Messages in this thread | | | Subject | Re: [announce] [patch] Voluntary Kernel Preemption Patch | From | Redeeman <> | Date | Fri, 09 Jul 2004 23:13:30 +0200 |
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this all seems pretty cool... do you think you could make a patch against mm for this? it would be greatly apreciated
On Fri, 2004-07-09 at 20:26 +0200, Ingo Molnar wrote: > as most of you are probably aware of it, there have been complaints on > lkml that the 2.6 kernel is not suitable for serious audio work due to > high scheduling latencies (e.g. the Jackit people complained). I took a > look at latencies and indeed 2.6.7 is pretty bad - latencies up to 50 > msec (!) can be easily triggered using common workloads, on fast 2GHz+ > x86 system - even when using the fully preemptible kernel! > > to solve this problem, Arjan van de Ven and I went over various kernel > functions to determine their preemptability and we re-created from > scratch a patch that is equivalent in performance to the 2.4 lowlatency > patches but is different in design, impact and approach: > > http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.7-bk20-H2 > > (Note to kernel patch reviewers: the split voluntary_resched type of > APIs, the feature #ifdefs and runtime flags are temporary and were > only introduced to enable a easy benchmarking/comparisons. I'll split > this up into small pieces once there's testing feedback and actual > audio users had their say!) > > unlike the lowlatency patches, this patch doesn't add a lot of new > scheduling points to the source code, it rather reuses a rich but > currently inactive set of scheduling points that already exist in the > 2.6 tree: the might_sleep() debugging checks. Any code point that does > might_sleep() is in fact ready to sleep at that point. So the patch > activates these debugging checks to be scheduling points. This reduces > complexity and impact quite significantly. > > but even using these (over one hundred) might_sleep() points there were > still a number of latency sources in the kernel - we identified and > fixed them by hand, either via additional might_sleep() checks, or via > explicit rescheduling points. Sometimes lock-break was necessary as > well. > > as a practical goal, this patch aims to fix all latency sources that > generate higher than ~1 msec latencies. We'd love to learn about > workloads that still cause audio skipping even with this patch applied, > but i've been unable to generate any load that creates higher than 1msec > latencies. (not counting driver initialization routines.) > > this patch is also more configurable than the 2.4 lowlatency patches > were: there's a .config option to enable voluntary preemption, and there > are runtime /proc/sys knobs and boot-time flags to turn voluntary > preemption (CONFIG_VOLUNTARY_PREEMPT) and kernel preemption > (CONFIG_PREEMPT) on/off: > > # turn on/off voluntary preemption (if CONFIG_VOLUNTARY_PREEMPT) > echo 1 > /proc/sys/kernel/voluntary_preemption > echo 0 > /proc/sys/kernel/voluntary_preemption > > # turn on/off the preemptible kernel feature (if CONFIG_PREEMPT) > /proc/sys/kernel/kernel_preemption > /proc/sys/kernel/kernel_preemption > > the 'voluntary-preemption=0/1' and 'kernel-preemption=0/1' boot options > can be used to control these flags at boot-time. > > all 4 combinations make sense if both CONFIG_PREEMPT and > CONFIG_VOLUNTARY_PREEMPT are enabled - great for performance/latency > testing and comparisons. > > The stock 2.6 kernel is equivalent to: > > voluntary_preemption:0 kernel_preemption:0 > > the 2.6 kernel with voluntary kernel preemption is equivalent to: > > voluntary_preemption:1 kernel_preemption:0 > > the 2.6 kernel with preemptible kernel enabled is: > > voluntary_preemption:0 kernel_preemption:1 > > and the preemptible kernel enhanced with additional lock-breaks is > enabled via: > > voluntary_preemption:1 kernel_preemption:1 > > it is safe to change these flags anytime. > > The patch is against 2.6.7-bk20, and it also includes fixes for kernel > bugs that were uncovered while developing this patch. While it works for > me, be careful when using this patch! > > Testreports, comments, suggestions are more than welcome, > > Ingo > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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