[Codel] OpenWRT wrong adjustment of fq_codel defaults (Was: fq_codel_drop vs a udp flood)

[Roman Yeryomin]

On 7 May 2016 at 12:57, Kevin Darbyshire-Bryant
<kevin at darbyshire-bryant.me.uk> wrote:
>
>
> On 06/05/16 10:42, Jesper Dangaard Brouer wrote:
>> Hi Felix,
>>
>> This is an important fix for OpenWRT, please read!
>>
>> OpenWRT changed the default fq_codel sch->limit from 10240 to 1024,
>> without also adjusting q->flows_cnt.  Eric explains below that you must
>> also adjust the buckets (q->flows_cnt) for this not to break. (Just
>> adjust it to 128)
>>
>> Problematic OpenWRT commit in question:
>>  http://git.openwrt.org/?p=openwrt.git;a=patch;h=12cd6578084e
>>  12cd6578084e ("kernel: revert fq_codel quantum override to prevent it from causing too much cpu load with higher speed (#21326)")
> I 'pull requested' this to the lede-staging tree on github.
> https://github.com/lede-project/staging/pull/11
>
> One way or another Felix & co should see the change :-)

If you would follow the white rabbit, you would see that it doesn't help

>>
>>
>> I also highly recommend you cherry-pick this very recent commit:
>>  net-next: 9d18562a2278 ("fq_codel: add batch ability to fq_codel_drop()")
>>  https://git.kernel.org/davem/net-next/c/9d18562a227
>>
>> This should fix very high CPU usage in-case fq_codel goes into drop mode.
>> The problem is that drop mode was considered rare, and implementation
>> wise it was chosen to be more expensive (to save cycles on normal mode).
>> Unfortunately is it easy to trigger with an UDP flood. Drop mode is
>> especially expensive for smaller devices, as it scans a 4K big array,
>> thus 64 cache misses for small devices!
>>
>> The fix is to allow drop-mode to bulk-drop more packets when entering
>> drop-mode (default 64 bulk drop).  That way we don't suddenly
>> experience a significantly higher processing cost per packet, but
>> instead can amortize this.
> I haven't done the above cherry-pick patch & backport patch creation for
> 4.4/4.1/3.18 yet - maybe if $dayjob permits time and no one else beats
> me to it :-)
>
> Kevin
>