netdev - Re: [PATCH iproute2-next] tc: pie: change maximum integer value of tc_pie

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Date:   Mon, 9 Mar 2020 10:54:45 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Ahern <dsahern@...il.com>,
        Leslie Monis <lesliemonis@...il.com>,
        Linux NetDev <netdev@...r.kernel.org>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        "Mohit P . Tahiliani" <tahiliani@...k.edu.in>,
        Gautam Ramakrishnan <gautamramk@...il.com>
Subject: Re: [PATCH iproute2-next] tc: pie: change maximum integer value of
 tc_pie_xstats->prob



On 3/9/20 10:48 AM, Eric Dumazet wrote:
> 
> 
> On 3/8/20 7:49 PM, David Ahern wrote:
>> On 3/5/20 9:25 AM, Leslie Monis wrote:
>>> Kernel commit 105e808c1da2 ("pie: remove pie_vars->accu_prob_overflows"),
>>> changes the maximum value of tc_pie_xstats->prob from (2^64 - 1) to
>>> (2^56 - 1).
>>>
>>> Signed-off-by: Mohit P. Tahiliani <tahiliani@...k.edu.in>
>>> Signed-off-by: Gautam Ramakrishnan <gautamramk@...il.com>
>>> Signed-off-by: Leslie Monis <lesliemonis@...il.com>
>>> ---
>>>  tc/q_pie.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>
>> applied to iproute2-next. Thanks
>>
>>
> 
> This means that iproute2 is incompatible with old kernels.
> 
> commit 105e808c1da2 ("pie: remove pie_vars->accu_prob_overflows") was wrong,
> it should not have changed user ABI.
> 
> The rule is : iproute2 v-X should work with linux-<whatever-version>
> 
> Since pie MAX_PROB was implicitly in the user ABI, it can not be changed,
> at least from user point of view.
> 

So this kernel patch might be needed :

diff --git a/net/sched/sch_pie.c b/net/sched/sch_pie.c
index f52442d39bf57a7cf7af2595638a277e9c1ecf60..c65077f0c0f39832ee97f4e89f25639306b19281 100644
--- a/net/sched/sch_pie.c
+++ b/net/sched/sch_pie.c
@@ -493,7 +493,7 @@ static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
 {
        struct pie_sched_data *q = qdisc_priv(sch);
        struct tc_pie_xstats st = {
-               .prob           = q->vars.prob,
+               .prob           = q->vars.prob << BITS_PER_BYTE,
                .delay          = ((u32)PSCHED_TICKS2NS(q->vars.qdelay)) /
                                   NSEC_PER_USEC,
                .packets_in     = q->stats.packets_in,