scx_rustland: mitigate sub-optimal performance with offline CPUs #189
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Most of the schedulers assume that the amount of possible CPUs in the
system represents the actual number of CPUs available.
This is not always true: some CPUs may be offline or certain CPU models
(AMD CPUs for example) may include unavailable CPUs in this number.
This can lead to sub-optimal performance or even errors in the scheduler
(see for example [1][2]).
Ideally, we need to attack this issue in a more generic way, such as
having a proper API provided by a C library, that can be used by all
schedulers and the topology Rust module (scx_utils crate).
But for now, let's try to mitigate most of the common sub-optimal cases
separately inside each scheduler.
For rustland we can apply some mitigations both in select_cpu() (for the
BPF part) and in the user-space part:
- the former is fixed in the sched-ext kernel by commit 94dc0c01b957
("scx: Use cpu_online_mask when resetting idle masks"). However,
adding an extra check `cpu < num_possible_cpus` in select_cpu(),
allows to properly support AMD CPUs, even with kernels that don't
have the cpu_online_mask fix yet (this doesn't always guarantee the
validity of cpu, but it should be enough to mitigate the majority of
the potential sub-optimal cases, without introducing any significant
overhead)
- the latter can be fixed relying on topology.span(), instead of
topology.nr_cpus(), to count the amount of available CPUs in the
system.
[1] sched-ext/sched_ext#69
[2] #147
Link: sched-ext/sched_ext@94dc0c0
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Byte-Lab
reviewed
Mar 14, 2024
| @@ -264,14 +264,15 @@ impl<'a> Scheduler<'a> { | |||
| let init_page_faults: u64 = 0; | |||
|
|
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| // Low-level BPF connector. | |||
| let nr_online_cpus = topo.span().weight() + 1; | |||
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FYI, there's a small bug in topo where it computes span, that I'm about to send a fix for. Once that's addressed, you shouldn't need to do + 1 here. I'll merge this and update your code here as part of that fix.
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FYI, there's a small bug in topo where it computes span, that I'm about to send a fix for. Once that's addressed, you shouldn't need to do + 1 here. I'll merge this and update your code here as part of that fix.
I actually seemed like a bug, but I wasn't sure, thanks for clarifying it!
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Most of the schedulers assume that the amount of possible CPUs in the system represents the actual number of CPUs available.
This is not always true: some CPUs may be offline or certain CPU models (AMD CPUs for example) may include unavailable CPUs in this number.
This can lead to sub-optimal performance or even errors in the scheduler (see for example [1][2]).
Ideally, we need to attack this issue in a more generic way, such as having a proper API provided by a C library, that can be used by all schedulers and the topology Rust module (scx_utils crate).
But for now, let's try to mitigate most of the common sub-optimal cases separately inside each scheduler.
For rustland we can apply some mitigations both in select_cpu() (for the BPF part) and in the user-space part:
the former is fixed in the sched-ext kernel by commit 94dc0c01b957 ("scx: Use cpu_online_mask when resetting idle masks"). However, adding an extra check
cpu < num_possible_cpusin select_cpu(), allows to properly support AMD CPUs, even with kernels that don't have the cpu_online_mask fix yet (this doesn't always guarantee the validity of cpu, but it should be enough to mitigate the majority of the potential sub-optimal cases, without introducing any significant overhead)the latter can be fixed relying on topology.span(), instead of topology.nr_cpus(), to count the amount of available CPUs in the system.
[1] sched-ext/sched_ext#69
[2] #147
Link: sched-ext/sched_ext@94dc0c0