[linuxkernelnewbies] KS2008: Minisummit reports [LWN.net]

[Peter Teoh]

http://lwn.net/Articles/298521/?format=printable KS2008: Minisummit reports By Jonathan Corbet September 16, 2008 LWN's 2008 Kernel Summit coverage There is an increasing trend toward the use of "minisummits" for the detailed discussion of issues specific to a kernel subsystem. Kernel summits typically include a slot where the results from these sessions can be reported back to the group as a whole. The results from three such events were discussed at the 2008 kernel summit. Power management Len Brown went over the power management summit held last July in Ottawa; some notes from that gathering were posted here in August. The talk started with a quick recap of recent events in the power management area; these include the mainstream adoption of the tickless kernel, the establishment of lesswatts.org, and the creation of acpica.org, which, among other things, contains public bugzilla and git servers for the ACPI reference implementation. The number of unresolved bugs in the ACPI subsystem is dropping; it was 222 in 2004, but is 59 now (though one should count the 45 bugs which have been pushed out to a separate suspend/resume category). New bugs continue to come in at a steady rate, but the ACPI developers have been working at addressing them and simultaneously taking care of the backlog. Most of the bugs, Len notes, are problems which have always been present in the code; very few of them are regressions being added by current work. Andi Kleen (who was the ACPI maintainer while Len took a short sabbatical) made the claim that ACPI is the only kernel subsystem which knows how many bugs it has. There was some talk of the TI OMAP/ARM architecture which, after some effort, is now running entirely on current kernel releases. The flow of patches back upstream is still small, though, in need of improvement. Even suspend and resume work for this architecture, but they are too slow for current needs. USB autosuspend was mentioned briefly. It works, except for the systems which it breaks completely. As a result, it is currently disabled by default. That, says Len, is an unfortunate situation; disabled-by-default code, in many cases, might as well have never been written. Wireless networking John Linville summarized the wireless networking summit, also held in Ottawa. One topic of interest is the cfg80211 API, a wireless configuration interface which is intended to replace the much-maligned wireless extensions interface. One idea being considered is to use DBus to carry cfg80211 messages, which currently travel via netlink. That change would require putting a DBus implementation into the kernel itself, which, says John, might not be quite as crazy as it sounds. The wireless regulatory framework was covered briefly. Power management is an issue for wireless networking as well. The wireless protocols allow a device to announce its intention to go to sleep for a while; the access point will then buffer packets until the interface wakes up again. Linux needs support for this feature, as well as for some more basic things. The mac80211 layer, for example, still lacks support for suspend and resume. Vendor support is getting better, especially with Atheros hiring a community developer and beginning to contribute to the ath9k driver (though not, yet, to the older ath5k driver). Broadcom, on the other hand, remains as uncooperative as ever. There will be another wireless networking summit held in the next year, almost certainly in Europe. Containers The final topic of the session was containers. Several developers in this area got together to talk about outstanding issues. Namespaces in general were dealt with quickly; there are no real changes planned in this area. On the other hand, the group decided to shift the checkpoint/restart functionality over to a "one big syscall" approach; that work has since been covered in this article. The checkpoint developers are still working on getting a simple case - checkpointing a single process with no outstanding signals or other complicated situations - working before dealing with the more complex issues. The biggest area of interest would appear to be in resource management - the general task of keeping containers within a set of resource usage boundaries. The biggest problems in this area appear to be in the control group interface. The current interface does not offer any sort of transactional semantics, and it is hard for user-space administrative processes to learn about resource-oriented events. Some of these problems may be addressed via a new FIFO attached to each control group. There is a lot of work going into I/O bandwidth controllers. Too much work, in fact; there are four independent implementations circulating and they do not appear to be converging. Some sort of consensus on the way forward will need to be reached, but it is not, yet, clear what that consensus will be. Other work in progress includes a swap controller (which will be merged with the memory controller), the beginning of a network traffic controller, and some early effort toward a user-space library for working with control groups. A member of the group asked whether the memory controller still requires the addition of a pointer to the page structure. The kernel keeps a page structure for every page of memory; there are a lot of these structures, so struct page may be the most ruthlessly compressed data structures in the system. Adding a new pointer is not a price that the developers will willingly pay. Balbir Singh replied that this pointer is still there for now, but there is a patch which removes it. The problem is that this patch comes with a 4% performance loss; work toward lessening that impact continues.