[PATCH 32/39] sched_ext: Implement sched_ext_ops.cpu_online/offline()
From: Tejun Heo
Date: Wed May 01 2024 - 11:20:59 EST
Add ops.cpu_online/offline() which are invoked when CPUs come online and
offline respectively. As the enqueue path already automatically bypasses
tasks to the local dsq on a deactivated CPU, BPF schedulers are guaranteed
to see tasks only on CPUs which are between online() and offline().
If the BPF scheduler doesn't implement ops.cpu_online/offline(), the
scheduler is automatically exited with SCX_ECODE_RESTART |
SCX_ECODE_RSN_HOTPLUG. Userspace can implement CPU hotpplug support
trivially by simply reinitializing and reloading the scheduler.
scx_qmap is updated to print out online CPUs on hotplug events. Other
schedulers are updated to restart based on ecode.
v2: - To accommodate lock ordering change between scx_cgroup_rwsem and
cpus_read_lock(), CPU hotplug operations are put into its own SCX_OPI
block and enabled eariler during scx_ope_enable() so that
cpus_read_lock() can be dropped before acquiring scx_cgroup_rwsem.
- Auto exit with ECODE added.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: David Vernet <dvernet@xxxxxxxx>
Acked-by: Josh Don <joshdon@xxxxxxxxxx>
Acked-by: Hao Luo <haoluo@xxxxxxxxxx>
Acked-by: Barret Rhoden <brho@xxxxxxxxxx>
---
kernel/sched/ext.c | 135 ++++++++++++++++++-
tools/sched_ext/include/scx/compat.h | 29 ++++
tools/sched_ext/include/scx/user_exit_info.h | 28 ++++
tools/sched_ext/scx_central.c | 9 +-
tools/sched_ext/scx_flatcg.c | 8 +-
tools/sched_ext/scx_qmap.bpf.c | 60 +++++++++
tools/sched_ext/scx_qmap.c | 4 +
tools/sched_ext/scx_simple.c | 8 +-
8 files changed, 271 insertions(+), 10 deletions(-)
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 9bc03533cf5e..ed2452d42862 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -30,6 +30,29 @@ enum scx_exit_kind {
SCX_EXIT_ERROR_STALL, /* watchdog detected stalled runnable tasks */
};
+/*
+ * An exit code can be specified when exiting with scx_bpf_exit() or
+ * scx_ops_exit(), corresponding to exit_kind UNREG_BPF and UNREG_KERN
+ * respectively. The codes are 64bit of the format:
+ *
+ * Bits: [63 .. 48 47 .. 32 31 .. 0]
+ * [ SYS ACT ] [ SYS RSN ] [ USR ]
+ *
+ * SYS ACT: System-defined exit actions
+ * SYS RSN: System-defined exit reasons
+ * USR : User-defined exit codes and reasons
+ *
+ * Using the above, users may communicate intention and context by ORing system
+ * actions and/or system reasons with a user-defined exit code.
+ */
+enum scx_exit_code {
+ /* Reasons */
+ SCX_ECODE_RSN_HOTPLUG = 1LLU << 32,
+
+ /* Actions */
+ SCX_ECODE_ACT_RESTART = 1LLU << 48,
+};
+
/*
* scx_exit_info is passed to ops.exit() to describe why the BPF scheduler is
* being disabled.
@@ -504,7 +527,29 @@ struct sched_ext_ops {
#endif /* CONFIG_CGROUPS */
/*
- * All online ops must come before ops.init().
+ * All online ops must come before ops.cpu_online().
+ */
+
+ /**
+ * cpu_online - A CPU became online
+ * @cpu: CPU which just came up
+ *
+ * @cpu just came online. @cpu doesn't call ops.enqueue() or run tasks
+ * associated with other CPUs beforehand.
+ */
+ void (*cpu_online)(s32 cpu);
+
+ /**
+ * cpu_offline - A CPU is going offline
+ * @cpu: CPU which is going offline
+ *
+ * @cpu is going offline. @cpu doesn't call ops.enqueue() or run tasks
+ * associated with other CPUs afterwards.
+ */
+ void (*cpu_offline)(s32 cpu);
+
+ /*
+ * All CPU hotplug ops must come before ops.init().
*/
/**
@@ -543,6 +588,15 @@ struct sched_ext_ops {
*/
u32 exit_dump_len;
+ /**
+ * hotplug_seq - A sequence number that may be set by the scheduler to
+ * detect when a hotplug event has occurred during the loading process.
+ * If 0, no detection occurs. Otherwise, the scheduler will fail to
+ * load if the sequence number does not match @scx_hotplug_seq on the
+ * enable path.
+ */
+ u64 hotplug_seq;
+
/**
* name - BPF scheduler's name
*
@@ -556,7 +610,9 @@ struct sched_ext_ops {
enum scx_opi {
SCX_OPI_BEGIN = 0,
SCX_OPI_NORMAL_BEGIN = 0,
- SCX_OPI_NORMAL_END = SCX_OP_IDX(init),
+ SCX_OPI_NORMAL_END = SCX_OP_IDX(cpu_online),
+ SCX_OPI_CPU_HOTPLUG_BEGIN = SCX_OP_IDX(cpu_online),
+ SCX_OPI_CPU_HOTPLUG_END = SCX_OP_IDX(init),
SCX_OPI_END = SCX_OP_IDX(init),
};
@@ -746,6 +802,7 @@ static atomic_t scx_exit_kind = ATOMIC_INIT(SCX_EXIT_DONE);
static struct scx_exit_info *scx_exit_info;
static atomic_long_t scx_nr_rejected = ATOMIC_LONG_INIT(0);
+static atomic_long_t scx_hotplug_seq = ATOMIC_LONG_INIT(0);
/*
* The maximum amount of time in jiffies that a task may be runnable without
@@ -2172,7 +2229,8 @@ static int balance_scx(struct rq *rq, struct task_struct *prev,
* emitted in scx_next_task_picked().
*/
if (SCX_HAS_OP(cpu_acquire))
- SCX_CALL_OP(0, cpu_acquire, cpu_of(rq), NULL);
+ SCX_CALL_OP(SCX_KF_UNLOCKED, cpu_acquire, cpu_of(rq),
+ NULL);
rq->scx.cpu_released = false;
}
@@ -2700,6 +2758,34 @@ void __scx_update_idle(struct rq *rq, bool idle)
#endif
}
+static void handle_hotplug(struct rq *rq, bool online)
+{
+ int cpu = cpu_of(rq);
+
+ atomic_long_inc(&scx_hotplug_seq);
+
+ if (online && SCX_HAS_OP(cpu_online))
+ SCX_CALL_OP(SCX_KF_REST, cpu_online, cpu);
+ else if (!online && SCX_HAS_OP(cpu_offline))
+ SCX_CALL_OP(SCX_KF_REST, cpu_offline, cpu);
+ else
+ scx_ops_exit(SCX_ECODE_ACT_RESTART | SCX_ECODE_RSN_HOTPLUG,
+ "cpu %d going %s, exiting scheduler", cpu,
+ online ? "online" : "offline");
+}
+
+static void rq_online_scx(struct rq *rq, enum rq_onoff_reason reason)
+{
+ if (reason == RQ_ONOFF_HOTPLUG)
+ handle_hotplug(rq, true);
+}
+
+static void rq_offline_scx(struct rq *rq, enum rq_onoff_reason reason)
+{
+ if (reason == RQ_ONOFF_HOTPLUG)
+ handle_hotplug(rq, false);
+}
+
#else /* CONFIG_SMP */
static bool test_and_clear_cpu_idle(int cpu) { return false; }
@@ -3328,6 +3414,9 @@ DEFINE_SCHED_CLASS(ext) = {
.balance = balance_scx,
.select_task_rq = select_task_rq_scx,
.set_cpus_allowed = set_cpus_allowed_scx,
+
+ .rq_online = rq_online_scx,
+ .rq_offline = rq_offline_scx,
#endif
.task_tick = task_tick_scx,
@@ -3584,10 +3673,18 @@ static ssize_t scx_attr_nr_rejected_show(struct kobject *kobj,
}
SCX_ATTR(nr_rejected);
+static ssize_t scx_attr_hotplug_seq_show(struct kobject *kobj,
+ struct kobj_attribute *ka, char *buf)
+{
+ return sysfs_emit(buf, "%ld\n", atomic_long_read(&scx_hotplug_seq));
+}
+SCX_ATTR(hotplug_seq);
+
static struct attribute *scx_global_attrs[] = {
&scx_attr_state.attr,
&scx_attr_switch_all.attr,
&scx_attr_nr_rejected.attr,
+ &scx_attr_hotplug_seq.attr,
NULL,
};
@@ -4110,6 +4207,25 @@ static struct kthread_worker *scx_create_rt_helper(const char *name)
return helper;
}
+static void check_hotplug_seq(const struct sched_ext_ops *ops)
+{
+ unsigned long long global_hotplug_seq;
+
+ /*
+ * If a hotplug event has occurred between when a scheduler was
+ * initialized, and when we were able to attach, exit and notify user
+ * space about it.
+ */
+ if (ops->hotplug_seq) {
+ global_hotplug_seq = atomic_long_read(&scx_hotplug_seq);
+ if (ops->hotplug_seq != global_hotplug_seq) {
+ scx_ops_exit(SCX_ECODE_ACT_RESTART | SCX_ECODE_RSN_HOTPLUG,
+ "expected hotplug seq %llu did not match actual %llu",
+ ops->hotplug_seq, global_hotplug_seq);
+ }
+ }
+}
+
static int validate_ops(const struct sched_ext_ops *ops)
{
/*
@@ -4192,6 +4308,10 @@ static int scx_ops_enable(struct sched_ext_ops *ops)
}
}
+ for (i = SCX_OPI_CPU_HOTPLUG_BEGIN; i < SCX_OPI_CPU_HOTPLUG_END; i++)
+ if (((void (**)(void))ops)[i])
+ static_branch_enable_cpuslocked(&scx_has_op[i]);
+
cpus_read_unlock();
ret = validate_ops(ops);
@@ -4239,6 +4359,8 @@ static int scx_ops_enable(struct sched_ext_ops *ops)
cpus_read_lock();
scx_cgroup_lock();
+ check_hotplug_seq(ops);
+
for (i = SCX_OPI_NORMAL_BEGIN; i < SCX_OPI_NORMAL_END; i++)
if (((void (**)(void))ops)[i])
static_branch_enable_cpuslocked(&scx_has_op[i]);
@@ -4563,6 +4685,9 @@ static int bpf_scx_init_member(const struct btf_type *t,
ops->exit_dump_len =
*(u32 *)(udata + moff) ?: SCX_EXIT_DUMP_DFL_LEN;
return 1;
+ case offsetof(struct sched_ext_ops, hotplug_seq):
+ ops->hotplug_seq = *(u64 *)(udata + moff);
+ return 1;
}
return 0;
@@ -4659,6 +4784,8 @@ static void cgroup_move_stub(struct task_struct *p, struct cgroup *from, struct
static void cgroup_cancel_move_stub(struct task_struct *p, struct cgroup *from, struct cgroup *to) {}
static void cgroup_set_weight_stub(struct cgroup *cgrp, u32 weight) {}
#endif
+static void cpu_online_stub(s32 cpu) {}
+static void cpu_offline_stub(s32 cpu) {}
static s32 init_stub(void) { return -EINVAL; }
static void exit_stub(struct scx_exit_info *info) {}
@@ -4689,6 +4816,8 @@ static struct sched_ext_ops __bpf_ops_sched_ext_ops = {
.cgroup_cancel_move = cgroup_cancel_move_stub,
.cgroup_set_weight = cgroup_set_weight_stub,
#endif
+ .cpu_online = cpu_online_stub,
+ .cpu_offline = cpu_offline_stub,
.init = init_stub,
.exit = exit_stub,
};
diff --git a/tools/sched_ext/include/scx/compat.h b/tools/sched_ext/include/scx/compat.h
index 2be79bd88a25..7155b69150ff 100644
--- a/tools/sched_ext/include/scx/compat.h
+++ b/tools/sched_ext/include/scx/compat.h
@@ -8,6 +8,9 @@
#define __SCX_COMPAT_H
#include <bpf/btf.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
struct btf *__COMPAT_vmlinux_btf __attribute__((weak));
@@ -120,6 +123,28 @@ static inline bool __COMPAT_struct_has_field(const char *type, const char *field
#define __COMPAT_HAS_CPUMASKS \
__COMPAT_has_ksym("scx_bpf_nr_cpu_ids")
+static inline long scx_hotplug_seq(void)
+{
+ int fd;
+ char buf[32];
+ ssize_t len;
+ long val;
+
+ fd = open("/sys/kernel/sched_ext/hotplug_seq", O_RDONLY);
+ if (fd < 0)
+ return -ENOENT;
+
+ len = read(fd, buf, sizeof(buf) - 1);
+ SCX_BUG_ON(len <= 0, "read failed (%ld)", len);
+ buf[len] = 0;
+ close(fd);
+
+ val = strtoul(buf, NULL, 10);
+ SCX_BUG_ON(val < 0, "invalid num hotplug events: %lu", val);
+
+ return val;
+}
+
/*
* struct sched_ext_ops can change over time. If compat.bpf.h::SCX_OPS_DEFINE()
* is used to define ops and compat.h::SCX_OPS_LOAD/ATTACH() are used to load
@@ -128,12 +153,16 @@ static inline bool __COMPAT_struct_has_field(const char *type, const char *field
*
* - ops.tick(): Ignored on older kernels with a warning.
* - ops.exit_dump_len: Cleared to zero on older kernels with a warning.
+ * - ops.hotplug_seq: Ignored on older kernels.
*/
#define SCX_OPS_OPEN(__ops_name, __scx_name) ({ \
struct __scx_name *__skel; \
\
__skel = __scx_name##__open(); \
SCX_BUG_ON(!__skel, "Could not open " #__scx_name); \
+ \
+ if (__COMPAT_struct_has_field("sched_ext_ops", "hotplug_seq")) \
+ __skel->struct_ops.__ops_name->hotplug_seq = scx_hotplug_seq(); \
__skel; \
})
diff --git a/tools/sched_ext/include/scx/user_exit_info.h b/tools/sched_ext/include/scx/user_exit_info.h
index cf4293cb250e..2d86d01a9575 100644
--- a/tools/sched_ext/include/scx/user_exit_info.h
+++ b/tools/sched_ext/include/scx/user_exit_info.h
@@ -77,7 +77,35 @@ struct user_exit_info {
if (__uei->msg[0] != '\0') \
fprintf(stderr, " (%s)", __uei->msg); \
fputs("\n", stderr); \
+ __uei->exit_code; \
})
+/*
+ * We can't import vmlinux.h while compiling user C code. Let's duplicate
+ * scx_exit_code definition.
+ */
+enum scx_exit_code {
+ /* Reasons */
+ SCX_ECODE_RSN_HOTPLUG = 1LLU << 32,
+
+ /* Actions */
+ SCX_ECODE_ACT_RESTART = 1LLU << 48,
+};
+
+enum uei_ecode_mask {
+ UEI_ECODE_USER_MASK = ((1LLU << 32) - 1),
+ UEI_ECODE_SYS_RSN_MASK = ((1LLU << 16) - 1) << 32,
+ UEI_ECODE_SYS_ACT_MASK = ((1LLU << 16) - 1) << 48,
+};
+
+/*
+ * These macro interpret the ecode returned from UEI_REPORT().
+ */
+#define UEI_ECODE_USER(__ecode) ((__ecode) & UEI_ECODE_USER_MASK)
+#define UEI_ECODE_SYS_RSN(__ecode) ((__ecode) & UEI_ECODE_SYS_RSN_MASK)
+#define UEI_ECODE_SYS_ACT(__ecode) ((__ecode) & UEI_ECODE_SYS_ACT_MASK)
+
+#define UEI_ECODE_RESTART(__ecode) (UEI_ECODE_SYS_ACT((__ecode)) == SCX_ECODE_ACT_RESTART)
+
#endif /* __bpf__ */
#endif /* __USER_EXIT_INFO_H */
diff --git a/tools/sched_ext/scx_central.c b/tools/sched_ext/scx_central.c
index 2908add16880..df692dc0ccb1 100644
--- a/tools/sched_ext/scx_central.c
+++ b/tools/sched_ext/scx_central.c
@@ -46,14 +46,14 @@ int main(int argc, char **argv)
{
struct scx_central *skel;
struct bpf_link *link;
- __u64 seq = 0;
+ __u64 seq = 0, ecode;
__s32 opt;
cpu_set_t *cpuset;
libbpf_set_print(libbpf_print_fn);
signal(SIGINT, sigint_handler);
signal(SIGTERM, sigint_handler);
-
+restart:
skel = SCX_OPS_OPEN(central_ops, scx_central);
skel->rodata->central_cpu = 0;
@@ -126,7 +126,10 @@ int main(int argc, char **argv)
}
bpf_link__destroy(link);
- UEI_REPORT(skel, uei);
+ ecode = UEI_REPORT(skel, uei);
scx_central__destroy(skel);
+
+ if (UEI_ECODE_RESTART(ecode))
+ goto restart;
return 0;
}
diff --git a/tools/sched_ext/scx_flatcg.c b/tools/sched_ext/scx_flatcg.c
index bb0a832a0cfd..20c5a132b610 100644
--- a/tools/sched_ext/scx_flatcg.c
+++ b/tools/sched_ext/scx_flatcg.c
@@ -127,11 +127,12 @@ int main(int argc, char **argv)
__u64 last_stats[FCG_NR_STATS] = {};
unsigned long seq = 0;
__s32 opt;
+ __u64 ecode;
libbpf_set_print(libbpf_print_fn);
signal(SIGINT, sigint_handler);
signal(SIGTERM, sigint_handler);
-
+restart:
skel = SCX_OPS_OPEN(flatcg_ops, scx_flatcg);
skel->rodata->nr_cpus = libbpf_num_possible_cpus();
@@ -219,7 +220,10 @@ int main(int argc, char **argv)
}
bpf_link__destroy(link);
- UEI_REPORT(skel, uei);
+ ecode = UEI_REPORT(skel, uei);
scx_flatcg__destroy(skel);
+
+ if (UEI_ECODE_RESTART(ecode))
+ goto restart;
return 0;
}
diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index 7c3b0dcae1e0..c2edc080d7e5 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -308,11 +308,69 @@ s32 BPF_STRUCT_OPS(qmap_init_task, struct task_struct *p,
return -ENOMEM;
}
+/*
+ * Print out the online and possible CPU map using bpf_printk() as a
+ * demonstration of using the cpumask kfuncs and ops.cpu_on/offline().
+ */
+static void print_cpus(void)
+{
+ const struct cpumask *possible, *online;
+ s32 cpu;
+ char buf[128] = "", *p;
+ int idx;
+
+ if (!__COMPAT_HAS_CPUMASKS)
+ return;
+
+ possible = scx_bpf_get_possible_cpumask();
+ online = scx_bpf_get_online_cpumask();
+
+ idx = 0;
+ bpf_for(cpu, 0, scx_bpf_nr_cpu_ids()) {
+ if (!(p = MEMBER_VPTR(buf, [idx++])))
+ break;
+ if (bpf_cpumask_test_cpu(cpu, online))
+ *p++ = 'O';
+ else if (bpf_cpumask_test_cpu(cpu, possible))
+ *p++ = 'X';
+ else
+ *p++ = ' ';
+
+ if ((cpu & 7) == 7) {
+ if (!(p = MEMBER_VPTR(buf, [idx++])))
+ break;
+ *p++ = '|';
+ }
+ }
+ buf[sizeof(buf) - 1] = '\0';
+
+ scx_bpf_put_cpumask(online);
+ scx_bpf_put_cpumask(possible);
+
+ bpf_printk("CPUS: |%s", buf);
+}
+
+void BPF_STRUCT_OPS(qmap_cpu_online, s32 cpu)
+{
+ bpf_printk("CPU %d coming online", cpu);
+ /* @cpu is already online at this point */
+ print_cpus();
+}
+
+void BPF_STRUCT_OPS(qmap_cpu_offline, s32 cpu)
+{
+ bpf_printk("CPU %d going offline", cpu);
+ /* @cpu is still online at this point */
+ print_cpus();
+}
+
s32 BPF_STRUCT_OPS_SLEEPABLE(qmap_init)
{
if (!switch_partial)
__COMPAT_scx_bpf_switch_all();
+ print_cpus();
+
return scx_bpf_create_dsq(SHARED_DSQ, -1);
}
@@ -328,6 +386,8 @@ SCX_OPS_DEFINE(qmap_ops,
.dispatch = (void *)qmap_dispatch,
.cpu_release = (void *)qmap_cpu_release,
.init_task = (void *)qmap_init_task,
+ .cpu_online = (void *)qmap_cpu_online,
+ .cpu_offline = (void *)qmap_cpu_offline,
.init = (void *)qmap_init,
.exit = (void *)qmap_exit,
.timeout_ms = 5000U,
diff --git a/tools/sched_ext/scx_qmap.c b/tools/sched_ext/scx_qmap.c
index 048b31eed17d..e82f58b5c131 100644
--- a/tools/sched_ext/scx_qmap.c
+++ b/tools/sched_ext/scx_qmap.c
@@ -119,5 +119,9 @@ int main(int argc, char **argv)
bpf_link__destroy(link);
UEI_REPORT(skel, uei);
scx_qmap__destroy(skel);
+ /*
+ * scx_qmap implements ops.cpu_on/offline() and doesn't need to restart
+ * on CPU hotplug events.
+ */
return 0;
}
diff --git a/tools/sched_ext/scx_simple.c b/tools/sched_ext/scx_simple.c
index 9ffa8d084228..acee683a3ec9 100644
--- a/tools/sched_ext/scx_simple.c
+++ b/tools/sched_ext/scx_simple.c
@@ -62,11 +62,12 @@ int main(int argc, char **argv)
struct scx_simple *skel;
struct bpf_link *link;
__u32 opt;
+ __u64 ecode;
libbpf_set_print(libbpf_print_fn);
signal(SIGINT, sigint_handler);
signal(SIGTERM, sigint_handler);
-
+restart:
skel = SCX_OPS_OPEN(simple_ops, scx_simple);
while ((opt = getopt(argc, argv, "vh")) != -1) {
@@ -93,7 +94,10 @@ int main(int argc, char **argv)
}
bpf_link__destroy(link);
- UEI_REPORT(skel, uei);
+ ecode = UEI_REPORT(skel, uei);
scx_simple__destroy(skel);
+
+ if (UEI_ECODE_RESTART(ecode))
+ goto restart;
return 0;
}
--
2.44.0