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scx_layered: Add more comments

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This commit is contained in:
Tejun Heo 2024-03-13 09:56:28 -10:00
parent a9457a408e
commit 60b346c1fc
1 changed files with 21 additions and 1 deletions
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22
scheds/rust/scx_layered/src/bpf/main.bpf.c
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@ -504,6 +504,12 @@ void BPF_STRUCT_OPS(layered_enqueue, struct task_struct *p, u64 enq_flags)
if (!(cand_cctx = lookup_cpu_ctx(cpu)) || cand_cctx->current_preempt)
continue;
/*
* If exclusive, we want to make sure the sibling CPU, if
* there's one, is idle. However, if the sibling CPU is already
* running a preempt task, we shouldn't kick it out.
*/
if (layer->exclusive && (sib = sibling_cpu(cpu)) >= 0 &&
(!(sib_cctx = lookup_cpu_ctx(sib)) ||
sib_cctx->current_preempt)) {
@ -512,6 +518,14 @@ void BPF_STRUCT_OPS(layered_enqueue, struct task_struct *p, u64 enq_flags)
}
scx_bpf_kick_cpu(cpu, SCX_KICK_PREEMPT);
/*
* $sib_cctx is set iff @p is an exclusive task, a sibling CPU
* exists which is not running a preempt task. Let's preempt the
* sibling CPU so that it can become idle. The ->maybe_idle test
* is inaccurate and racy but should be good enough for
* best-effort optimization.
*/
if (sib_cctx && !sib_cctx->maybe_idle) {
lstat_inc(LSTAT_EXCL_PREEMPT, layer, cctx);
scx_bpf_kick_cpu(sib, SCX_KICK_PREEMPT);
@ -539,6 +553,11 @@ void BPF_STRUCT_OPS(layered_dispatch, s32 cpu, struct task_struct *prev)
if (!(cctx = lookup_cpu_ctx(-1)))
return;
/*
* If the sibling CPU is running an exclusive task, keep this CPU idle.
* This test is a racy test but should be good enough for best-effort
* optimization.
*/
if (sib >= 0 && (sib_cctx = lookup_cpu_ctx(sib)) &&
sib_cctx->current_exclusive) {
gstat_inc(GSTAT_EXCL_IDLE, cctx);
@ -731,7 +750,8 @@ void BPF_STRUCT_OPS(layered_running, struct task_struct *p)
/*
* %SCX_KICK_IDLE would be great here but we want to support
* older kernels. Let's use racy custom idle flag instead.
* older kernels. Let's use racy and inaccruate custom idle flag
* instead.
*/
if (sib >= 0 && (sib_cctx = lookup_cpu_ctx(sib)) &&
sib_cctx->maybe_idle) {