linux-kernel - Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU)

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Message-ID: <CADLDEKsxx=MSFu=4_4JLX1afUMr3GVjNxSQ-726NrbLn8KQaQg@mail.gmail.com>
Date:   Thu, 13 Sep 2018 08:11:49 +0200
From:   Juerg Haefliger <juergh@...il.com>
To:     Julian Stecklina <jsteckli@...zon.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        deepa.srinivasan@...cle.com, Jim Mattson <jmattson@...gle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-mm <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        joao.m.martins@...cle.com, pradeep.vincent@...cle.com,
        Andi Kleen <ak@...ux.intel.com>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        kanth.ghatraju@...cle.com, Liran Alon <liran.alon@...cle.com>,
        Kees Cook <keescook@...gle.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        chris.hyser@...cle.com, Tyler Hicks <tyhicks@...onical.com>,
        John Haxby <john.haxby@...cle.com>,
        Jon Masters <jcm@...hat.com>
Subject: Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs
 in mind (for KVM to isolate its guests per CPU)

On Wed, Sep 12, 2018 at 5:37 PM, Julian Stecklina <jsteckli@...zon.de> wrote:
> Julian Stecklina <jsteckli@...zon.de> writes:
>
>> Linus Torvalds <torvalds@...ux-foundation.org> writes:
>>
>>> On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <jsteckli@...zon.de> wrote:
>>>>
>>>> I've been spending some cycles on the XPFO patch set this week. For the
>>>> patch set as it was posted for v4.13, the performance overhead of
>>>> compiling a Linux kernel is ~40% on x86_64[1]. The overhead comes almost
>>>> completely from TLB flushing. If we can live with stale TLB entries
>>>> allowing temporary access (which I think is reasonable), we can remove
>>>> all TLB flushing (on x86). This reduces the overhead to 2-3% for
>>>> kernel compile.
>>>
>>> I have to say, even 2-3% for a kernel compile sounds absolutely horrendous.
>>
>> Well, it's at least in a range where it doesn't look hopeless.
>>
>>> Kernel bullds are 90% user space at least for me, so a 2-3% slowdown
>>> from a kernel is not some small unnoticeable thing.
>>
>> The overhead seems to come from the hooks that XPFO adds to
>> alloc/free_pages. These hooks add a couple of atomic operations per
>> allocated (4K) page for book keeping. Some of these atomic ops are only
>> for debugging and could be removed. There is also some opportunity to
>> streamline the per-page space overhead of XPFO.
>
> I've updated my XPFO branch[1] to make some of the debugging optional
> and also integrated the XPFO bookkeeping with struct page, instead of
> requiring CONFIG_PAGE_EXTENSION, which removes some checks in the hot
> path.

FWIW, that was my original design but there was some resistance to
adding more to the page struct and page extension was suggested
instead.


> These changes push the overhead down to somewhere between 1.5 and
> 2% for my quad core box in kernel compile. This is close to the
> measurement noise, so I take suggestions for a better benchmark here.
>
> Of course, if you hit contention on the xpfo spinlock then performance
> will suffer. I guess this is what happened on Khalid's large box.
>
> I'll try to remove the spinlocks and add fixup code to the pagefault
> handler to see whether this improves the situation on large boxes. This
> might turn out to be ugly, though.

I'm wondering how much performance we're loosing by having to split
hugepages. Any chance this can be quantified somehow? Maybe we can
have a pool of some sorts reserved for userpages and group allocations
so that we can track the XPFO state at the hugepage level instead of
at the 4k level to prevent/reduce page splitting. Not sure if that
causes issues or has any unwanted side effects though...

...Juerg


> Julian
>
> [1] http://git.infradead.org/users/jsteckli/linux-xpfo.git/shortlog/refs/heads/xpfo-master
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