netdev - Re: [PATCH net 1/1] net sched: fix reporting the first-time use timestamp

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Date:   Mon, 18 May 2020 10:58:34 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Roman Mashak <mrv@...atatu.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        kernel@...atatu.com, Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH net 1/1] net sched: fix reporting the first-time use timestamp

On Mon, May 18, 2020 at 5:43 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
>
> On 2020-05-17 9:10 p.m., Roman Mashak wrote:
> > Cong Wang <xiyou.wangcong@...il.com> writes:
> >
> >> On Sun, May 17, 2020 at 5:47 AM Roman Mashak <mrv@...atatu.com> wrote:
> >>>
> >>> When a new action is installed, firstuse field of 'tcf_t' is explicitly set
> >>> to 0. Value of zero means "new action, not yet used"; as a packet hits the
> >>> action, 'firstuse' is stamped with the current jiffies value.
> >>>
> >>> tcf_tm_dump() should return 0 for firstuse if action has not yet been hit.
> >>
> >> Your patch makes sense to me.
> >>
> >> Just one more thing, how about 'lastuse'? It is initialized with jiffies,
> >> not 0, it seems we should initialize it to 0 too, as it is not yet used?
> >
> > Yes, exactly. I was planning to send a separate patch for this.
> >
> > Thanks for review, Cong.
> >
>
> For these corner cases, firstuse using zero to indicate
> "has not been used" is not ambigious.
> lastuse has ambiguity because zero now has two meanings
> if you check for the corner case in the kernel.
> 1)Zero is a legit value when dumping or
> getting (example an action was just hit when you dumped).
> 2) zero also now means "has not been used".

Well, technically firstuse could be a legit 0 too, when the
action was just hit for the first time right when dumping.
So the ambiguity is same for both.

>
> My suggestion is to leave this alone in the kernel.
> In user space/iproute2 check if lastused and created
> are equal and declare "has not been used".

Sounds a good idea to me.

Thanks.