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[PATCH RFC] sched: boost throttled entities on wakeups [message #48453] Thu, 18 October 2012 07:32 Go to next message
Vladimir Davydov is currently offline  Vladimir Davydov
Messages: 5
Registered: July 2012
Junior Member
From: *parallels.com
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(-)
Re: [PATCH RFC] sched: boost throttled entities on wakeups [message #48479 is a reply to message #48453] Thu, 18 October 2012 10:39 Go to previous messageGo to next message
Vladimir Davydov is currently offline  Vladimir Davydov
Messages: 5
Registered: July 2012
Junior Member
From: *parallels.com
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>
Re: [PATCH RFC] sched: boost throttled entities on wakeups [message #48537 is a reply to message #48453] Fri, 19 October 2012 14:24 Go to previous messageGo to next message
Peter Zijlstra is currently offline  Peter Zijlstra
Messages: 61
Registered: September 2006
Member
From: *parallels.com
On Thu, 2012-10-18 at 11:32 +0400, Vladimir Davydov wrote:
>
> 1) Do you agree that the problem exists and should be sorted out?

This is two questions.. yes it exists, I'm absolutely sure I pointed it
out as soon as people even started talking about this nonsense (bw
cruft).

Should it be sorted, dunno, in general !PREEMPT_RT is very susceptible
to all this and in general we don't fix it.

> 2) If so, does the general approach proposed (unthrottling on wakeups) suits
> you? Why or why not?

its a quick hack similar to existing hacks done for rt, preferably we'd
do smarter things though.

> 3) If you think that the approach proposed is sane, what you dislike about the
> patch?

its not inlined, its got coding style issues, but worst of all, you
added yet another callback from the schedule() path and did it wrong ;-)

Also, it adds even more bw cruft overhead to regular scheduling paths,
we took some pains to limit that when we introduced the fail^Wfeature.
Re: [PATCH RFC] sched: boost throttled entities on wakeups [message #48538 is a reply to message #48537] Fri, 19 October 2012 15:40 Go to previous message
Vladimir Davydov is currently offline  Vladimir Davydov
Messages: 5
Registered: July 2012
Junior Member
From: *parallels.com
Thank you for the answer.

On Oct 19, 2012, at 6:24 PM, Peter Zijlstra wrote:

> its a quick hack similar to existing hacks done for rt, preferably we'd
> do smarter things though.

If you have any ideas how to fix this in a better way, please share.
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