
the destructor is invoked through ksoftirqd, we could end up with
a misbalance and if the process runs long enough eventually
overflow for one particular CPU. We could work around that, but I
think it's not worth the effort.

Dave, please drop the 3rd patch of the series, thanks.

> Then at least you need to comment in the code.
>
> Thanks.
>