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

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Message-ID: <877e4bbyw2.fsf@vitty.brq.redhat.com>
Date:   Thu, 07 Nov 2019 14:26:53 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        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>,
        Michael Kelley <mikelley@...rosoft.com>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v3] x86/hyper-v: micro-optimize send_ipi_one case

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> 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

Hi Sasha,

do you have plans to pick this up for hyperv-next or should we ask x86
folks to?

Thanks!

-- 
Vitaly