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Subject[PATCH 1/4] task_work: use the single-linked list to shrink sizeof(task_work)
Change struct task_work to have a single forward pointer. The pending
task_work's create the circular list, and task->last_twork points to
the last element. This way we can access head/tail in O(1).

Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
include/linux/sched.h | 3 +-
include/linux/task_work.h | 5 +--
include/linux/tracehook.h | 4 +-
kernel/fork.c | 2 +-
kernel/task_work.c | 62 +++++++++++++++++++++++++-------------------
5 files changed, 42 insertions(+), 34 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4059c0f..5486299 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -101,6 +101,7 @@ struct bio_list;
struct fs_struct;
struct perf_event_context;
struct blk_plug;
+struct task_work;

/*
* List of flags we want to share for kernel threads,
@@ -1405,7 +1406,7 @@ struct task_struct {
int (*notifier)(void *priv);
void *notifier_data;
sigset_t *notifier_mask;
- struct hlist_head task_works;
+ struct task_work *last_twork;

struct audit_context *audit_context;
#ifdef CONFIG_AUDITSYSCALL
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 294d5d5..03640fb 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -1,14 +1,13 @@
#ifndef _LINUX_TASK_WORK_H
#define _LINUX_TASK_WORK_H

-#include <linux/list.h>
#include <linux/sched.h>

struct task_work;
typedef void (*task_work_func_t)(struct task_work *);

struct task_work {
- struct hlist_node hlist;
+ struct task_work *next;
task_work_func_t func;
void *data;
};
@@ -26,7 +25,7 @@ void task_work_run(void);

static inline void exit_task_work(struct task_struct *task)
{
- if (unlikely(!hlist_empty(&task->task_works)))
+ if (unlikely(task->last_twork))
task_work_run();
}

diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 6a4d82b..89e88fe 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -189,10 +189,10 @@ static inline void tracehook_notify_resume(struct pt_regs *regs)
/*
* The caller just cleared TIF_NOTIFY_RESUME. This barrier
* pairs with task_work_add()->set_notify_resume() after
- * hlist_add_head(task->task_works);
+ * addition to task->last_twork list.
*/
smp_mb__after_clear_bit();
- if (unlikely(!hlist_empty(&current->task_works)))
+ if (unlikely(current->last_twork))
task_work_run();
}

diff --git a/kernel/fork.c b/kernel/fork.c
index ab5211b..075000b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1415,7 +1415,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
*/
p->group_leader = p;
INIT_LIST_HEAD(&p->thread_group);
- INIT_HLIST_HEAD(&p->task_works);
+ p->last_twork = NULL;

/* Now that the task is set up, run cgroup callbacks if
* necessary. We need to run them before the task is visible
diff --git a/kernel/task_work.c b/kernel/task_work.c
index 82d1c79..a0a3133 100644
--- a/kernel/task_work.c
+++ b/kernel/task_work.c
@@ -2,10 +2,15 @@
#include <linux/task_work.h>
#include <linux/tracehook.h>

+/*
+ * task->last_twork points to the last node in the circular single-linked list.
+ */
+
int
task_work_add(struct task_struct *task, struct task_work *twork, bool notify)
{
unsigned long flags;
+ struct task_work *last;
int err = -ESRCH;

#ifndef TIF_NOTIFY_RESUME
@@ -19,7 +24,10 @@ task_work_add(struct task_struct *task, struct task_work *twork, bool notify)
*/
raw_spin_lock_irqsave(&task->pi_lock, flags);
if (likely(!(task->flags & PF_EXITING))) {
- hlist_add_head(&twork->hlist, &task->task_works);
+ last = task->last_twork ?: twork;
+ task->last_twork = twork;
+ twork->next = last->next;
+ last->next = twork;
err = 0;
}
raw_spin_unlock_irqrestore(&task->pi_lock, flags);
@@ -34,15 +42,26 @@ struct task_work *
task_work_cancel(struct task_struct *task, task_work_func_t func)
{
unsigned long flags;
- struct task_work *twork;
- struct hlist_node *pos;
+ struct task_work *last, *prev, *twork;

raw_spin_lock_irqsave(&task->pi_lock, flags);
- hlist_for_each_entry(twork, pos, &task->task_works, hlist) {
- if (twork->func == func) {
- hlist_del(&twork->hlist);
+ last = task->last_twork;
+ if (last) {
+ twork = last;
+ do {
+ prev = twork;
+ twork = twork->next;
+ if (twork->func != func)
+ continue;
+
+ prev->next = twork->next;
+ if (twork == last) {
+ if (prev == twork)
+ prev = NULL;
+ task->last_twork = prev;
+ }
goto found;
- }
+ } while (twork != last);
}
twork = NULL;
found:
@@ -54,31 +73,20 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
void task_work_run(void)
{
struct task_struct *task = current;
- struct hlist_head task_works;
- struct hlist_node *pos;
+ struct task_work *last, *next, *twork;

raw_spin_lock_irq(&task->pi_lock);
- hlist_move_list(&task->task_works, &task_works);
+ last = task->last_twork;
+ task->last_twork = NULL;
raw_spin_unlock_irq(&task->pi_lock);

- if (unlikely(hlist_empty(&task_works)))
+ if (unlikely(!last))
return;
- /*
- * We use hlist to save the space in task_struct, but we want fifo.
- * Find the last entry, the list should be short, then process them
- * in reverse order.
- */
- for (pos = task_works.first; pos->next; pos = pos->next)
- ;

- for (;;) {
- struct hlist_node **pprev = pos->pprev;
- struct task_work *twork = container_of(pos, struct task_work,
- hlist);
+ next = last->next;
+ do {
+ twork = next;
+ next = twork->next;
twork->func(twork);
-
- if (pprev == &task_works.first)
- break;
- pos = container_of(pprev, struct hlist_node, next);
- }
+ } while (twork != last);
}
--
1.5.5.1



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