Currently, the primary bottleneck in scx_rustland lies within its custom
memory allocator, which is used to prevent page faults in the user-space
scheduler.
This is pretty evident looking at perf top:
39.95% scx_rustland [.] <scx_rustland::bpf::alloc::RustLandAllocator as core::alloc::global::GlobalAlloc>::alloc
3.41% [kernel] [k] _copy_from_user
3.20% [kernel] [k] __kmem_cache_alloc_node
2.59% [kernel] [k] __sys_bpf
2.30% [kernel] [k] __kmem_cache_free
1.48% libc.so.6 [.] syscall
1.45% [kernel] [k] __virt_addr_valid
1.42% scx_rustland [.] <scx_rustland::bpf::alloc::RustLandAllocator as core::alloc::global::GlobalAlloc>::dealloc
1.31% [kernel] [k] _copy_to_user
1.23% [kernel] [k] entry_SYSRETQ_unsafe_stack
However, there's no need to reinvent the wheel here, rather than relying
on an overly simplistic and inefficient allocator, we can rely on
buddy-alloc [1], which is also capable of operating on a preallocated
memory buffer.
After switching to buddy-alloc, the performance profile under the same
workload conditions looks like the following:
6.01% [kernel] [k] _copy_from_user
5.21% [kernel] [k] __kmem_cache_alloc_node
4.45% [kernel] [k] __sys_bpf
3.80% [kernel] [k] __kmem_cache_free
2.79% libc.so.6 [.] syscall
2.34% [kernel] [k] __virt_addr_valid
2.26% [kernel] [k] _copy_to_user
2.14% [kernel] [k] __check_heap_object
2.10% [kernel] [k] __check_object_size.part.0
2.02% [kernel] [k] entry_SYSRETQ_unsafe_stack
With this change in place, the primary overhead is now moved to the
bpf() syscall and the copies between kernel and user-space (this could
potentially be optimized in the future using BPF ring buffers, instead
of BPF FIFO queues).
A better focus at the allocator overhead before vs after this change:
[before]
39.95% scx_rustland [.] core::alloc::global::GlobalAlloc>::alloc
1.42% scx_rustland [.] core::alloc::global::GlobalAlloc>::dealloc
[after]
1.50% scx_rustland [.] core::alloc::global::GlobalAlloc>::alloc
0.76% scx_rustland [.] core::alloc::global::GlobalAlloc>::dealloc
[1] https://crates.io/crates/buddy-alloc
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
After updates to reflect the updated init and direct dispatch API, the
schedulers aren't compatible with older kernels. Bump versions and publish
releases.
Rename scx_rustlite to scx_rustland to better represent the mirroring of
scx_userland (in C), but implemented in Rust.
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
