netdev - Re: [PATCH v3 net-next 3/5] tcp: Access &tcp

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Date:   Fri, 2 Sep 2022 20:25:39 -0700
From:   Kuniyuki Iwashima <kuniyu@...zon.com>
To:     <edumazet@...gle.com>
CC:     <davem@...emloft.net>, <kuba@...nel.org>, <kuni1840@...il.com>,
        <kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v3 net-next 3/5] tcp: Access &tcp_hashinfo via net.

From:   Eric Dumazet <edumazet@...gle.com>
Date:   Fri, 2 Sep 2022 20:16:11 -0700
> On Fri, Sep 2, 2022 at 7:50 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> 
> > Ah, exactly.
> > Then I will add ____cacheline_aligned_in_smp to tw_refcount, instead of
> > keeping the original order, to avoid invalidation by sysctl_max_tw_buckets
> > change, which wouldn't be so frequently done though.
> >
> >  struct inet_timewait_death_row {
> > -       refcount_t              tw_refcount;
> > -
> > -       struct inet_hashinfo    *hashinfo ____cacheline_aligned_in_smp;
> > +       struct inet_hashinfo    *hashinfo;
> > +       refcount_t              tw_refcount ____cacheline_aligned_in_smp;
> >         int                     sysctl_max_tw_buckets;
> 
> This would move sysctl_max_tw_buckets in a separate/dedicated cache line :/
> 
> -->
> 
> {
>    refcount_t              tw_refcount;
>    /* Padding to avoid false sharing, tw_refcount can be often written */
>     struct inet_hashinfo    *hashinfo ____cacheline_aligned_in_smp;
>    int                     sysctl_max_tw_buckets;
> 
>   .. other read only fields could fit here.
> 
> >  };
> 
> Explicit alignment of the structure or first field is not needed,
> they will already be cache line aligned.

I got it.  I'll add that change.
Thank you so much!