6.6-stable review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra (Intel) peterz@infradead.org
commit 33cf66d88306663d16e4759e9d24766b0aaa2e17 upstream.
Add a randomized algorithm that runs newidle balancing proportional to its success rate.
This improves schbench significantly:
6.18-rc4: 2.22 Mrps/s 6.18-rc4+revert: 2.04 Mrps/s 6.18-rc4+revert+random: 2.18 Mrps/S
Conversely, per Adam Li this affects SpecJBB slightly, reducing it by 1%:
6.17: -6% 6.17+revert: 0% 6.17+revert+random: -1%
Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Tested-by: Dietmar Eggemann dietmar.eggemann@arm.com Tested-by: Chris Mason clm@meta.com Link: https://lkml.kernel.org/r/6825c50d-7fa7-45d8-9b81-c6e7e25738e2@meta.com Link: https://patch.msgid.link/20251107161739.770122091@infradead.org [ Ajay: Modified to apply on v6.6 ] Signed-off-by: Ajay Kaher ajay.kaher@broadcom.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/sched/topology.h | 3 ++ kernel/sched/core.c | 3 ++ kernel/sched/fair.c | 44 +++++++++++++++++++++++++++++++++++++---- kernel/sched/features.h | 5 ++++ kernel/sched/sched.h | 7 ++++++ kernel/sched/topology.c | 6 +++++ 6 files changed, 64 insertions(+), 4 deletions(-)
--- a/include/linux/sched/topology.h +++ b/include/linux/sched/topology.h @@ -106,6 +106,9 @@ struct sched_domain { unsigned int nr_balance_failed; /* initialise to 0 */
/* idle_balance() stats */ + unsigned int newidle_call; + unsigned int newidle_success; + unsigned int newidle_ratio; u64 max_newidle_lb_cost; unsigned long last_decay_max_lb_cost;
--- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -116,6 +116,7 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_util_ EXPORT_TRACEPOINT_SYMBOL_GPL(sched_update_nr_running_tp);
DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues); +DEFINE_PER_CPU(struct rnd_state, sched_rnd_state);
#ifdef CONFIG_SCHED_DEBUG /* @@ -9872,6 +9873,8 @@ void __init sched_init_smp(void) { sched_init_numa(NUMA_NO_NODE);
+ prandom_init_once(&sched_rnd_state); + /* * There's no userspace yet to cause hotplug operations; hence all the * CPU masks are stable and all blatant races in the below code cannot --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -11716,11 +11716,27 @@ void update_max_interval(void) max_load_balance_interval = HZ*num_online_cpus()/10; }
-static inline bool update_newidle_cost(struct sched_domain *sd, u64 cost) +static inline void update_newidle_stats(struct sched_domain *sd, unsigned int success) +{ + sd->newidle_call++; + sd->newidle_success += success; + + if (sd->newidle_call >= 1024) { + sd->newidle_ratio = sd->newidle_success; + sd->newidle_call /= 2; + sd->newidle_success /= 2; + } +} + +static inline bool +update_newidle_cost(struct sched_domain *sd, u64 cost, unsigned int success) { unsigned long next_decay = sd->last_decay_max_lb_cost + HZ; unsigned long now = jiffies;
+ if (cost) + update_newidle_stats(sd, success); + if (cost > sd->max_newidle_lb_cost) { /* * Track max cost of a domain to make sure to not delay the @@ -11768,7 +11784,7 @@ static void rebalance_domains(struct rq * Decay the newidle max times here because this is a regular * visit to all the domains. */ - need_decay = update_newidle_cost(sd, 0); + need_decay = update_newidle_cost(sd, 0, 0); max_cost += sd->max_newidle_lb_cost;
/* @@ -12406,6 +12422,22 @@ static int sched_balance_newidle(struct break;
if (sd->flags & SD_BALANCE_NEWIDLE) { + unsigned int weight = 1; + + if (sched_feat(NI_RANDOM)) { + /* + * Throw a 1k sided dice; and only run + * newidle_balance according to the success + * rate. + */ + u32 d1k = sched_rng() % 1024; + weight = 1 + sd->newidle_ratio; + if (d1k > weight) { + update_newidle_stats(sd, 0); + continue; + } + weight = (1024 + weight/2) / weight; + }
pulled_task = load_balance(this_cpu, this_rq, sd, CPU_NEWLY_IDLE, @@ -12413,10 +12445,14 @@ static int sched_balance_newidle(struct
t1 = sched_clock_cpu(this_cpu); domain_cost = t1 - t0; - update_newidle_cost(sd, domain_cost); - curr_cost += domain_cost; t0 = t1; + + /* + * Track max cost of a domain to make sure to not delay the + * next wakeup on the CPU. + */ + update_newidle_cost(sd, domain_cost, weight * !!pulled_task); }
/* --- a/kernel/sched/features.h +++ b/kernel/sched/features.h @@ -88,4 +88,9 @@ SCHED_FEAT(UTIL_EST_FASTUP, true)
SCHED_FEAT(LATENCY_WARN, false)
+/* + * Do newidle balancing proportional to its success rate using randomization. + */ +SCHED_FEAT(NI_RANDOM, true) + SCHED_FEAT(HZ_BW, true) --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -5,6 +5,7 @@ #ifndef _KERNEL_SCHED_SCHED_H #define _KERNEL_SCHED_SCHED_H
+#include <linux/prandom.h> #include <linux/sched/affinity.h> #include <linux/sched/autogroup.h> #include <linux/sched/cpufreq.h> @@ -1205,6 +1206,12 @@ static inline bool is_migration_disabled }
DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues); +DECLARE_PER_CPU(struct rnd_state, sched_rnd_state); + +static inline u32 sched_rng(void) +{ + return prandom_u32_state(this_cpu_ptr(&sched_rnd_state)); +}
#define cpu_rq(cpu) (&per_cpu(runqueues, (cpu))) #define this_rq() this_cpu_ptr(&runqueues) --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -1600,6 +1600,12 @@ sd_init(struct sched_domain_topology_lev
.last_balance = jiffies, .balance_interval = sd_weight, + + /* 50% success rate */ + .newidle_call = 512, + .newidle_success = 256, + .newidle_ratio = 512, + .max_newidle_lb_cost = 0, .last_decay_max_lb_cost = jiffies, .child = child,