[patch] scheduler, migration startup fixes, 2.5.29

From: Ingo Molnar (mingo@elte.hu)
Date: Sat Jul 27 2002 - 05:54:33 EST

the attached patch fixes the scheduler's migration thread startup bug that
got unearthed by Rusty's recent CPU-startup enhancements.

the fix is to let a startup-helper thread migrate the migration thread,
instead of the migration thread calling set_cpus_allowed() itself.
Migrating a not running thread is a simple and robust thing, and needs no
cooperation from migration threads - thus the catch-22 problem of how to
migrate the migration threads is solved finally.

the patch is against Rusty's initcall fix/hack which calls
migration_init() before other CPUs are brought up - this ordering is
clearly the clean way of doing migration init. [the patch also fixes a UP
compiliation bug in Rusty's hack.]

tested on x86 SMP and UP.

        Ingo

--- linux/kernel/sched.c.orig Sat Jul 27 12:25:07 2002
+++ linux/kernel/sched.c Sat Jul 27 12:48:33 2002
@@ -16,19 +16,23 @@
  * by Davide Libenzi, preemptible kernel bits by Robert Love.
  */
 
+#define __KERNEL_SYSCALLS__
+
 #include <linux/mm.h>
 #include <linux/nmi.h>
 #include <linux/init.h>
-#include <asm/uaccess.h>
+#include <linux/delay.h>
+#include <linux/unistd.h>
 #include <linux/highmem.h>
+#include <linux/security.h>
+#include <linux/notifier.h>
 #include <linux/smp_lock.h>
-#include <asm/mmu_context.h>
 #include <linux/interrupt.h>
 #include <linux/completion.h>
 #include <linux/kernel_stat.h>
-#include <linux/security.h>
-#include <linux/notifier.h>
-#include <linux/delay.h>
+
+#include <asm/uaccess.h>
+#include <asm/mmu_context.h>
 
 /*
  * Convert user-nice values [ -20 ... 0 ... 19 ]
@@ -1816,18 +1820,57 @@
         preempt_enable();
 }
 
+/*
+ * The migration thread startup relies on the following property
+ * of set_cpus_allowed(): if the thread is not running currently
+ * then we can just put it into the target runqueue.
+ */
+DECLARE_MUTEX_LOCKED(migration_startup);
+
+typedef struct migration_startup_data {
+ int cpu;
+ task_t *thread;
+} migration_startup_t;
+
+static int migration_startup_thread(void * data)
+{
+ migration_startup_t *startup = data;
+
+ wait_task_inactive(startup->thread);
+ set_cpus_allowed(startup->thread, 1UL << startup->cpu);
+ up(&migration_startup);
+
+ return 0;
+}
+
 static int migration_thread(void * bind_cpu)
 {
         int cpu = (int) (long) bind_cpu;
         struct sched_param param = { sched_priority: MAX_RT_PRIO-1 };
+ migration_startup_t startup;
         runqueue_t *rq;
- int ret;
+ int ret, pid;
 
         daemonize();
         sigfillset(&current->blocked);
         set_fs(KERNEL_DS);
 
- set_cpus_allowed(current, 1UL << cpu);
+ startup.cpu = cpu;
+ startup.thread = current;
+ pid = kernel_thread(migration_startup_thread, &startup,
+ CLONE_FS | CLONE_FILES | CLONE_SIGNAL);
+ down(&migration_startup);
+
+ /* we need to waitpid() to release the helper thread */
+ waitpid(pid, NULL, __WCLONE);
+
+ /*
+ * At this point the startup helper thread must have
+ * migrated us to the proper CPU already:
+ */
+ if (smp_processor_id() != (int)bind_cpu)
+ BUG();
+
         printk("migration_task %d on cpu=%d\n", cpu, smp_processor_id());
         ret = setscheduler(0, SCHED_FIFO, &param);
 
@@ -1888,13 +1931,14 @@
                           unsigned long action,
                           void *hcpu)
 {
+ unsigned long cpu = (unsigned long)hcpu;
+
         switch (action) {
         case CPU_ONLINE:
- printk("Starting migration thread for cpu %li\n",
- (long)hcpu);
- kernel_thread(migration_thread, hcpu,
+ printk("Starting migration thread for cpu %li\n", cpu);
+ kernel_thread(migration_thread, (void *)cpu,
                               CLONE_FS | CLONE_FILES | CLONE_SIGNAL);
- while (!cpu_rq((long)hcpu)->migration_thread)
+ while (!cpu_rq(cpu)->migration_thread)
                         yield();
                 break;
         }
--- linux/init/main.c.orig Sat Jul 27 12:50:10 2002
+++ linux/init/main.c Sat Jul 27 12:50:46 2002
@@ -526,10 +526,14 @@
 
 static void do_pre_smp_initcalls(void)
 {
+#if CONFIG_SMP
         extern int migration_init(void);
+#endif
         extern int spawn_ksoftirqd(void);
 
+#if CONFIG_SMP
         migration_init();
+#endif
         spawn_ksoftirqd();
 }
 

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