netdev - Re: [PATCH net-next 03/13] net: sched: extend Qdisc with rcu

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Message-ID: <vbf1sa7x9m9.fsf@reg-r-vrt-018-180.mtr.labs.mlnx>
Date:   Thu, 06 Sep 2018 12:23:26 +0300
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
        jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        davem@...emloft.net, stephen@...workplumber.org,
        paulmck@...ux.vnet.ibm.com, nicolas.dichtel@...nd.com,
        leon@...nel.org, gregkh@...uxfoundation.org, mark.rutland@....com,
        fw@...len.de, dsahern@...il.com, lucien.xin@...il.com,
        jakub.kicinski@...ronome.com, christian.brauner@...ntu.com,
        jbenc@...hat.com
Subject: Re: [PATCH net-next 03/13] net: sched: extend Qdisc with rcu


On Thu 06 Sep 2018 at 08:39, Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> On 06.09.2018 11:30, Eric Dumazet wrote:
>> 
>> 
>> On 09/06/2018 12:58 AM, Vlad Buslov wrote:
>> 
>> ...
>> 
>>> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
>>> index 18e22a5a6550..239c73f29471 100644
>>> --- a/include/net/sch_generic.h
>>> +++ b/include/net/sch_generic.h
>>> @@ -90,6 +90,7 @@ struct Qdisc {
>>>  	struct gnet_stats_queue	__percpu *cpu_qstats;
>>>  	int			padded;
>>>  	refcount_t		refcnt;
>>> +	struct rcu_head		rcu;
>>>  
>>>  	/*
>>>  	 * For performance sake on SMP, we put highly modified fields at the end
>> 
>> Probably better to move this at the end of struct Qdisc,
>> not risking unexpected performance regressions in fast path.
>
> Do you mean regressions on UP? On SMP it looks like this field
> fits in the unused gap created by:
>
> 	struct sk_buff_head     gso_skb ____cacheline_aligned_in_smp;
>
> Kirill

Hi Eric, Kirill

I intentionally put rcu_head here in order for it not to be in same
cache line with "highly modified fields" (according to comment).