scx_rustland: improve kernel/user-space communication #142
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added 2 commits
February 20, 2024 12:30
Instead of using a BPF_MAP_TYPE_ARRAY to store which tasks are running
on which CPU we can simply use a global array, mapped in the user-space
address space.
In this way we can avoid a lot of memory copies and call to sys_bpf(),
significantly reducing the scheduler's overhead.
Keep in mind that we don't need to be 100% correct while accessing this
information, so we can accept some fuzziness in order to significantly
reduce the scheduler's overhead.
Performance profile before this change:
5.52% [kernel] [k] __sys_bpf
4.84% [kernel] [k] __kmem_cache_alloc_node
4.71% [kernel] [k] map_lookup_elem
4.10% [kernel] [k] _copy_from_user
3.51% [kernel] [k] bpf_map_copy_value
3.12% [kernel] [k] check_heap_object
After this change:
2.20% [kernel] [k] __sys_bpf
1.91% [kernel] [k] map_lookup_and_delete_elem
1.60% [kernel] [k] __kmem_cache_alloc_node
1.10% [kernel] [k] _copy_from_user
0.12% [kernel] [k] check_heap_object
n/a bpf_map_copy_value
n/a map_lookup_elem
With this change we can reduce the overhead of sys_bpf() by ~2x and
the overhead of copy_from_user() by ~4x.
Signed-off-by: Andrea Righi <[email protected]>
Switch from a BPF_MAP_TYPE_QUEUE to a BPF_MAP_TYPE_RINGBUF to store the tasks that need to be processed by the user-space scheduler. A ring buffer allows to save a lot of memory copies and syscalls, since the memory is directly shared between the BPF and the user-space components. Performance profile before this change: 2.44% [kernel] [k] __memset 2.19% [kernel] [k] __sys_bpf 1.59% [kernel] [k] __kmem_cache_alloc_node 1.00% [kernel] [k] _copy_from_user After this change: 1.42% [kernel] [k] __memset 0.14% [kernel] [k] __sys_bpf 0.10% [kernel] [k] __kmem_cache_alloc_node 0.07% [kernel] [k] _copy_from_user Both the overhead of sys_bpf() and copy_from_user() are reduced by a factor of ~15x now (only the dispatch path is using sys_bpf() now). NOTE: despite being very effective, the current implementation is a bit of a hack. This is because the present ring buffer API exclusively permits consumption in a greedy manner, where multiple items can be consumed simultaneously. However, libbpf-rs does not provide precise information regarding the exact number of items consumed. By utilizing a more refined libbpf-rs API [1] we may be able to improve this code a bit. Moreover, libbpf-rs doesn't provide an API for the user_ring_buffer, so at the moment there's not a trivial way to apply the same change to the dispatched tasks. However, just with this change applied, the overhead of sys_bpf() and copy_from_user() is already minimal, so we won't get much benefits by changing the dispatch path to use a BPF ring buffer. [1] libbpf/libbpf-rs#680 Signed-off-by: Andrea Righi <[email protected]>
htejun
approved these changes
Feb 20, 2024
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Reduce the overhead of kernel/user-space communication by using a BPF_MAP_TYPE_RINGBUF for the enqueued tasks and change cpu_map (map of pids running on the available CPUs) from a BPF_MAP_TYPE_ARRAY to a global static array.
These changes allow to significantly reduce the amount of syscalls and memcpy operations performed by scx_rustland. Looking at perf top:
Before:
After: