Messages in this thread | | | Subject | Re: Redoing eXclusive Page Frame Ownership (XPFO) with isolated CPUs in mind (for KVM to isolate its guests per CPU) | From | Khalid Aziz <> | Date | Fri, 7 Sep 2018 15:30:10 -0600 |
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On 08/30/2018 10:00 AM, Julian Stecklina wrote: > Hey everyone, > > On Mon, 20 Aug 2018 15:27 Linus Torvalds <torvalds@linux-foundation.org> wrote: >> On Mon, Aug 20, 2018 at 3:02 PM Woodhouse, David <dwmw@amazon.co.uk> wrote: >>> >>> It's the *kernel* we don't want being able to access those pages, >>> because of the multitude of unfixable cache load gadgets. >> >> Ahh. >> >> I guess the proof is in the pudding. Did somebody try to forward-port >> that patch set and see what the performance is like? > > 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. > > There were no problems in forward-porting the patch set to master. > You can find the result here, including a patch makes the TLB flushing > configurable: > http://git.infradead.org/users/jsteckli/linux-xpfo.git/shortlog/refs/heads/xpfo-master > > It survived some casual stress-ng runs. I can rerun the benchmarks on > this version, but I doubt there is any change. > >> It used to be just 500 LOC. Was that because they took horrible >> shortcuts? > > The patch is still fairly small. As for the horrible shortcuts, I let > others comment on that.
Looks like the performance impact can be whole lot worse. On my test system with 2 Xeon Platinum 8160 (HT enabled) CPUs and 768 GB of memory, I am seeing very high penalty with XPFO when building 4.18.6 kernel sources with "make -j60":
No XPFO patch XPFO patch(No TLB flush) XPFO(TLB Flush) sys time 52m 54.036s 55m 47.897s 434m 8.645s
That is ~8% worse with TLB flush disabled and ~720% worse with TLB flush enabled. This test was with kernel sources being compiled on an ext4 filesystem. XPFO seems to affect ext2 even more. With ext2 filesystem, impact was ~18.6% and ~900%.
-- Khalid
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