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scx_nest: Set timer callback after cancelling

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In scx_nest, we use a per-cpu BPF timer to schedule compaction for a
primary core before it goes idle. If a task comes along that could use
that core, we cancel the callback with bpf_timer_cancel().
bpf_timer_cancel() drops a refcnt on the prog and nullifies the
callback, so if we want to schedule the callback again, we must use
bpf_timer_set_callback() to reset the prog. This patch does that.

Reported-by: Julia Lawall <julia.lawall@inria.fr>
Signed-off-by: David Vernet <void@manifault.com>
This commit is contained in:
David Vernet 2024-01-16 14:01:39 -06:00
parent f28e5fb259
commit 607119d8a4
No known key found for this signature in database
GPG Key ID: 59E4B86965C4F364
1 changed files with 41 additions and 35 deletions
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76
scheds/c/scx_nest.bpf.c
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@ -197,6 +197,40 @@ static void update_attached(struct task_ctx *tctx, s32 prev_cpu, s32 new_cpu)
tctx->prev_cpu = prev_cpu;
}
static int compact_primary_core(void *map, int *key, struct bpf_timer *timer)
{
struct bpf_cpumask *primary, *reserve;
s32 cpu = bpf_get_smp_processor_id();
struct pcpu_ctx *pcpu_ctx;
stat_inc(NEST_STAT(CALLBACK_COMPACTED));
/*
* If we made it to this callback, it means that the timer callback was
* never cancelled, and so the core needs to be demoted from the
* primary nest.
*/
pcpu_ctx = bpf_map_lookup_elem(&pcpu_ctxs, &cpu);
if (!pcpu_ctx) {
scx_bpf_error("Couldn't lookup pcpu ctx");
return 0;
}
bpf_rcu_read_lock();
primary = primary_cpumask;
reserve = reserve_cpumask;
if (!primary || !reserve) {
scx_bpf_error("Couldn't find primary or reserve");
bpf_rcu_read_unlock();
return 0;
}
bpf_cpumask_clear_cpu(cpu, primary);
try_make_core_reserved(cpu, reserve, false);
bpf_rcu_read_unlock();
pcpu_ctx->num_schedulings = 0;
pcpu_ctx->scheduled_compaction = false;
return 0;
}
s32 BPF_STRUCT_OPS(nest_select_cpu, struct task_struct *p, s32 prev_cpu,
u64 wake_flags)
{
@ -344,6 +378,8 @@ migrate_primary:
if (pcpu_ctx->scheduled_compaction) {
if (bpf_timer_cancel(&pcpu_ctx->timer) < 0)
scx_bpf_error("Failed to cancel pcpu timer");
if (bpf_timer_set_callback(&pcpu_ctx->timer, compact_primary_core))
scx_bpf_error("Failed to re-arm pcpu timer");
pcpu_ctx->scheduled_compaction = false;
stat_inc(NEST_STAT(CANCELLED_COMPACTION));
}
@ -514,40 +550,6 @@ void BPF_STRUCT_OPS(nest_enable, struct task_struct *p)
p->scx.dsq_vtime = vtime_now;
}
static int compact_primary_core(void *map, int *key, struct bpf_timer *timer)
{
struct bpf_cpumask *primary, *reserve;
s32 cpu = bpf_get_smp_processor_id();
struct pcpu_ctx *pcpu_ctx;
stat_inc(NEST_STAT(CALLBACK_COMPACTED));
/*
* If we made it to this callback, it means that the timer callback was
* never cancelled, and so the core needs to be demoted from the
* primary nest.
*/
pcpu_ctx = bpf_map_lookup_elem(&pcpu_ctxs, &cpu);
if (!pcpu_ctx) {
scx_bpf_error("Couldn't lookup pcpu ctx");
return 0;
}
bpf_rcu_read_lock();
primary = primary_cpumask;
reserve = reserve_cpumask;
if (!primary || !reserve) {
scx_bpf_error("Couldn't find primary or reserve");
bpf_rcu_read_unlock();
return 0;
}
bpf_cpumask_clear_cpu(cpu, primary);
try_make_core_reserved(cpu, reserve, false);
bpf_rcu_read_unlock();
pcpu_ctx->num_schedulings = 0;
pcpu_ctx->scheduled_compaction = false;
return 0;
}
static int stats_timerfn(void *map, int *key, struct bpf_timer *timer)
{
s32 cpu;
@ -637,7 +639,11 @@ s32 BPF_STRUCT_OPS_SLEEPABLE(nest_init)
return -EINVAL;
}
ctx->num_schedulings = 0;
bpf_timer_set_callback(&ctx->timer, compact_primary_core);
err = bpf_timer_set_callback(&ctx->timer, compact_primary_core);
if (err) {
scx_bpf_error("Failed to set pcpu timer callback");
return -EINVAL;
}
}
timer = bpf_map_lookup_elem(&stats_timer, &key);