"It took me quite a while to realize the real root cause of the VAIO - and probably many other machines - suspend/resume regressions, which were unearthed by the dyntick / clockevents patches," Thomas Gleixner explained regarding two patches for fixing suspend issues that Andrew Morton experienced with his VAIO laptop. He continued, "we disable a lot of ACPI/BIOS functionality during suspend, but we keep the lower idle C-states functionality active across suspend/resume. It seems that this causes trouble with certain BIOSes, but I assume that the problem is more wide spread and just not surfacing due to the various scenarios in which a machine goes into suspend/resume." Thomas concluded, "I really hope that this two patches finally set an end to the 'jinxed VAIO heisenbug series', which started when we removed the periodic tick with the clockevents/dyntick patches."

Linus Torvalds expressed some concerns, "the patches look fine, but I somehow have this slight feeling that you gave up a bit too soon on the '*why* does this happen?' question." He agreed that at that point there was a problem with ACPI, but cautioned that this could be triggered by another bug, "in particular, I also suspect that this may not really fix the problem - maybe it just makes the window sufficiently small that it no longer triggers. Because we don't necessarily understand what the real background for the problem is, I'm not sure we can say that it is solved." Linus concluded, "but hey, I think I'll apply the patches as-is. I'd just feel even better if we actually understood *why* doing the CPU Cx states is not something we can do around the suspend code!"

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 2.6.21.2) 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."

Linus Torvalds announced the first release candidate for the upcoming 2.6.21 kernel, ending the two-week merge window [story], "there's a lot of changes, as is usual for an -rc1 thing, but at least so far it would seem that 2.6.20 has been a good base, and I don't think we have anything *really* scary here." Linus noted that the tickless kernel patch [story] was finally merged into the mainline kernel, "the most interesting core change may be the dyntick/nohz one, where timer ticks will only happen when needed. It's been brewing for a _loong_ time, but it's in the standard kernel now as an option." Thomas Gleixner explained a year ago how this could result in cooler CPUs and power savings, "the tickless kernel feature (CONFIG_NO_HZ) enables 'on-demand' timer interrupts: if there is no timer to be expired for say 1.5 seconds when the system goes idle, then the system will stay totally idle for 1.5 seconds."

As for the rest of the changes, Linus added, "there's a ton of architecture updates (arm, mips, powerpc, x86, you name it), ACPI updates, and lots of driver work. And just a lot of cleanups." Release candidate kernels can be downloaded from your nearest kernel.org mirror. You can browse through all the changes using the gitweb interface. Kernel Newbiews maintains a useful summary of all the changes going into the latest version of the Linux kernel.