linux-kernel - Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

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Date:   Wed, 15 Jun 2022 09:38:34 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
Cc:     "zhangfei.gao@...mail.com" <zhangfei.gao@...mail.com>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matthew Wilcox <willy@...radead.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        Auger Eric <eric.auger@...hat.com>,
        "chenxiang (M)" <chenxiang66@...ilicon.com>
Subject: Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and blocking readers from consuming CPU) cause qemu boot slow

On Mon, 13 Jun 2022 16:02:30 +0100,
Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com> wrote:
> 
> 
> 
> > -----Original Message-----
> > From: zhangfei.gao@...mail.com [mailto:zhangfei.gao@...mail.com]
> > Sent: 13 June 2022 07:56
> > To: paulmck@...nel.org
> > Cc: Paolo Bonzini <pbonzini@...hat.com>; Zhangfei Gao
> > <zhangfei.gao@...aro.org>; linux-kernel@...r.kernel.org;
> > rcu@...r.kernel.org; Lai Jiangshan <jiangshanlai@...il.com>; Josh Triplett
> > <josh@...htriplett.org>; Mathieu Desnoyers
> > <mathieu.desnoyers@...icios.com>; Matthew Wilcox <willy@...radead.org>;
> > Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>;
> > mtosatti@...hat.com; Auger Eric <eric.auger@...hat.com>
> > Subject: Re: Commit 282d8998e997 (srcu: Prevent expedited GPs and
> > blocking readers from consuming CPU) cause qemu boot slow
> > 
>  
> > By the way, the issue should be only related with qemu apci. not related
> > with rmr feature
> > Test with: https://github.com/qemu/qemu/tree/stable-6.1
> > 
> > Looks it caused by too many kvm_region_add & kvm_region_del if
> > acpi=force,
> 
> Based on the setup I have, I think it has nothing to do with Guest
> kernel booting with ACPI per se(ie, acpi=force in Qemu kernel cmd
> line).  It is more to do with Qemu having the "-bios QEMU_EFI.fd"
> which sets up pflash devices resulting in large number of pflash
> read/write calls(before Guest kernel even boots) which in turn seems
> to be triggering the below kvm_region_add/del calls.

Indeed, this is all about memslots being added/removed at an alarming
rate (well, not more alarming today than yesterday, but QEMU's flash
emulation is... interesting).

	M.

-- 
Without deviation from the norm, progress is not possible.