async: Asynchronous function calls to speed up kernel boot

Right now, most of the kernel boot is strictly synchronous, such that
various hardware delays are done sequentially.

In order to make the kernel boot faster, this patch introduces
infrastructure to allow doing some of the initialization steps
asynchronously, which will hide significant portions of the hardware delays
in practice.

In order to not change device order and other similar observables, this
patch does NOT do full parallel initialization.

Rather, it operates more in the way an out of order CPU does; the work may
be done out of order and asynchronous, but the observable effects
(instruction retiring for the CPU) are still done in the original sequence.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

1 files changed, 321 insertions, 0 deletions

diff --git a/kernel/async.c b/kernel/async.c
new file mode 100644
index 000000000000..afaa8a653d5a
--- /dev/null
+++ b/kernel/async.c
@@ -0,0 +1,321 @@
+/*
+ * async.c: Asynchronous function calls for boot performance
+ *
+ * (C) Copyright 2009 Intel Corporation
+ * Author: Arjan van de Ven <arjan@linux.intel.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; version 2
+ * of the License.
+ */
+
+
+/*
+
+Goals and Theory of Operation
+
+The primary goal of this feature is to reduce the kernel boot time,
+by doing various independent hardware delays and discovery operations
+decoupled and not strictly serialized.
+
+More specifically, the asynchronous function call concept allows
+certain operations (primarily during system boot) to happen
+asynchronously, out of order, while these operations still
+have their externally visible parts happen sequentially and in-order.
+(not unlike how out-of-order CPUs retire their instructions in order)
+
+Key to the asynchronous function call implementation is the concept of
+a "sequence cookie" (which, although it has an abstracted type, can be
+thought of as a monotonically incrementing number).
+
+The async core will assign each scheduled event such a sequence cookie and
+pass this to the called functions.
+
+The asynchronously called function should before doing a globally visible
+operation, such as registering device numbers, call the
+async_synchronize_cookie() function and pass in its own cookie. The
+async_synchronize_cookie() function will make sure that all asynchronous
+operations that were scheduled prior to the operation corresponding with the
+cookie have completed.
+
+Subsystem/driver initialization code that scheduled asynchronous probe
+functions, but which shares global resources with other drivers/subsystems
+that do not use the asynchronous call feature, need to do a full
+synchronization with the async_synchronize_full() function, before returning
+from their init function. This is to maintain strict ordering between the
+asynchronous and synchronous parts of the kernel.
+
+*/
+
+#include <linux/async.h>
+#include <linux/module.h>
+#include <linux/wait.h>
+#include <linux/sched.h>
+#include <linux/init.h>
+#include <linux/kthread.h>
+#include <asm/atomic.h>
+
+static async_cookie_t next_cookie = 1;
+
+#define MAX_THREADS     256
+#define MAX_WORK        32768
+
+static LIST_HEAD(async_pending);
+static LIST_HEAD(async_running);
+static DEFINE_SPINLOCK(async_lock);
+
+struct async_entry {
+        struct list_head list;
+        async_cookie_t   cookie;
+        async_func_ptr   *func;
+        void             *data;
+        struct list_head *running;
+};
+
+static DECLARE_WAIT_QUEUE_HEAD(async_done);
+static DECLARE_WAIT_QUEUE_HEAD(async_new);
+
+static atomic_t entry_count;
+static atomic_t thread_count;
+
+extern int initcall_debug;
+
+
+/*
+ * MUST be called with the lock held!
+ */
+static async_cookie_t  __lowest_in_progress(struct list_head *running)
+{
+        struct async_entry *entry;
+        if (!list_empty(&async_pending)) {
+                entry = list_first_entry(&async_pending,
+                        struct async_entry, list);
+                return entry->cookie;
+        } else if (!list_empty(running)) {
+                entry = list_first_entry(running,
+                        struct async_entry, list);
+                return entry->cookie;
+        } else {
+                /* nothing in progress... next_cookie is "infinity" */
+                return next_cookie;
+        }
+
+}
+/*
+ * pick the first pending entry and run it
+ */
+static void run_one_entry(void)
+{
+        unsigned long flags;
+        struct async_entry *entry;
+        ktime_t calltime, delta, rettime;
+
+        /* 1) pick one task from the pending queue */
+
+        spin_lock_irqsave(&async_lock, flags);
+        if (list_empty(&async_pending))
+                goto out;
+        entry = list_first_entry(&async_pending, struct async_entry, list);
+
+        /* 2) move it to the running queue */
+        list_del(&entry->list);
+        list_add_tail(&entry->list, &async_running);
+        spin_unlock_irqrestore(&async_lock, flags);
+
+        /* 3) run it (and print duration)*/
+        if (initcall_debug) {
+                printk("calling  %lli_%pF @ %i\n", entry->cookie, entry->func, task_pid_nr(current));
+                calltime = ktime_get();
+        }
+        entry->func(entry->data, entry->cookie);
+        if (initcall_debug) {
+                rettime = ktime_get();
+                delta = ktime_sub(rettime, calltime);
+                printk("initcall %lli_%pF returned 0 after %lld usecs\n", entry->cookie,
+                        entry->func, ktime_to_ns(delta) >> 10);
+        }
+
+        /* 4) remove it from the running queue */
+        spin_lock_irqsave(&async_lock, flags);
+        list_del(&entry->list);
+
+        /* 5) free the entry  */
+        kfree(entry);
+        atomic_dec(&entry_count);
+
+        spin_unlock_irqrestore(&async_lock, flags);
+
+        /* 6) wake up any waiters. */
+        wake_up(&async_done);
+        return;
+
+out:
+        spin_unlock_irqrestore(&async_lock, flags);
+}
+
+
+static async_cookie_t __async_schedule(async_func_ptr *ptr, void *data, struct list_head *running)
+{
+        struct async_entry *entry;
+        unsigned long flags;
+        async_cookie_t newcookie;
+        
+
+        /* allow irq-off callers */
+        entry = kzalloc(sizeof(struct async_entry), GFP_ATOMIC);
+
+        /*
+         * If we're out of memory or if there's too much work
+         * pending already, we execute synchronously.
+         */
+        if (!entry || atomic_read(&entry_count) > MAX_WORK) {
+                kfree(entry);
+                spin_lock_irqsave(&async_lock, flags);
+                newcookie = next_cookie++;
+                spin_unlock_irqrestore(&async_lock, flags);
+
+                /* low on memory.. run synchronously */
+                ptr(data, newcookie);
+                return newcookie;
+        }
+        entry->func = ptr;
+        entry->data = data;
+        entry->running = running;
+
+        spin_lock_irqsave(&async_lock, flags);
+        newcookie = entry->cookie = next_cookie++;
+        list_add_tail(&entry->list, &async_pending);
+        atomic_inc(&entry_count);
+        spin_unlock_irqrestore(&async_lock, flags);
+        wake_up(&async_new);
+        return newcookie;
+}
+
+async_cookie_t async_schedule(async_func_ptr *ptr, void *data)
+{
+        return __async_schedule(ptr, data, &async_pending);
+}
+EXPORT_SYMBOL_GPL(async_schedule);
+
+async_cookie_t async_schedule_special(async_func_ptr *ptr, void *data, struct list_head *running)
+{
+        return __async_schedule(ptr, data, running);
+}
+EXPORT_SYMBOL_GPL(async_schedule_special);
+
+void async_synchronize_full(void)
+{
+        async_synchronize_cookie(next_cookie);
+}
+EXPORT_SYMBOL_GPL(async_synchronize_full);
+
+void async_synchronize_full_special(struct list_head *list)
+{
+        async_synchronize_cookie_special(next_cookie, list);
+}
+EXPORT_SYMBOL_GPL(async_synchronize_full_special);
+
+void async_synchronize_cookie_special(async_cookie_t cookie, struct list_head *running)
+{
+        ktime_t starttime, delta, endtime;
+
+        if (initcall_debug) {
+                printk("async_waiting @ %i\n", task_pid_nr(current));
+                starttime = ktime_get();
+        }
+
+        wait_event(async_done, __lowest_in_progress(running) >= cookie);
+
+        if (initcall_debug) {
+                endtime = ktime_get();
+                delta = ktime_sub(endtime, starttime);
+
+                printk("async_continuing @ %i after %lli usec\n",
+                        task_pid_nr(current), ktime_to_ns(delta) >> 10);
+        }
+}
+EXPORT_SYMBOL_GPL(async_synchronize_cookie_special);
+
+void async_synchronize_cookie(async_cookie_t cookie)
+{
+        async_synchronize_cookie_special(cookie, &async_running);
+}
+EXPORT_SYMBOL_GPL(async_synchronize_cookie);
+
+
+static int async_thread(void *unused)
+{
+        DECLARE_WAITQUEUE(wq, current);
+        add_wait_queue(&async_new, &wq);
+
+        while (!kthread_should_stop()) {
+                int ret = HZ;
+                set_current_state(TASK_INTERRUPTIBLE);
+                /*
+                 * check the list head without lock.. false positives
+                 * are dealt with inside run_one_entry() while holding
+                 * the lock.
+                 */
+                rmb();
+                if (!list_empty(&async_pending))
+                        run_one_entry();
+                else
+                        ret = schedule_timeout(HZ);
+
+                if (ret == 0) {
+                        /*
+                         * we timed out, this means we as thread are redundant.
+                         * we sign off and die, but we to avoid any races there
+                         * is a last-straw check to see if work snuck in.
+                         */
+                        atomic_dec(&thread_count);
+                        wmb(); /* manager must see our departure first */
+                        if (list_empty(&async_pending))
+                                break;
+                        /*
+                         * woops work came in between us timing out and us
+                         * signing off; we need to stay alive and keep working.
+                         */
+                        atomic_inc(&thread_count);
+                }
+        }
+        remove_wait_queue(&async_new, &wq);
+
+        return 0;
+}
+
+static int async_manager_thread(void *unused)
+{
+        DECLARE_WAITQUEUE(wq, current);
+        add_wait_queue(&async_new, &wq);
+
+        while (!kthread_should_stop()) {
+                int tc, ec;
+
+                set_current_state(TASK_INTERRUPTIBLE);
+
+                tc = atomic_read(&thread_count);
+                rmb();
+                ec = atomic_read(&entry_count);
+
+                while (tc < ec && tc < MAX_THREADS) {
+                        kthread_run(async_thread, NULL, "async/%i", tc);
+                        atomic_inc(&thread_count);
+                        tc++;
+                }
+
+                schedule();
+        }
+        remove_wait_queue(&async_new, &wq);
+
+        return 0;
+}
+
+static int __init async_init(void)
+{
+        kthread_run(async_manager_thread, NULL, "async/mgr");
+        return 0;
+}
+
+core_initcall(async_init);