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scx_rustland_core: use SCX_DSQ_LOCAL only with kthreads

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Tasks dispatched directly using SCX_DSQ_LOCAL may receive excessively
high priority compared to those dispatched by the user-space scheduler.

To avoid this priority disparity, dispatch tasks to the per-CPU DSQs
using direct dispatch and reserve SCX_DSQ_LOCAL for per-CPU kthreads
only.

Tested-by: Tested-by: SoulHarsh007 <harsh.peshwani@outlook.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
This commit is contained in:
Andrea Righi 2024-06-10 14:20:08 +02:00
parent 8c6fe540eb
commit 2d2c6083d7
1 changed files with 16 additions and 15 deletions
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31
rust/scx_rustland_core/assets/bpf/main.bpf.c
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@ -367,7 +367,6 @@ dispatch_task(struct task_struct *p, u64 dsq_id,
s32 cpu;
switch (dsq_id) {
case SCX_DSQ_LOCAL:
case SHARED_DSQ:
scx_bpf_dispatch(p, dsq_id, slice, enq_flags);
dbg_msg("dispatch: pid=%d (%s) dsq=%llu enq_flags=%llx slice=%llu",
@ -461,6 +460,18 @@ static void dispatch_user_scheduler(void)
bpf_task_release(p);
}
/*
* Directly dispatch a task on a target CPU bypassing the user-space scheduler.
*/
static void
dispatch_direct_cpu(struct task_struct *p, s32 cpu, u64 slice_ns, u64 enq_flags)
{
scx_bpf_dispatch(p, cpu_to_dsq(cpu), slice_ns, enq_flags);
scx_bpf_kick_cpu(cpu, __COMPAT_SCX_KICK_IDLE);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
}
/*
* Select the target CPU where a task can be executed.
*
@ -495,14 +506,10 @@ s32 BPF_STRUCT_OPS(rustland_select_cpu, struct task_struct *p, s32 prev_cpu,
/*
* If the previously used CPU is still available, keep using it to take
* advantage of the cached working set.
*
* NOTE: assign a shorter time slice (slice_ns / 4) to a task directly
* dispatched to prevent it from gaining excessive CPU bandwidth.
*/
if (scx_bpf_test_and_clear_cpu_idle(prev_cpu)) {
tctx->allow_migration = false;
dispatch_task(p, SCX_DSQ_LOCAL, 0, slice_ns / 4, 0);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
dispatch_direct_cpu(p, prev_cpu, slice_ns, 0);
return prev_cpu;
}
@ -523,8 +530,7 @@ s32 BPF_STRUCT_OPS(rustland_select_cpu, struct task_struct *p, s32 prev_cpu,
cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, &is_idle);
if (is_idle) {
tctx->allow_migration = false;
dispatch_task(p, SCX_DSQ_LOCAL, 0, slice_ns / 4, 0);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
dispatch_direct_cpu(p, cpu, slice_ns, 0);
return cpu;
}
tctx->allow_migration = true;
@ -595,7 +601,7 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
* long (i.e., ksoftirqd/N, rcuop/N, etc.).
*/
if (is_kthread(p) && p->nr_cpus_allowed == 1) {
dispatch_task(p, SCX_DSQ_LOCAL, 0, slice_ns, enq_flags);
scx_bpf_dispatch(p, SCX_DSQ_LOCAL, slice_ns, enq_flags);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
return;
}
@ -605,12 +611,7 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
* FIFO mode.
*/
if (!full_user && is_fifo_enabled) {
s32 cpu = scx_bpf_task_cpu(p);
scx_bpf_dispatch(p, cpu_to_dsq(cpu), slice_ns, enq_flags);
scx_bpf_kick_cpu(cpu, __COMPAT_SCX_KICK_IDLE);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
dispatch_direct_cpu(p, scx_bpf_task_cpu(p), slice_ns, enq_flags);
return;
}