[PATCH v2 11/11] scx_qmap: Implement highpri boosting

Tejun Heo
2024-08-30T20:59:57Z

Implement a silly boosting mechanism for nice -20 tasks. The only purpose is
demonstrating and testing scx_bpf_dispatch_from_dsq(). The boosting only
works within SHARED_DSQ and makes only minor differences with increased
dispatch batch (-b).

This exercises moving tasks to a user DSQ and all local DSQs from
ops.dispatch() and BPF timerfn.

v2: Drop the workaround for the iterated tasks not being trusted by the
verifier. The issue is fixed from BPF side.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Daniel Hodges <hodges.daniel.scott@gmail.com>
Cc: David Vernet <void@manifault.com>
Cc: Changwoo Min <multics69@gmail.com>
Cc: Andrea Righi <andrea.righi@linux.dev>
Cc: Dan Schatzberg <schatzberg.dan@gmail.com>
---
tools/sched_ext/scx_qmap.bpf.c | 126 ++++++++++++++++++++++++++++++++++++-----
tools/sched_ext/scx_qmap.c | 11 +++
2 files changed, 123 insertions(+), 14 deletions(-)

--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -27,6 +27,8 @@
enum consts {
ONE_SEC_IN_NS = 1000000000,
SHARED_DSQ = 0,
+ HIGHPRI_DSQ = 1,
+ HIGHPRI_WEIGHT = 8668, /* this is what -20 maps to */
};

char _license[] SEC("license") = "GPL";
@@ -36,10 +38,12 @@ const volatile u32 stall_user_nth;
const volatile u32 stall_kernel_nth;
const volatile u32 dsp_inf_loop_after;
const volatile u32 dsp_batch;
+const volatile bool highpri_boosting;
const volatile bool print_shared_dsq;
const volatile s32 disallow_tgid;
const volatile bool suppress_dump;

+u64 nr_highpri_queued;
u32 test_error_cnt;

UEI_DEFINE(uei);
@@ -95,6 +99,7 @@ static u64 core_sched_tail_seqs[5];
/* Per-task scheduling context */
struct task_ctx {
bool force_local; /* Dispatch directly to local_dsq */
+ bool highpri;
u64 core_sched_seq;
};

@@ -122,6 +127,7 @@ struct {
/* Statistics */
u64 nr_enqueued, nr_dispatched, nr_reenqueued, nr_dequeued, nr_ddsp_from_enq;
u64 nr_core_sched_execed;
+u64 nr_expedited_local, nr_expedited_remote, nr_expedited_lost, nr_expedited_from_timer;
u32 cpuperf_min, cpuperf_avg, cpuperf_max;
u32 cpuperf_target_min, cpuperf_target_avg, cpuperf_target_max;

@@ -140,17 +146,25 @@ static s32 pick_direct_dispatch_cpu(stru
return -1;
}

+static struct task_ctx *lookup_task_ctx(struct task_struct *p)
+{
+ struct task_ctx *tctx;
+
+ if (!(tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0))) {
+ scx_bpf_error("task_ctx lookup failed");
+ return NULL;
+ }
+ return tctx;
+}
+
s32 BPF_STRUCT_OPS(qmap_select_cpu, struct task_struct *p,
s32 prev_cpu, u64 wake_flags)
{
struct task_ctx *tctx;
s32 cpu;

- tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
- if (!tctx) {
- scx_bpf_error("task_ctx lookup failed");
+ if (!(tctx = lookup_task_ctx(p)))
return -ESRCH;
- }

cpu = pick_direct_dispatch_cpu(p, prev_cpu);

@@ -197,11 +211,8 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct
if (test_error_cnt && !--test_error_cnt)
scx_bpf_error("test triggering error");

- tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
- if (!tctx) {
- scx_bpf_error("task_ctx lookup failed");
+ if (!(tctx = lookup_task_ctx(p)))
return;
- }

/*
* All enqueued tasks must have their core_sched_seq updated for correct
@@ -256,6 +267,10 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct
return;
}

+ if (highpri_boosting && p->scx.weight >= HIGHPRI_WEIGHT) {
+ tctx->highpri = true;
+ __sync_fetch_and_add(&nr_highpri_queued, 1);
+ }
__sync_fetch_and_add(&nr_enqueued, 1);
}

@@ -272,13 +287,73 @@ void BPF_STRUCT_OPS(qmap_dequeue, struct

static void update_core_sched_head_seq(struct task_struct *p)
{
- struct task_ctx *tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
int idx = weight_to_idx(p->scx.weight);
+ struct task_ctx *tctx;

- if (tctx)
+ if ((tctx = lookup_task_ctx(p)))
core_sched_head_seqs[idx] = tctx->core_sched_seq;
- else
- scx_bpf_error("task_ctx lookup failed");
+}
+
+/*
+ * To demonstrate the use of scx_bpf_dispatch_from_dsq(), implement silly
+ * selective priority boosting mechanism by scanning SHARED_DSQ looking for
+ * highpri tasks, moving them to HIGHPRI_DSQ and then consuming them first. This
+ * makes minor difference only when dsp_batch is larger than 1.
+ *
+ * scx_bpf_dispatch[_vtime]_from_dsq() are allowed both from ops.dispatch() and
+ * non-rq-lock holding BPF programs. As demonstration, this function is called
+ * from qmap_dispatch() and monitor_timerfn().
+ */
+static bool dispatch_highpri(bool from_timer)
+{
+ struct task_struct *p;
+ s32 this_cpu = bpf_get_smp_processor_id();
+
+ /* scan SHARED_DSQ and move highpri tasks to HIGHPRI_DSQ */
+ bpf_for_each(scx_dsq, p, SHARED_DSQ, 0) {
+ struct task_ctx *tctx;
+
+ if (!(tctx = lookup_task_ctx(p)))
+ return false;
+
+ if (tctx->highpri)
+ scx_bpf_dispatch_from_dsq(BPF_FOR_EACH_ITER, p,
+ HIGHPRI_DSQ, 0, 0);
+ }
+
+ /*
+ * Scan HIGHPRI_DSQ and dispatch until a task that can run on this CPU
+ * is found.
+ */
+ bpf_for_each(scx_dsq, p, HIGHPRI_DSQ, 0) {
+ bool dispatched = false;
+ s32 cpu;
+
+ if (bpf_cpumask_test_cpu(this_cpu, p->cpus_ptr))
+ cpu = this_cpu;
+ else
+ cpu = scx_bpf_pick_any_cpu(p->cpus_ptr, 0);
+
+ if (scx_bpf_dispatch_from_dsq(BPF_FOR_EACH_ITER, p,
+ SCX_DSQ_LOCAL_ON | cpu, 0,
+ SCX_ENQ_PREEMPT)) {
+ if (cpu == this_cpu) {
+ dispatched = true;
+ __sync_fetch_and_add(&nr_expedited_local, 1);
+ } else {
+ __sync_fetch_and_add(&nr_expedited_remote, 1);
+ }
+ if (from_timer)
+ __sync_fetch_and_add(&nr_expedited_from_timer, 1);
+ } else {
+ __sync_fetch_and_add(&nr_expedited_lost, 1);
+ }
+
+ if (dispatched)
+ return true;
+ }
+
+ return false;
}

void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)
@@ -289,7 +364,10 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 c
void *fifo;
s32 i, pid;

- if (scx_bpf_consume(SHARED_DSQ))
+ if (dispatch_highpri(false))
+ return;
+
+ if (!nr_highpri_queued && scx_bpf_consume(SHARED_DSQ))
return;

if (dsp_inf_loop_after && nr_dispatched > dsp_inf_loop_after) {
@@ -326,6 +404,8 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 c

/* Dispatch or advance. */
bpf_repeat(BPF_MAX_LOOPS) {
+ struct task_ctx *tctx;
+
if (bpf_map_pop_elem(fifo, &pid))
break;

@@ -333,13 +413,25 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 c
if (!p)
continue;

+ if (!(tctx = lookup_task_ctx(p))) {
+ bpf_task_release(p);
+ return;
+ }
+
+ if (tctx->highpri)
+ __sync_fetch_and_sub(&nr_highpri_queued, 1);
+
update_core_sched_head_seq(p);
__sync_fetch_and_add(&nr_dispatched, 1);
+
scx_bpf_dispatch(p, SHARED_DSQ, slice_ns, 0);
bpf_task_release(p);
+
batch--;
cpuc->dsp_cnt--;
if (!batch || !scx_bpf_dispatch_nr_slots()) {
+ if (dispatch_highpri(false))
+ return;
scx_bpf_consume(SHARED_DSQ);
return;
}
@@ -649,6 +741,10 @@ static void dump_shared_dsq(void)

static int monitor_timerfn(void *map, int *key, struct bpf_timer *timer)
{
+ bpf_rcu_read_lock();
+ dispatch_highpri(true);
+ bpf_rcu_read_unlock();
+
monitor_cpuperf();

if (print_shared_dsq)
@@ -670,6 +766,10 @@ s32 BPF_STRUCT_OPS_SLEEPABLE(qmap_init)
if (ret)
return ret;

+ ret = scx_bpf_create_dsq(HIGHPRI_DSQ, -1);
+ if (ret)
+ return ret;
+
timer = bpf_map_lookup_elem(&monitor_timer, &key);
if (!timer)
return -ESRCH;
--- a/tools/sched_ext/scx_qmap.c
+++ b/tools/sched_ext/scx_qmap.c
@@ -29,6 +29,7 @@ const char help_fmt[] =
" -l COUNT Trigger dispatch infinite looping after COUNT dispatches\n"
" -b COUNT Dispatch upto COUNT tasks together\n"
" -P Print out DSQ content to trace_pipe every second, use with -b\n"
+" -H Boost nice -20 tasks in SHARED_DSQ, use with -b\n"
" -d PID Disallow a process from switching into SCHED_EXT (-1 for self)\n"
" -D LEN Set scx_exit_info.dump buffer length\n"
" -S Suppress qmap-specific debug dump\n"
@@ -63,7 +64,7 @@ int main(int argc, char **argv)

skel = SCX_OPS_OPEN(qmap_ops, scx_qmap);

- while ((opt = getopt(argc, argv, "s:e:t:T:l:b:Pd:D:Spvh")) != -1) {
+ while ((opt = getopt(argc, argv, "s:e:t:T:l:b:PHd:D:Spvh")) != -1) {
switch (opt) {
case 's':
skel->rodata->slice_ns = strtoull(optarg, NULL, 0) * 1000;
@@ -86,6 +87,9 @@ int main(int argc, char **argv)
case 'P':
skel->rodata->print_shared_dsq = true;
break;
+ case 'H':
+ skel->rodata->highpri_boosting = true;
+ break;
case 'd':
skel->rodata->disallow_tgid = strtol(optarg, NULL, 0);
if (skel->rodata->disallow_tgid < 0)
@@ -121,6 +125,11 @@ int main(int argc, char **argv)
skel->bss->nr_reenqueued, skel->bss->nr_dequeued,
skel->bss->nr_core_sched_execed,
skel->bss->nr_ddsp_from_enq);
+ printf(" exp_local=%"PRIu64" exp_remote=%"PRIu64" exp_timer=%"PRIu64" exp_lost=%"PRIu64"\n",
+ skel->bss->nr_expedited_local,
+ skel->bss->nr_expedited_remote,
+ skel->bss->nr_expedited_from_timer,
+ skel->bss->nr_expedited_lost);
if (__COMPAT_has_ksym("scx_bpf_cpuperf_cur"))
printf("cpuperf: cur min/avg/max=%u/%u/%u target min/avg/max=%u/%u/%u\n",
skel->bss->cpuperf_min,