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scx_rustland: get rid of force_local

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Now that we can dispatch directly from select_cpu() we can make the code
more compact and readable by removing the force_local logic.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
This commit is contained in:
Andrea Righi 2024-01-09 21:51:08 +01:00
parent 6ead675fb6
commit 1da2983804
1 changed files with 14 additions and 98 deletions
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112
scheds/rust/scx_rustland/src/bpf/main.bpf.c
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@ -129,31 +129,6 @@ struct {
__uint(max_entries, MAX_ENQUEUED_TASKS);
} dispatched SEC(".maps");
/*
* Per-task local storage.
*
* This contain all the per-task information used internally by the BPF code.
*/
struct task_ctx {
/*
* Set this flag to dispatch directly from .enqueueu() to the local DSQ
* of the cpu where the task is going to run (bypassing the scheduler).
*
* This can be used for example when the selected cpu is idle; in this
* case we can simply dispatch the task on the same target cpu and
* avoid unnecessary calls to the user-space scheduler.
*/
bool force_local;
};
/* Map that contains task-local storage. */
struct {
__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
__uint(map_flags, BPF_F_NO_PREALLOC);
__type(key, int);
__type(value, struct task_ctx);
} task_ctx_stor SEC(".maps");
/*
* Heartbeat timer used to periodically trigger the check to run the user-space
* scheduler.
@ -375,66 +350,23 @@ static void dispatch_task(struct task_struct *p, s32 cpu, u64 enq_flags)
s32 BPF_STRUCT_OPS(rustland_select_cpu, struct task_struct *p, s32 prev_cpu,
u64 wake_flags)
{
struct task_ctx *tctx;
tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
if (!tctx) {
scx_bpf_error("Failed to look up task-local storage for %s", p->comm);
return -ESRCH;
}
/*
* Always try to keep the tasks on the same CPU (unless the user-space
* scheduler decides otherwise).
*
* Check if the previously used CPU is idle, in this case we can
* dispatch directly from .enqueue(), bypassing the user-space
* scheduler.
*/
tctx->force_local = get_cpu_owner(prev_cpu) == 0;
return prev_cpu;
}
/*
* Return true if the selected CPU for the task is immediately available
* (user-space scheduler not required), false otherwise (user-space scheduler
* required).
*
* To determine if the CPU is available we rely on tctx->force_idle (set in
* .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, u64 enq_flags)
{
struct task_ctx *tctx;
/*
* Always dispatch per-CPU kthreads on the same CPU, bypassing the
* user-space scheduler (in this way we can to prioritize critical
* kernel threads that may potentially slow down the entire system if
* they are blocked for too long).
*/
if (is_kthread(p) && p->nr_cpus_allowed == 1)
return true;
/*
* Do not bypass the user-space scheduler if there are pending
* activity, otherwise, we may risk disrupting the scheduler's
* dispatch directly here, bypassing the user-space scheduler.
*
* However, do not bypass the user-space scheduler if there are pending
* activities, otherwise, we may risk disrupting the scheduler's
* decisions and negatively affecting the overall system performance.
*/
if (is_usersched_needed())
return false;
/*
* For regular tasks always rely on force_local to determine if we can
* bypass the scheduler.
*/
tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
if (!tctx) {
scx_bpf_error("Failed to lookup task ctx for %s", p->comm);
return false;
if (!is_usersched_needed() && !get_cpu_owner(prev_cpu)) {
dispatch_local(p, 0);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
}
return tctx->force_local;
return prev_cpu;
}
/*
@ -484,10 +416,12 @@ void BPF_STRUCT_OPS(rustland_enqueue, struct task_struct *p, u64 enq_flags)
return;
/*
* Directly dispatch the task to the local DSQ if the selected task's
* CPU is available (no scheduling decision required).
* Always dispatch per-CPU kthreads on the same CPU, bypassing the
* user-space scheduler (in this way we can to prioritize critical
* kernel threads that may potentially slow down the entire system if
* they are blocked for too long).
*/
if (is_task_cpu_available(p, enq_flags)) {
if (is_kthread(p) && p->nr_cpus_allowed == 1) {
dispatch_local(p, enq_flags);
__sync_fetch_and_add(&nr_kernel_dispatches, 1);
return;
@ -647,23 +581,6 @@ void BPF_STRUCT_OPS(rustland_update_idle, s32 cpu, bool idle)
}
}
/*
* A new task @p is being created.
*
* Allocate and initialize all the internal structures for the task (this
* function is allowed to block, so it can be used to preallocate memory).
*/
s32 BPF_STRUCT_OPS(rustland_init_task, struct task_struct *p,
struct scx_init_task_args *args)
{
/* Allocate task's local storage */
if (bpf_task_storage_get(&task_ctx_stor, p, 0,
BPF_LOCAL_STORAGE_GET_F_CREATE))
return 0;
else
return -ENOMEM;
}
/*
* Task @p is exiting.
*
@ -777,7 +694,6 @@ struct sched_ext_ops rustland = {
.running = (void *)rustland_running,
.stopping = (void *)rustland_stopping,
.update_idle = (void *)rustland_update_idle,
.init_task = (void *)rustland_init_task,
.exit_task = (void *)rustland_exit_task,
.init = (void *)rustland_init,
.exit = (void *)rustland_exit,