netdev - Re: [patch 1/3] net: dst: Prevent false sharing vs. dst_entry::_

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Date:   Tue, 28 Feb 2023 22:31:40 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Wangyang Guo <wangyang.guo@...el.com>,
        Arjan van De Ven <arjan@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: [patch 1/3] net: dst: Prevent false sharing vs.
 dst_entry::__refcnt

Eric!

On Tue, Feb 28 2023 at 16:17, Eric Dumazet wrote:
> On Tue, Feb 28, 2023 at 3:33 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>> perf top identified two affected read accesses. dst_entry::lwtstate and
>> rtable::rt_genid.
>>
>> dst_entry:__refcnt is located at offset 64 of dst_entry, which puts it into
>> a seperate cacheline vs. the read mostly members located at the beginning
>> of the struct.
>
> This will probably increase struct rt6_info past the 4 cache line
> size, right ?

pahole says: /* size: 256, cachelines: 4, members: 11 */

> It would be nice to allow sharing the 'hot' cache line with seldom
> used fields.

Sure.

> Instead of mere pads, add some unions, and let rt6i_uncached/rt6i_uncached_list
> use them.

If I understand correctly, you suggest to move

   rt6_info::rt6i_uncached[_list], rtable::rt_uncached[_list]
   
into struct dst_entry and fixup the usage sites, right?

I don't see why that would need a union. dst_entry::rt_uncached[_list]
would work for both, no?

Thanks,

        tglx