On Wed, Sep 12, 2018 at 5:37 PM, Julian Stecklina <jsteckli@xxxxxxxxx> wrote: > Julian Stecklina <jsteckli@xxxxxxxxx> writes: > >> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: >> >>> On Fri, Aug 31, 2018 at 12:45 AM Julian Stecklina <jsteckli@xxxxxxxxx> 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 > -- > Amazon Development Center Germany GmbH > Berlin - Dresden - Aachen > main office: Krausenstr. 38, 10117 Berlin > Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger > Ust-ID: DE289237879 > Eingetragen am Amtsgericht Charlottenburg HRB 149173 B >
