netdev - Re: [PATCH net-next] net/sched: get rid of qdisc->padded

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Message-ID: <20201008171846.335b435a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 8 Oct 2020 17:18:46 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Allen Pais <allen.lkml@...il.com>
Subject: Re: [PATCH net-next] net/sched: get rid of qdisc->padded

On Wed,  7 Oct 2020 09:51:11 -0700 Eric Dumazet wrote:
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index 6c762457122fd0091cb0f2bf41bda73babc4ac12..d8fd8676fc724110630904909f64d7789f3a4b47 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -91,7 +91,7 @@ struct Qdisc {
>  	struct net_rate_estimator __rcu *rate_est;
>  	struct gnet_stats_basic_cpu __percpu *cpu_bstats;
>  	struct gnet_stats_queue	__percpu *cpu_qstats;
> -	int			padded;
> +	int			pad;
>  	refcount_t		refcnt;
>  
>  	/*

Hi Eric!

Why keep the pad field? the member to lines down is
__cacheline_aligned, so we shouldn't have to manually 
push things out?

        struct gnet_stats_queue __percpu *cpu_qstats;                           
        int                     pad;                                            
        refcount_t              refcnt;                                         
                                                                                
        /*                                                                      
         * For performance sake on SMP, we put highly modified fields at the end                                                                               
         */                                                                     
        struct sk_buff_head     gso_skb ____cacheline_aligned_in_smp;