[PATCH v2 1/9] sched: Add vCPU debooster infrastructure

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

Introduce foundational infrastructure for the vCPU debooster mechanism
to improve yield_to() effectiveness in virtualization workloads.

Add per-rq tracking fields for rate limiting (yield_deboost_last_time_ns)
and debouncing (yield_deboost_last_src/dst_pid, last_pair_time_ns).
Introduce global sysctl knob sysctl_sched_vcpu_debooster_enabled for
runtime control, defaulting to enabled. Add debugfs interface for
observability and initialization in sched_init().

The infrastructure is inert at this stage as no deboost logic is
implemented yet, allowing independent verification that existing
behavior remains unchanged.

v1 -> v2:
- Rename debugfs entry from sched_vcpu_debooster_enabled to
  vcpu_debooster_enabled for consistency with other sched debugfs entries
- Add explicit initialization of yield_deboost_last_time_ns to 0 in
  sched_init() for clarity
- Improve comments to follow kernel documentation style

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 kernel/sched/core.c  |  9 +++++++--
 kernel/sched/debug.c |  2 ++
 kernel/sched/fair.c  |  7 +++++++
 kernel/sched/sched.h | 12 ++++++++++++
 4 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 41ba0be16911..9f0936b9c1c9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8606,9 +8606,14 @@ void __init sched_init(void)
 #endif /* CONFIG_CGROUP_SCHED */
 
 	for_each_possible_cpu(i) {
-		struct rq *rq;
+		struct rq *rq = cpu_rq(i);
+
+		/* Initialize vCPU debooster per-rq state */
+		rq->yield_deboost_last_time_ns = 0;
+		rq->yield_deboost_last_src_pid = -1;
+		rq->yield_deboost_last_dst_pid = -1;
+		rq->yield_deboost_last_pair_time_ns = 0;
 
-		rq = cpu_rq(i);
 		raw_spin_lock_init(&rq->__lock);
 		rq->nr_running = 0;
 		rq->calc_load_active = 0;
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 41caa22e0680..13e67617549d 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -508,6 +508,8 @@ static __init int sched_init_debug(void)
 	debugfs_create_file("tunable_scaling", 0644, debugfs_sched, NULL, &sched_scaling_fops);
 	debugfs_create_u32("migration_cost_ns", 0644, debugfs_sched, &sysctl_sched_migration_cost);
 	debugfs_create_u32("nr_migrate", 0644, debugfs_sched, &sysctl_sched_nr_migrate);
+	debugfs_create_u32("vcpu_debooster_enabled", 0644, debugfs_sched,
+			   &sysctl_sched_vcpu_debooster_enabled);
 
 	sched_domains_mutex_lock();
 	update_sched_domain_debugfs();
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index da46c3164537..87c30db2c853 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -81,6 +81,13 @@ static unsigned int normalized_sysctl_sched_base_slice	= 700000ULL;
 
 __read_mostly unsigned int sysctl_sched_migration_cost	= 500000UL;
 
+/*
+ * vCPU debooster: runtime toggle for yield_to() vruntime penalty mechanism.
+ * When enabled (default), yield_to() applies bounded vruntime penalties to
+ * improve lock holder scheduling in virtualized environments.
+ */
+unsigned int sysctl_sched_vcpu_debooster_enabled __read_mostly = 1;
+
 static int __init setup_sched_thermal_decay_shift(char *str)
 {
 	pr_warn("Ignoring the deprecated sched_thermal_decay_shift= option\n");
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d30cca6870f5..b7aa0d35c793 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1294,6 +1294,16 @@ struct rq {
 	unsigned int		push_busy;
 	struct cpu_stop_work	push_work;
 
+	/*
+	 * vCPU debooster: per-rq state for yield_to() optimization.
+	 * Used to rate-limit and debounce vruntime penalties applied
+	 * when a vCPU yields to a lock holder.
+	 */
+	u64			yield_deboost_last_time_ns;
+	pid_t			yield_deboost_last_src_pid;
+	pid_t			yield_deboost_last_dst_pid;
+	u64			yield_deboost_last_pair_time_ns;
+
 #ifdef CONFIG_SCHED_CORE
 	/* per rq */
 	struct rq		*core;
@@ -2958,6 +2968,8 @@ extern int sysctl_resched_latency_warn_once;
 
 extern unsigned int sysctl_sched_tunable_scaling;
 
+extern unsigned int sysctl_sched_vcpu_debooster_enabled;
+
 extern unsigned int sysctl_numa_balancing_scan_delay;
 extern unsigned int sysctl_numa_balancing_scan_period_min;
 extern unsigned int sysctl_numa_balancing_scan_period_max;
-- 
2.43.0