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Subject: Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs
 in mind (for KVM to isolate its guests per CPU)
To:     Julian Stecklina <jsteckli@amazon.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Woodhouse <dwmw@amazon.co.uk>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
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        Jim Mattson <jmattson@google.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-mm <linux-mm@kvack.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Liran Alon <liran.alon@oracle.com>,
        Kees Cook <keescook@google.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        chris.hyser@oracle.com, Tyler Hicks <tyhicks@canonical.com>,
        John Haxby <john.haxby@oracle.com>,
        Jon Masters <jcm@redhat.com>
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From:   Khalid Aziz <khalid.aziz@oracle.com>
Organization: Oracle Corp
Message-ID: <5efc291c-b0ed-577e-02d1-285d080c293d@oracle.com>
Date:   Fri, 14 Sep 2018 11:06:53 -0600
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On 09/12/2018 09:37 AM, Julian Stecklina wrote:
> Julian Stecklina <jsteckli@amazon.de> writes:
> 
>> Linus Torvalds <torvalds@linux-foundation.org> writes:
>>
>>> On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <jsteckli@amazon.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. 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.
> 

Hi Julian,

I ran tests with your updated code and gathered lock statistics. Change in system time for "make -j60" was in the noise margin (It actually went up by about 2%). There is some contention on xpfo_lock. Average wait time does not look high compared to other locks. Max hold time looks a little long. From /proc/lock_stat:

              &(&page->xpfo_lock)->rlock:         29698          29897           0.06         134.39       15345.58           0.51      422474670      960222532           0.05       30362.05   195807002.62           0.20

Nevertheless even a smaller average wait time can add up.

--
Khalid