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Date:   Thu, 7 Nov 2019 22:21:35 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
cc:     Sasha Levin <sashal@kernel.org>, linux-hyperv@vger.kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Roman Kagan <rkagan@virtuozzo.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Joe Perches <joe@perches.com>
Subject: Re: [PATCH v3] x86/hyper-v: micro-optimize send_ipi_one case
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References: <20191027151938.7296-1-vkuznets@redhat.com> <877e4bbyw2.fsf@vitty.brq.redhat.com>
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On Thu, 7 Nov 2019, Vitaly Kuznetsov wrote:

> Vitaly Kuznetsov <vkuznets@redhat.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@redhat.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?

I'm picking up the constant TSC one anyway, so I can just throw that in as
well.

Thanks,

	tglx