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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 28 Feb 2023 16:17:34 +0100
Message-ID: <CANn89iKM6yMP2Doy0MuCrfX1LASPFt_OnpPY-aNg+hu=F3W7AA@mail.gmail.com>
Subject: Re: [patch 1/3] net: dst: Prevent false sharing vs. dst_entry::__refcnt
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>, x86@kernel.org,
        Wangyang Guo <wangyang.guo@intel.com>,
        Arjan van De Ven <arjan@linux.intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Marc Zyngier <maz@kernel.org>
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On Tue, Feb 28, 2023 at 3:33=E2=80=AFPM Thomas Gleixner <tglx@linutronix.de=
> wrote:
>
> From: Wangyang Guo <wangyang.guo@intel.com>
>
> dst_entry::__refcnt is highly contended in scenarios where many connectio=
ns
> happen from and to the same IP. The reference count is an atomic_t, so th=
e
> reference count operations have to take the cache-line exclusive.
>
> Aside of the unavoidable reference count contention there is another
> significant problem which is caused by that: False sharing.
>
> 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 in=
to
> 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, rig=
ht ?

It would be nice to allow sharing the 'hot' cache line with seldom used fie=
lds.

Instead of mere pads, add some unions, and let rt6i_uncached/rt6i_uncached_=
list
use them.