linux-kernel - Re: [PATCH v2] x86/apic: reduce cache line misses in __x2apic_send_IPI

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Message-ID: <CANn89iLdxNeZR_67ZOTX+K7GbOw44kZXU=ebpODySDs6x5BZNw@mail.gmail.com>
Date:   Thu, 28 Oct 2021 17:14:02 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH v2] x86/apic: reduce cache line misses in __x2apic_send_IPI_mask()

On Tue, Oct 12, 2021 at 5:47 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Oct 07, 2021 at 07:35:56AM -0700, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > Using per-cpu storage for @x86_cpu_to_logical_apicid
> > is not optimal.
> >
> > Broadcast IPI will need at least one cache line
> > per cpu to access this field.
> >
> > __x2apic_send_IPI_mask() is using standard bitmask operators.
> >
> > By converting x86_cpu_to_logical_apicid to an array,
> > we divide by 16x number of needed cache lines, because
> > we find 16 values per cache line. CPU prefetcher can
> > kick nicely.
> >
> > Also move @cluster_masks to READ_MOSTLY section to avoid false sharing.
> >
> > Tested on a dual socket host with 256 cpus,
> > cost for a full broadcast is now 11 usec instead of 33 usec.
> >
> > v2: use a dynamically allocated array, as suggested by Peter.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Has this patch been merged x86 maintainers tree yet ?

Thanks for reviewing !