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Message-ID: <YrmDo7Sx1jNQ4WFd@MiWiFi-R3L-srv>
Date: Mon, 27 Jun 2022 18:17:07 +0800
From: Baoquan He <bhe@...hat.com>
To: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Ard Biesheuvel <ardb@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
Eric Biederman <ebiederm@...ssion.com>,
Rob Herring <robh+dt@...nel.org>,
Frank Rowand <frowand.list@...il.com>,
devicetree@...r.kernel.org, Dave Young <dyoung@...hat.com>,
Vivek Goyal <vgoyal@...hat.com>, kexec@...ts.infradead.org,
linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
linux-arm-kernel@...ts.infradead.org,
Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
Randy Dunlap <rdunlap@...radead.org>,
Feng Zhou <zhoufeng.zf@...edance.com>,
Chen Zhou <dingguo.cz@...group.com>,
John Donnelly <John.p.donnelly@...cle.com>,
Dave Kleikamp <dave.kleikamp@...cle.com>,
liushixin <liushixin2@...wei.com>
Subject: Re: [PATCH 5/5] arm64: kdump: Don't defer the reservation of crash
high memory
On 06/27/22 at 05:17pm, Leizhen (ThunderTown) wrote:
>
>
> On 2022/6/27 10:52, Baoquan He wrote:
> > On 06/23/22 at 03:07pm, Catalin Marinas wrote:
> >> On Wed, Jun 22, 2022 at 04:35:16PM +0800, Baoquan He wrote:
> >>> On 06/21/22 at 07:04pm, Catalin Marinas wrote:
> >>>> The problem with splitting is that you can end up with two entries in
> >>>> the TLB for the same VA->PA mapping (e.g. one for a 4KB page and another
> >>>> for a 2MB block). In the lucky case, the CPU will trigger a TLB conflict
> >>>> abort (but can be worse like loss of coherency).
> >>>
> >>> Thanks for this explanation. Is this a drawback of arm64 design? X86
> >>> code do the same thing w/o issue, is there way to overcome this on
> >>> arm64 from hardware or software side?
> >>
> >> It is a drawback of the arm64 implementations. Having multiple TLB
> >> entries for the same VA would need additional logic in hardware to
> >> detect, so the microarchitects have pushed back. In ARMv8.4, some
> >> balanced was reached with FEAT_BBM so that the only visible side-effect
> >> is a potential TLB conflict abort that could be resolved by software.
> >
> > I see, thx.
> >
> >>
> >>> I ever got a arm64 server with huge memory, w or w/o crashkernel setting
> >>> have different bootup time. And the more often TLB miss and flush will
> >>> cause performance cost. It is really a pity if we have very powerful
> >>> arm64 cpu and system capacity, but bottlenecked by this drawback.
> >>
> >> Is it only the boot time affected or the runtime performance as well?
> >
> > Sorry for late reply. What I observerd is the boot time serious latecy
> > with huge memory. Since the timestamp is not available at that time,
> > we can't tell the number. I didn't notice the runtime performance.
>
> There's some data here, and I see you're not on the cc list.
>
> https://lore.kernel.org/linux-mm/1656241815-28494-1-git-send-email-guanghuifeng@linux.alibaba.com/T/
Thanks, Zhen Lei. I also saw the patch. That seems to be a good way,
since there's only one process running at that time. Not sure if there's
still risk of multiple TLB entries for the same VA existing.
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