** Summary changed:
- apply sched-ext patch set to linux-unstable
+ provide a sched-ext enabled kernel for the 24.10 release
** Also affects: linux (Ubuntu Oracular)
Importance: Undecided
Status: New
** No longer affects: linux (Ubuntu Noble)
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https://bugs.launchpad.net/bugs/2045503
Title:
provide a sched-ext enabled kernel for the 24.10 release
Status in linux package in Ubuntu:
New
Status in linux source package in Oracular:
New
Bug description:
[Impact]
sched-ext is a new scheduling class introduced in the Linux kernel
that provides a mechanism to implement scheduling policies as eBPF
programs (https://lwn.net/Articles/922405/). Such programs can also be
connected to user-space counterparts to defer scheduling decisions to
regular user-space processes.
The idea of "pluggable" schedulers is not new, it was initially
proposed in 2004 (https://lwn.net/Articles/109458/), but at that time
it was strongly rejected, to prioritize the creation of a single
generic scheduler (one to rule them all), that ended up being the
“completely fair scheduler” (CFS).
However, with BPF and the sched-ext scheduling class, we now have the
possibility to easily and quickly implement and test scheduling
policies, making the “pluggable” approach an effective tool for easy
experimentation.
The ability to implement custom scheduling policies via BPF greatly
lowers the difficulty of testing new scheduling ideas (much easier
than changing CFS or replacing it with a different scheduler). With
this feature researchers or developers can test their own scheduler in
a safe way, without even needing to reboot the system.
Shipping this feature in the Ubuntu kernel can provide a significant
benefit to researchers and companies that want to experiment (or ship)
their own scheduling policy, implemented as an eBPF/user-space
program.
Targeting linux-unstable only for now is probably a good compromise to
allow users to start some experiments, collect feedbacks, help the
upstream community to find and fix bugs and at the same time avoid to
introduce too much maintenance burden on us.
[Test case]
Basic test cases for this feature are provided by the sched-ext patch
set. Tests and custom scheduler implementations are available in
tools/sched_ext or in https://github.com/sched-ext/scx.
[Fix]
Apply this patch set as SAUCE to linux-unstable:
https://lore.kernel.org/bpf/zvpjtc5znenny...@slm.duckdns.org/T/
On top of the patch set we want to apply also the following patches
(still as SAUCE):
- UBUNTU: SAUCE: sched_ext: use proper atomic operator for scx.ops_state
(extra fix to properly build sched-ext on armhf)
- UBUNTU: SAUCE: sched-ext: taint kernel when a custom scheduler is loaded
(set TAINT_OOT_MODULE in /proc/sys/kernel/tainted to easily determine when
a custom scheduler has been used, that can be useful for bug reports: we can
easily detect when a custom scheduler has been used and treat the bug report
accordingly)
- UBUNTU: [Config] enable sched_ext in annotations
(enable sched-ext in the config across all the supported architectures)
Soon there will be a branch against any kernel that we need here (we will
only need 6.7 for now):
https://github.com/sched-ext/sched_ext
[Regression potential]
This feature is not going to be merged upstream in the near future,
some upstream maintainers are worried that giving the possibility to
inject in the kernel a custom scheduler can introduce performance
regressions that are hard to track down.
For this reason we should apply this feature only to linux-unstable
for now, making sure that the patch is unapplied or reverted when
linux-unstable becomes linux.
In the meantime we can also figure out a reasonable way to determine
when a custom scheduler is used (i.e., taint the kernel?) to easily
determine when any potential performance regression may have been
introduced by a custom sched-ext scheduler.
From a maintenance perspective, having this patch set applied may also
be problematic (potential conflicts) when we apply new stable updates.
However, the upstream maintainers of sched-ext have expressed interest
to help us maintaining the patch set against the target kernel(s) that
we need. And targeting linux-unstable only can definitely mitigate the
maintenance problem a lot (since we won't have the urgency to apply
critical security fixes to linux-unstable).
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