Messages in this thread | | | Date | Sun, 28 Mar 2010 22:04:14 +0200 | From | Frederic Weisbecker <> | Subject | Re: [GIT, RFC] Killing the Big Kernel Lock |
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On Wed, Mar 24, 2010 at 10:40:54PM +0100, Arnd Bergmann wrote: > I've spent some time continuing the work of the people on Cc and many others > to remove the big kernel lock from Linux and I now have bkl-removal branch > in my git tree at git://git.kernel.org/pub/scm/linux/kernel/git/arnd/playground.git > that lets me run a kernel on my quad-core machine with the only users of the BKL > being mostly obscure device driver modules. > > The oldest patch in this series is roughly eight years old and is Willy's patch > to remove the BKL from fs/locks.c, and I took a series of patches from Jan that > removes it from most of the VFS. > > The other non-obvious changes are: > > - all file operations that either have an .ioctl method or do not have their > own .llseek method used to implicitly require the BKL. I've changed that > so they need to explicitly set .llseek = default_llseek, .unlocked_ioctl = > default_ioctl, and changed all the code that either has supplied a .ioctl > method or looks like it needs the BKL somewhere else, meaning the > default_llseek function might actually do something. > > - The block layer now has a global bkldev_mutex that is used in all block > drivers in place of the BKL. The only recursive instance of the BKL was > __blkdev_get(), which is now called with the blkdev_mutex held instead of > grabbing the BKL. This has some possible performance implications that > need to be looked into. > > - The init/main.c code no longer take the BKL. I figured that this was > completely unnecessary because there is no other code running at the > same time that takes the BKL. > > - The most invasive change is in the TTY layer, which has a new global > mutex (sorry!). I know that Alan has plans of his own to remove the BKL > from this subsystem, so my patches may not go anywhere, but they seem > to work fine for me. > I've called the new lock the 'Big TTY Mutex' (BTM), a name that probably > makes more sense if you happen to speak German. > The basic idea here is to make recursive locking and the release-on-sleep > explicit, so every mutex_lock, wait_event, workqueue_flush and schedule > in the TTY layer now explicitly releases the BTM before blocking. > > - All drivers that still require the BKL are now listed as 'depends on BKL' > in Kconfig, and you can set that symbol to 'y', 'm' or 'n'. If the lock > itself is a module, only other modules can use it, and /proc/modules > will tell you exactly which ones those are. I've thought about adding > a module_init function in that module that will taint the kernel, but so > far I haven't done that. > > - Included is a debugfs file that gives statistics over the BKL usage from > early boot on. This is now obsolete and will not get merged, but I'm > including it for reference. > > Frederic has volunteered to help merging all of this upstream, which I > very much welcome. The shape that the tree is in now is very inconsistent, > especially some of the bits at the end are a bit dodgy and all of it needs > more testing. > > I've built-tested an allmodconfig kernel with CONFIG_BKL disabled > on x86_64, i386, powerpc64, powerpc32, s390 and arm to make sure I > catch all the modules that depend on BKL, and I've been running > various versions of this tree on my desktop machine over the last few > weeks while adding stuff. > > Arnd > > --- > > Arnd Bergmann (44): > input: kill BKL, fix input_open_file locking > ptrace: kill BKL > procfs: kill BKL in llseek > random: forbid llseek on random chardev > x86/microcode: use nonseekable_open > perf_event: use nonseekable_open
I just queued the perf_event one. It looks pretty good. I'm also looking at some of the most trivials (ehm..less hards) in the list and see which we can submit right away.
Thanks.
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