On 2024-04-25 16:04, Ferruh Yigit wrote:
On 4/25/2024 10:26 AM, Mattias Rönnblom wrote:
On 2024-04-25 01:55, Stephen Hemminger wrote:
On Thu, 25 Apr 2024 00:27:36 +0200
Mattias Rönnblom <hof...@lysator.liu.se> wrote:
On 2024-04-24 21:13, Stephen Hemminger wrote:
On Wed, 24 Apr 2024 18:50:50 +0100
Ferruh Yigit <ferruh.yi...@amd.com> wrote:
I don't know how slow af_packet is, but if you care about
performance,
you don't want to use atomic add for statistics.
There are a few soft drivers already using atomics adds for
updating stats.
If we document expectations from 'rte_eth_stats_reset()', we can
update
those usages.
Using atomic add is lots of extra overhead. The statistics are not
guaranteed
to be perfect. If nothing else, the bytes and packets can be skewed.
The sad thing here is that in case the counters are reset within the
load-modify-store cycle of the lcore counter update, the reset may end
up being a nop. So, it's not like you missed a packet or two, or suffer
some transient inconsistency, but you completed and permanently ignored
the reset request.
That is one of the many reasons the Linux kernel intentionally did not
implement a reset statistics operation.
DPDK should deprecate statistics reset, it seems to me.
The current API is broken anyway, if you care about correctness. A reset
function must return the current state of the counters, at the point
them being reset. Otherwise, a higher layer may miss counter updates.
The af_packet PMD should return -ENOTSUP on reset (which is allowed by
the API).
Maintaining an offset-since-last-reset for counters is a control plane
thing, and shouldn't be in PMDs. (If MT-safe reset for SW-managed
counters are to be expected from the PMDs, we should have some helper
API to facilitate its efficient & correct-enough implementation.)
statistics reset works for HW devices, instead of removing statics reset
I am for documenting API that it may be not reliable, I can send a patch
for it.
With API you mean <rte_ethdev.h>?
If rte_ethdev_stats_reset() sometimes reset the counters, and sometimes
doesn't, it should also have a name that reflect those semantics.
rte_ethdev_stats_reset_or_perhaps_not()
rte_ethdev_stats_usually_reset()
Rather than expecting the application to deal with unspecified
non-determinism is seems better to specify under which conditions the
reset is reliable (i.e., it's not MT safe). A non-MT-safe reset will
limit it usefulness though. Also, it will make having an MT safe reset
in a PMD pretty useless, except if the app is programmed not toward the
API, but toward some particular PMD.
With above change, we can be more relax on stats update specially for
soft drivers, and can convert atomic_add stats updates to "atomic load +
add + atomic store".
Does this plan make sense?
Not really.
Short-term the -ENOTSUP seems like the best option. Second best option
is to implement a proper MT safe reset.
What is unfortunate is that the API is silent on MT safety. I've
*assumed* that many users will have assumed it MT safe, but there's
nothing in the documentation to support that. Rather the opposite, the
generic section of DPDK MT safety only mentions PMD RX and TX functions.
This issue is not limited to this PMD or even to ethdev.
rte_event_dev_xstats_reset() and some of the event devs have the same
problem.
