linux-kernel - RE: [PATCH v3] x86/hyper-v: micro-optimize send_ipi

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Message-ID: <DM5PR21MB0137360776CEDD19E6FECC75D7670@DM5PR21MB0137.namprd21.prod.outlook.com>
Date:   Sun, 27 Oct 2019 16:49:31 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     vkuznets <vkuznets@...hat.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Joe Perches <joe@...ches.com>
Subject: RE: [PATCH v3] x86/hyper-v: micro-optimize send_ipi_one case

From: Vitaly Kuznetsov <vkuznets@...hat.com> Sent: Sunday, October 27, 2019 8:20 AM
> 
> When sending an IPI to a single CPU there is no need to deal with cpumasks.
> 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.
> 
> 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 CPU
> cycles for __send_ipi_mask_ex() call, 26000 for orig_apic.send_IPI(cpu,
> vector)).
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
> Changes since v2:
>  - Check VP number instead of CPU number against >= 64 [Michael]
>  - Check for VP_INVAL
>

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