Messages in this thread | | | From | Vladimir Davydov <> | Date | Thu, 18 Oct 2012 14:39:01 +0400 | Subject | Re: [Devel] [PATCH RFC] sched: boost throttled entities on wakeups |
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There is an error in the test script: I forgot to initialize cpuset.mems of test cgroups - without it it is impossible to add a task into a cpuset cgroup.
Sorry for that.
Fixed version of the test script is attached.
On Oct 18, 2012, at 11:32 AM, Vladimir Davydov wrote:
> If several tasks in different cpu cgroups are contending for the same resource > (e.g. a semaphore) and one of those task groups is cpu limited (using cfs > bandwidth control), the priority inversion problem is likely to arise: if a cpu > limited task goes to sleep holding the resource (e.g. trying to take another > semaphore), it can be throttled (i.e. removed from the runqueue), which will > result in other, perhaps high-priority, tasks waiting until the low-priority > task continues its execution. > > The patch tries to solve this problem by boosting tasks in throttled groups on > wakeups, i.e. temporarily unthrottling the groups a woken task belongs to in > order to let the task finish its execution in kernel space. This obviously > should eliminate the priority inversion problem on voluntary preemptable > kernels. However, it does not solve the problem for fully preemptable kernels, > although I guess the patch can be extended to handle those kernels too (e.g. by > boosting forcibly preempted tasks thus not allowing to throttle). > > I wrote a simple test that demonstrates the problem (the test is attached). It > creates two cgroups each of which is bound to exactly one cpu using cpusets, > sets the limit of the first group to 10% and leaves the second group unlimited. > Then in both groups it starts processes reading the same (big enough) file > along with a couple of busyloops in the limited groups, and measures the read > time. > > I've run the test 10 times for a 1 Gb file on a server with > 10 Gb of RAM and > 4 cores x 2 hyperthreads (the kernel was with CONFIG_PREEMPT_VOLUNTARY=y). Here > are the results: > > without the patch 40.03 +- 7.04 s > with the patch 8.42 +- 0.48 s > > (Since the server's RAM can accommodate the whole file, the read time was the > same for both groups) > > I would appreciate if you could answer the following questions regarding the > priority inversion problem and the proposed approach: > > 1) Do you agree that the problem exists and should be sorted out? > > 2) If so, does the general approach proposed (unthrottling on wakeups) suits > you? Why or why not? > > 3) If you think that the approach proposed is sane, what you dislike about the > patch? > > Thank you! > > --- > include/linux/sched.h | 8 ++ > kernel/sched/core.c | 8 ++ > kernel/sched/fair.c | 182 ++++++++++++++++++++++++++++++++++++++++++++++- > kernel/sched/features.h | 2 + > kernel/sched/sched.h | 6 ++ > 5 files changed, 204 insertions(+), 2 deletions(-) > > <sched-boost-throttled-entities-on-wakeups.patch><ioprio_inv_test.sh><ATT00001.c>
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