Rusty Russell's lguest was recently merged into the upcoming 2.6.23 Linux kernel. The merge comment describes the project, "lguest is a simple hypervisor for Linux on Linux. Unlike kvm it doesn't need VT/SVM hardware. Unlike Xen it's simply 'modprobe and go'. Unlike both, it's 5000 lines and self-contained." The comment goes on to note:
"Performance is ok, but not great (-30% on kernel compile). But given its hackability, I expect this to improve, along with the paravirt_ops code which it supplies a complete example for. There's also a 64-bit version being worked on and other craziness.
"But most of all, lguest is awesome fun! Too much of the kernel is a big ball of hair. lguest is simple enough to dive into and hack, plus has some warts which scream 'fork me!'."
Two new documentation directories were merged into the upcoming 2.6.23 mainline kernel, containing translations of the HOWTO and stable_api_nonsense.txt documents in Japanese and Chinese. Greg KH explained, "here are some patches that add some translations of some procedural documentation files to the Documentation/ tree." Regarding some of the concerns that were expressed with merging translated documentation into the mainline kernel tarball, Greg noted, "these files change _very_ slowly over time, and are quite easy to keep up to date by the translators." He added:
"I know that kernel development is in English, but translations of a small subset of documentation files that go over procedures and how to get involved in the community is something that I feel is important and will bring in more developers in the end. Having these files in the kernel tree is a good way to keep a central location that all can see and easily find, instead of hiding them away on different web sites that might be harder to update by anyone who needs to do so."
In response to another merge request, Andrew Morton retorted, "argh. I have a backlog of maybe 300 patches here which I am cheerfully ignoring while concentrating on preventing 2.6.23 from being less of a disaster than it has already been." He noted that he was not planning to merge any new code into his -mm tree for 2.6.23 inclusion, "the door for new 2.6.23 material shut two weeks ago. Here, at least." He went on to note:
"Please, stop writing patches. Maybe do something to help get 2.6.23 off its back. Like, go review some of the code which people are cheerfully merging five minutes after having written it."
Recent merges into the upcoming 2.6.23 kernel can be found by browsing the gitweb interface to Linus' 2.6 kernel tree. The 2.6.23-rc1 kernel should be released on or shortly after Sunday the 22nd, two weeks after 2.6.22 was released, and at which time the merge window is closed.
A recently merged KVM patchset included support for guest SMP, various performance improvements, and suspend/resume fixes. KVM stands for Kernel-based Virtual Machine, "a full virtualization solution for Linux on x86 hardware containing virtualization extensions". In regards to the recently merged guest SMP support which will be part of the upcoming 2.6.23 kernel, Avi Kivity noted:
"Guest smp is fully operational. Kernel build on 2-way smp is 40% faster than on a up guest. Expect significant performance improvements from in-kernel apic and from further tuning."
H. Peter Anvin submitted a series of patches rewriting the x86 setup code, "this patch set replaces the x86 setup code, which is currently all in assembly, with a version written in C, using the '.code16gcc' feature of binutils (which has been present since at least 2001.)" He went on to explain why he did this, "the new code is vastly easier to read, and, I hope, debug. It should be noted that I found a fair number of minor bugs while going through this code, and have attempted to correct them."
Linus Torvalds reacted favorably, "I can't really argue against this on any sane grounds - not only is it removing more lines than it adds, but moving from mostly unreadable assembly to C seems a good idea." He went on to note, "so let's just get this merged. But the question is, do we put it in 2.6.23-rc1, or do we put it in -mm for a few weeks, which would imply waiting for the next merge window? Andrew?" Andrew Morton pointed out that the patches have been in -mm already for a couple of months, "this code has been in -mm since 11 May, as git-newsetup.patch. It has caused (for what it is) astonishingly few problems. Maybe a couple of build glitches and one runtime failure, all quickly fixed. I'd say it's ready." Linus agreed, "Ok. That makes it easy. I'll just merge it."
Included in Andrew Morton's potential 2.6.23 merge list [story] were a series of patches to make the x86-64 architecture tickless. Andi Kleen, the x86-64 maintainer replied, "I'm sceptical about the dynticks code. It just rips out the x86-64 timing code completely, which needs a lot more review and testing. Probably not .23." Linus Torvalds agreed, "we are *not* going to do another 'rip everything out, and replace it with new code' again. Over my dead body. We're going to do this thing gradually, or not at all." He went on to explain "the patch-set itself actually looks fine, as far as I'm concerned. But it does seem to have that 'enable everything in one go' problem. I'd much rather see one time source at a time being converted, and enabled then and there, so that when people report problems and do a bisection, if it was HPET that broke, you get the commit that changed HPET."
In response to the pains caused by the original dyntick merge in 2.6.21, Ingo Molnar acknowledged, "we had 12 -hrt/dynticks merge related regressions between 2.6.21-rc1 and -final, and 4 after final." He went on to point out, "it's all pretty quiet today on the dynticks regressions front. (there are no open regressions in either the upstream i386 code or in the devel patches we are aware of)." As to the source of the bugs, he explained, "the majority of the above bugs were in the infrastructure code. (the worst was the generic resume/suspend one fixed in 220.127.116.11) And sadly, a fair number of the infrastructure bugs we introduced during the frentic clockevents/dynticks rewrites/redesigns we did between .20 and .21. That was a royally stupid mistake for us to do - instead of patiently waiting for the bugs to be shaken out we destabilized the infrastructure. (it was a 'lets make this thing so nice that it's impossible to reject' instintic gut reaction.)" Linus replied, "one thing I'll happily talk about is that while 2.6.21 was painful, you and Thomas in particular were both very responsible about the thing, so no, I'm not at all complaining or worried about it in that sense! I just really _really_ wish we could have two fairly stable releases in a row. I think 2.6.22 has the potential to be a pretty good setup, and I'd really like to avoid having another 2.6.21 immediately afterwards."
Ingo Molnar [interview]'s Completely Fair Scheduler [story] has been merged into the Linux kernel for inclusion in the upcoming 2.6.23 release. A comment in the patch titled 'sched: cfs core code' noted, "apply the CFS core code. This change switches over the scheduler core to CFS's modular design and makes use of kernel/sched_fair/rt/idletask.c to implement Linux's scheduling policies." Another patch included documentation which described the new scheduler, "80% of CFS's design can be summed up in a single sentence: CFS basically models an 'ideal, precise multi-tasking CPU' on real hardware." It goes on to explain:
"CFS's task picking logic is based on this p->wait_runtime value and it is thus very simple: it always tries to run the task with the largest p->wait_runtime value. In other words, CFS tries to run the task with the 'gravest need' for more CPU time. So CFS always tries to split up CPU time between runnable tasks as close to 'ideal multitasking hardware' as possible.
"Most of the rest of CFS's design just falls out of this really simple concept, with a few add-on embellishments like nice levels, multiprocessing and various algorithm variants to recognize sleepers."
Following the release of the 2.6.22 kernel [story], Andrew Morton [interview] posted a list of a wide range of patches that are in his -mm kernel, summarizing for each his plans as to whether or not they will be pushed upstream for inclusion in the upcoming 2.6.23 kernel. Comments included simply noting "merge" or "hold", as well as "these appear to need some work,", "don't know, need to ping suitable developers over this work," and "sent to maintainer." Perhaps most entertaining was Andrew's response to the vmscan-give-referenced-active-and-unmapped-pages-a-second-trip-around-the-lru.patch, "this is scary. Will sit and admire it until it has been demonstrated to be a net gain." It is possible to track which patches are actually merged using the gitweb interface to Linus' kernel tree.