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scx_rustland: lowlatency improvements #59

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merged 4 commits into from
Dec 31, 2023
Merged

scx_rustland: lowlatency improvements #59

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49f2e7c
scx_rustland: enable SCX_OPS_ENQ_LAST
Dec 31, 2023
405a113
scx_rustland: always use dispatch_on_cpu() when possible
Dec 31, 2023
a7677fd
scx_rustland: bypass user-space scheduler for short-lived kthreads
Dec 31, 2023
1cdcb8a
scx_rustland: show the CPU where the scheduler is running
Dec 31, 2023
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11 changes: 9 additions & 2 deletions scheds/rust/scx_rustland/src/bpf/main.bpf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -309,7 +309,7 @@ s32 BPF_STRUCT_OPS(rustland_select_cpu, struct task_struct *p, s32 prev_cpu,
* .select_cpu()), since this function may be called on a different CPU (so we
* cannot check the current CPU directly).
*/
static bool is_task_cpu_available(struct task_struct *p)
static bool is_task_cpu_available(struct task_struct *p, u64 enq_flags)
{
struct task_ctx *tctx;

Expand All @@ -322,6 +322,12 @@ static bool is_task_cpu_available(struct task_struct *p)
if (is_kthread(p) && p->nr_cpus_allowed == 1)
return true;

/*
* No scheduling required if it's the last task running.
*/
if (enq_flags & SCX_ENQ_LAST)
return true;
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I don't quite understand what difference this commit makes given that the default behavior w/o SCX_ENQ_LAST is auto-enq on local dsq. Does this make any behavior difference?

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I don't quite understand what difference this commit makes given that the default behavior w/o SCX_ENQ_LAST is auto-enq on local dsq. Does this make any behavior difference?

For some reasons with this in place the CPU utilization of scx_rustland drops significantly when the system is idle (like 0.3-0.5%), without this change when the system is idle I can see some cpu usage spikes of up to 5-10%,but I'm not sure why it's happening... My theory was that with SCX_ENQ_LAST in place we could save some unnecessary usersched invocations.

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That's surprising given that the two essentially are doing the same thing. Maybe there's something timing dependent going on or I'm just confused.

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That's surprising given that the two essentially are doing the same thing. Maybe there's something timing dependent going on or I'm just confused.

I'll run some tests with and without that and collect more info, now I want to understand what's going on exactly :)

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@htejun it's SCX_OPS_ENQ_LAST set in sched_ext_ops.flags that seems to make a difference, the check in is_task_cpu_available() does't make any difference and can be dropped apparently.


/*
* For regular tasks always rely on force_local to determine if we can
* bypass the scheduler.
Expand Down Expand Up @@ -365,7 +371,7 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
* Dispatch the task on the local FIFO directly if the selected task's
* CPU is available (no scheduling decision required).
*/
if (is_task_cpu_available(p)) {
if (is_task_cpu_available(p, enq_flags)) {
dispatch_local(p, enq_flags);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
return;
Expand Down Expand Up @@ -626,6 +632,7 @@ struct sched_ext_ops rustland = {
.prep_enable = (void *)rustland_prep_enable,
.init = (void *)rustland_init,
.exit = (void *)rustland_exit,
.flags = SCX_OPS_ENQ_LAST,
.timeout_ms = 5000,
.name = "rustland",
};