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From:   Michael Kelley <mikelley@microsoft.com>
To:     vkuznets <vkuznets@redhat.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Roman Kagan <rkagan@virtuozzo.com>,
        Joe Perches <joe@perches.com>
Subject: RE: [PATCH v3] x86/hyper-v: micro-optimize send_ipi_one case
Thread-Topic: [PATCH v3] x86/hyper-v: micro-optimize send_ipi_one case
Thread-Index: AQHVjNn5fFTFLcbQ9UmGNbPSapBCP6dustcA
Date:   Sun, 27 Oct 2019 16:49:31 +0000
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References: <20191027151938.7296-1-vkuznets@redhat.com>
In-Reply-To: <20191027151938.7296-1-vkuznets@redhat.com>
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From: Vitaly Kuznetsov <vkuznets@redhat.com> Sent: Sunday, October 27, 2019=
 8:20 AM
>=20
> When sending an IPI to a single CPU there is no need to deal with cpumask=
s.
> With 2 CPU guest on WS2019 I'm seeing a minor (like 3%, 8043 -> 7761 CPU
> cycles) improvement with smp_call_function_single() loop benchmark. The
> optimization, however, is tiny and straitforward. Also, send_ipi_one() is
> important for PV spinlock kick.
>=20
> I was also wondering if it would make sense to switch to using regular
> APIC IPI send for CPU > 64 case but no, it is twice as expesive (12650 CP=
U
> cycles for __send_ipi_mask_ex() call, 26000 for orig_apic.send_IPI(cpu,
> vector)).
>=20
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> Changes since v2:
>  - Check VP number instead of CPU number against >=3D 64 [Michael]
>  - Check for VP_INVAL
>

Reviewed-by: Michael Kelley <mikelley@microsoft.com>