netdev - Re: [PATCH net-next 1/1] net sched actions: add time filter for action dumping

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Date:   Wed, 19 Apr 2017 06:44:50 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Jakub Kicinski <kubakici@...pl>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        xiyou.wangcong@...il.com, eric.dumazet@...il.com, jiri@...nulli.us
Subject: Re: [PATCH net-next 1/1] net sched actions: add time filter for
 action dumping

On 17-04-18 11:55 PM, Jakub Kicinski wrote:
> On Tue, 18 Apr 2017 21:37:12 -0400, Jamal Hadi Salim wrote:
>> On 17-04-18 06:12 PM, Jakub Kicinski wrote:

[..]
>> I see both being fine from that perspective - you dont need 100%
>> accuracy. Just something that is within reason of a small delta
>> of time.
>
> I'm just referring to the theoretical possibility that if the dumping
> process gets preempted for long enough you may loose samples.  Just
> because the dumping process cannot control when kernel executes this
> line:
>
> 	jiffy_wanted = jiffies - jiffy_msecs;
>
> It could in theory be few seconds after the request was made.  Perhaps
> using timestamps from a proper time sources instead of the notion of
> "last X seconds" would solve that?

Good point which i didnt mention as part of the drawbacks. This
is a tradeoff. We dont need to be 100% accurate[1].
The timestamps on the action entries in the kernel are in jiffies;
a lot simpler to do jiffy comparison. If you used a different
timestamp source you'd need to convert for every comparison you
make (i am not sure how costly is when you have many actions).

In use cases i am familiar with, there is a user process app which
opens the socket once and issues dumps every X seconds (ranging from
5-120 seconds). So we will re-issue the dump regardless. Yes
it would be an issue if said application keeps getting pre-empted
and that jiffy computation was always off - but not sure under
what circumstances that could be a common scenario.

cheers,
jamal

[1] As an example, dumps are never 100% accurate you could iterate
  something that then changes while you are in the middle of dumping
  which then renders an already dumped entity obsolete.