From: "Mattias Rönnblom" <hofors@lysator.liu.se>
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: "Ferruh Yigit" <ferruh.yigit@amd.com>,
"Mattias Rönnblom" <mattias.ronnblom@ericsson.com>,
"John W . Linville" <linville@tuxdriver.com>,
dev@dpdk.org, "Tyler Retzlaff" <roretzla@linux.microsoft.com>,
"Honnappa Nagarahalli" <Honnappa.Nagarahalli@arm.com>
Subject: Re: [PATCH] net/af_packet: cache align Rx/Tx structs
Date: Thu, 25 Apr 2024 11:26:00 +0200 [thread overview]
Message-ID: <4c0f1625-4387-4870-a60a-dc423885811e@lysator.liu.se> (raw)
In-Reply-To: <20240424165534.2ad8ae0a@hermes.local>
On 2024-04-25 01:55, Stephen Hemminger wrote:
> On Thu, 25 Apr 2024 00:27:36 +0200
> Mattias Rönnblom <hofors@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.yigit@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.)
next prev parent reply other threads:[~2024-04-25 9:26 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-23 9:08 Mattias Rönnblom
2024-04-23 11:15 ` Ferruh Yigit
2024-04-23 20:56 ` Mattias Rönnblom
2024-04-24 0:27 ` Honnappa Nagarahalli
2024-04-24 6:28 ` Mattias Rönnblom
2024-04-24 10:21 ` Ferruh Yigit
2024-04-24 10:28 ` Bruce Richardson
2024-04-24 18:02 ` Ferruh Yigit
2024-04-24 11:57 ` Mattias Rönnblom
2024-04-24 17:50 ` Ferruh Yigit
2024-04-24 19:13 ` Stephen Hemminger
2024-04-24 22:27 ` Mattias Rönnblom
2024-04-24 23:55 ` Stephen Hemminger
2024-04-25 9:26 ` Mattias Rönnblom [this message]
2024-04-25 9:49 ` Morten Brørup
2024-04-25 14:04 ` Ferruh Yigit
2024-04-25 15:06 ` Mattias Rönnblom
2024-04-25 16:21 ` Ferruh Yigit
2024-04-25 15:07 ` Stephen Hemminger
2024-04-25 14:08 ` Ferruh Yigit
2024-04-25 15:08 ` Mattias Rönnblom
2024-04-25 15:35 ` Ferruh Yigit
2024-04-26 7:25 ` Mattias Rönnblom
2024-04-26 7:38 ` Mattias Rönnblom
2024-04-26 8:27 ` Ferruh Yigit
2024-04-26 10:20 ` Mattias Rönnblom
2024-04-26 9:05 ` [PATCH v3] " Mattias Rönnblom
2024-04-26 9:22 ` Morten Brørup
2024-04-26 15:10 ` Stephen Hemminger
2024-04-26 15:41 ` Tyler Retzlaff
2024-04-29 8:46 ` Ferruh Yigit
2024-04-26 21:27 ` [PATCH] " Patrick Robb
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