linux-kernel - Re: [RFC v2 0/8] arm64: MMU enabled kexec relocation

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Message-ID: <CA+CK2bBOSC0iYjq_A18DNaNCYskTTJJTkM4N-WAqssoxpxuNPg@mail.gmail.com>
Date:   Wed, 31 Jul 2019 13:04:08 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     James Morris <jmorris@...ei.org>, Sasha Levin <sashal@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        kexec mailing list <kexec@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Bhupesh Sharma <bhsharma@...hat.com>
Subject: Re: [RFC v2 0/8] arm64: MMU enabled kexec relocation

On Wed, Jul 31, 2019 at 12:50 PM Mark Rutland <mark.rutland@....com> wrote:
>
> On Wed, Jul 31, 2019 at 12:40:51PM -0400, Pavel Tatashin wrote:
> > On Wed, Jul 31, 2019 at 12:33 PM Mark Rutland <mark.rutland@....com> wrote:
> > >
> > > Hi Pavel,
> > >
> > > Generally, the cover letter should state up-front what the goal is (or
> > > what problem you're trying to solve). It would be really helpful to have
> > > that so that we understand what you're trying to achieve, and why.
>
> [...]
>
> > > > Here is the current data from the real hardware:
> > > > (because of bug, I forced EL1 mode by setting el2_switch always to zero in
> > > > cpu_soft_restart()):
> > > >
> > > > For this experiment, the size of kernel plus initramfs is 25M. If initramfs
> > > > was larger, than the improvements would be even greater, as time spent in
> > > > relocation is proportional to the size of relocation.
> > > >
> > > > Previously:
> > > > kernel shutdown       0.022131328s
> > > > relocation    0.440510736s
> > > > kernel startup        0.294706768s
> > >
> > > In total this takes ~0.76s...
> > >
> > > >
> > > > Relocation was taking: 58.2% of reboot time
> > > >
> > > > Now:
> > > > kernel shutdown       0.032066576s
> > > > relocation    0.022158152s
> > > > kernel startup        0.296055880s
> > >
> > > ... and this takes ~0.35s
> > >
> > > So do we really need this complexity for a few blinks of an eye?
> >
> > Yes, we have an extremely tight reboot budget, 0.35s is not an acceptable waste.
>
> Could you please elaborate on your use-case?
>
> Understanfin what you're trying to achieve would help us to understand
> which solutions make sense.

An extremely high availability device with an update story utilizing
kexec functionality for a faster kernel update and also for being able
to preserve some state in memory without wasting the time of copying
it to and from a backing storage. We at Microsoft will be using a
fleet of these devices. The total reboot budget is less than half a
second, out of which 0.44s is currently spent in kexec relocation.

Pasha

>
> Thanks,
> Mark.