[dpdk-dev] [PATCH] sched: add 64-bit counter retrieval API

[Alan Dewar]

Hi Stephen,

Sorry about the delay in responding to your comment.

Maybe I'm missing something but I don't consider 64-bit counter
rollover to be a problem.
The DPDK QoS stats are just 32-bits wide and are zeroed when read.  So
if we read them frequently enough the 32-bit counters won't wrap.

This new API is just a convenient way to read the 32-bit counters into
variables that are 64-bit wide which allows Vyatta to have a common
API slightly above the DPDK.

I'd hope that at some point in time the DPDK will move to 64-bit
counters, but there may be technical reasons why this can't happen
that I'm not aware of (support for 32-bit CPUs??)

Regards
Alan

On Mon, Jul 23, 2018 at 5:52 PM, Stephen Hemminger
<stephen at networkplumber.org> wrote:
> On Wed, 18 Jul 2018 15:51:39 +0100
> alangordondewar at gmail.com wrote:
>
>> From: Alan Dewar <alan.dewar at att.com>
>>
>> Add new APIs to retrieve counters in 64-bit wide fields.
>>
>> Signed-off-by: Alan Dewar <alan.dewar at att.com>
>
> Do you want to consider 64 bit counter roll over on 32 bit platform?
> The problem is that incrementing an 64 bit value is not atomic on
> 32 bit cpu. The carry operation can race with reading.
>
> The kernel has special case code to do restartable sequence for
> accessing 64 bit counter on 32 bit CPU. These functions become
> nop's on 64bit.