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Subject[RFC] process wide itimer cruft
From
Date
On Mon, 2009-02-02 at 09:53 +0100, Peter Zijlstra wrote:

> Now the problems appears to be that I overlooked that we keep the itimer
> clock running at all times, not only when we have a pending timer, and I
> suppose that the standard mandates that behaviour :/

> I would rather go back to the old model where we iterate all threads,
> and find a way to not make programs with too many threads for their own
> good lock up the kernel, but instead get poor service.

OK, so I attempted something like that (sort-of boots) but exit.c makes
my head hurt.

I'm punting the sum-all-threads work off to a workqueue, now the problem
is that __exit_signal() needs ensurance that that worklet isn't
outstanding, or delay the freeing of signal struct until the worklet it
done.

We cannot use cancel_delayed_work_sync() because that can sleep, so I
tried the other approach, but it seems my put_signal() below is flawed
in that release_task() can be called by a 3rd party in case of ptrace.

The remaining option is to make signal struct itself rcu freed, but
before I do that, I thought I'd run this code by some folks.

Any comments?

Not-Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/init_task.h | 7 +++-
include/linux/sched.h | 10 ++++
kernel/exit.c | 33 ++++++++------
kernel/sched.c | 107 +++++++++++++++++++++++++++++++++++++++++++-
kernel/sched_fair.c | 1 -
kernel/sched_rt.c | 1 -
kernel/sched_stats.h | 91 --------------------------------------
7 files changed, 138 insertions(+), 112 deletions(-)

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index fa50bdb..538c89e 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -14,6 +14,8 @@
extern struct files_struct init_files;
extern struct fs_struct init_fs;

+extern void do_thread_group_cputime_update(struct work_struct *work);
+
#define INIT_KIOCTX(name, which_mm) \
{ \
.users = ATOMIC_INIT(1), \
@@ -53,7 +55,10 @@ extern struct fs_struct init_fs;
.stime = cputime_zero, \
.sum_exec_runtime = 0, \
.lock = __SPIN_LOCK_UNLOCKED(sig.cputime.totals.lock), \
- }, }, \
+ }, \
+ .work = __DELAYED_WORK_INITIALIZER(sig.cputime.work, \
+ do_thread_group_cputime_update), \
+ }, \
}

extern struct nsproxy init_nsproxy;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4cdec0d..1ced38c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -478,6 +478,7 @@ struct task_cputime {
*/
struct thread_group_cputime {
struct task_cputime totals;
+ struct delayed_work work;
};

/*
@@ -567,6 +568,7 @@ struct signal_struct {
* in __exit_signal, except for the group leader.
*/
cputime_t cutime, cstime;
+ u64 cruntime;
cputime_t gtime;
cputime_t cgtime;
unsigned long nvcsw, nivcsw, cnvcsw, cnivcsw;
@@ -1949,6 +1951,7 @@ extern void exit_thread(void);
extern void exit_files(struct task_struct *);
extern void __cleanup_signal(struct signal_struct *);
extern void __cleanup_sighand(struct sighand_struct *);
+extern void put_signal(struct signal_struct *sig);

extern void exit_itimers(struct signal_struct *);
extern void flush_itimer_signals(void);
@@ -2210,6 +2213,9 @@ static inline int spin_needbreak(spinlock_t *lock)
* Thread group CPU time accounting.
*/

+extern void __thread_group_cputime_update(struct task_struct *p);
+extern void do_thread_group_cputime_update(struct work_struct *work);
+
static inline
void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
{
@@ -2217,6 +2223,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
unsigned long flags;

spin_lock_irqsave(&totals->lock, flags);
+ __thread_group_cputime_update(tsk);
*times = *totals;
spin_unlock_irqrestore(&totals->lock, flags);
}
@@ -2230,6 +2237,9 @@ static inline void thread_group_cputime_init(struct signal_struct *sig)
};

spin_lock_init(&sig->cputime.totals.lock);
+
+ INIT_DELAYED_WORK(&sig->cputime.work,
+ do_thread_group_cputime_update);
}

static inline void thread_group_cputime_free(struct signal_struct *sig)
diff --git a/kernel/exit.c b/kernel/exit.c
index a1b18c0..899749e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -81,6 +81,15 @@ static void __unhash_process(struct task_struct *p)
list_del_init(&p->sibling);
}

+void put_signal(struct signal_struct *sig)
+{
+ if (atomic_dec_and_test(&sig->count)) {
+ flush_sigqueue(&sig->shared_pending);
+ taskstats_tgid_free(sig);
+ __cleanup_signal(sig);
+ }
+}
+
/*
* This function expects the tasklist_lock write-locked.
*/
@@ -96,14 +105,16 @@ static void __exit_signal(struct task_struct *tsk)
spin_lock(&sighand->siglock);

posix_cpu_timers_exit(tsk);
- if (atomic_dec_and_test(&sig->count))
+ if (!atomic_read(&sig->live)) {
posix_cpu_timers_exit_group(tsk);
- else {
+ sig->curr_target = NULL;
+ } else {
/*
* If there is any task waiting for the group exit
* then notify it:
*/
- if (sig->group_exit_task && atomic_read(&sig->count) == sig->notify_count)
+ if (sig->group_exit_task &&
+ atomic_read(&sig->live) == sig->notify_count)
wake_up_process(sig->group_exit_task);

if (tsk == sig->curr_target)
@@ -126,7 +137,6 @@ static void __exit_signal(struct task_struct *tsk)
sig->inblock += task_io_get_inblock(tsk);
sig->oublock += task_io_get_oublock(tsk);
task_io_accounting_add(&sig->ioac, &tsk->ioac);
- sig = NULL; /* Marker for below. */
}

__unhash_process(tsk);
@@ -142,17 +152,9 @@ static void __exit_signal(struct task_struct *tsk)
spin_unlock(&sighand->siglock);

__cleanup_sighand(sighand);
- clear_tsk_thread_flag(tsk,TIF_SIGPENDING);
- if (sig) {
- flush_sigqueue(&sig->shared_pending);
- taskstats_tgid_free(sig);
- /*
- * Make sure ->signal can't go away under rq->lock,
- * see account_group_exec_runtime().
- */
- task_rq_unlock_wait(tsk);
- __cleanup_signal(sig);
- }
+ clear_tsk_thread_flag(tsk, TIF_SIGPENDING);
+
+ put_signal(sig);
}

static void delayed_put_task_struct(struct rcu_head *rhp)
@@ -1329,6 +1331,7 @@ static int wait_task_zombie(struct task_struct *p, int options,
cputime_add(psig->cstime,
cputime_add(cputime.stime,
sig->cstime));
+ psig->cruntime += cputime.sum_exec_runtime + sig->cruntime;
psig->cgtime =
cputime_add(psig->cgtime,
cputime_add(p->gtime,
diff --git a/kernel/sched.c b/kernel/sched.c
index 96439a4..2c21c12 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4338,7 +4338,6 @@ void account_user_time(struct task_struct *p, cputime_t cputime,
/* Add user time to process. */
p->utime = cputime_add(p->utime, cputime);
p->utimescaled = cputime_add(p->utimescaled, cputime_scaled);
- account_group_user_time(p, cputime);

/* Add user time to cpustat. */
tmp = cputime_to_cputime64(cputime);
@@ -4367,7 +4366,6 @@ static void account_guest_time(struct task_struct *p, cputime_t cputime,
/* Add guest time to process. */
p->utime = cputime_add(p->utime, cputime);
p->utimescaled = cputime_add(p->utimescaled, cputime_scaled);
- account_group_user_time(p, cputime);
p->gtime = cputime_add(p->gtime, cputime);

/* Add guest time to cpustat. */
@@ -4396,7 +4394,6 @@ void account_system_time(struct task_struct *p, int hardirq_offset,
/* Add system time to process. */
p->stime = cputime_add(p->stime, cputime);
p->stimescaled = cputime_add(p->stimescaled, cputime_scaled);
- account_group_system_time(p, cputime);

/* Add system time to cpustat. */
tmp = cputime_to_cputime64(cputime);
@@ -4442,6 +4439,108 @@ void account_idle_time(cputime_t cputime)
#ifndef CONFIG_VIRT_CPU_ACCOUNTING

/*
+ * Must be called with IRQs disabled, or from hardirq context.
+ */
+void __thread_group_cputime_update(struct task_struct *p)
+{
+ struct rq *rq = this_rq();
+ struct signal_struct *sig;
+
+ /*
+ * Early boot might not have workqueues running yet.
+ */
+ if (unlikely(system_state != SYSTEM_RUNNING))
+ return;
+
+ /*
+ * Nobody cares about group cputimes for idle.
+ */
+ if (unlikely(rq->idle == p))
+ return;
+
+ /*
+ * Dead man walking.
+ */
+ if (unlikely(p->exit_state))
+ return;
+
+ sig = p->signal;
+ /*
+ * We raced with __exit_signal() and lost.
+ */
+ if (unlikely(!sig))
+ return;
+
+ /*
+ * __exit_signal() is ran with IRQs disabled, so when we have
+ * a signal pointer, it must still be valid.
+ */
+ if (schedule_delayed_work(&sig->cputime.work,
+ ilog2(atomic_read(&sig->live) / nr_cpu_ids)))
+ atomic_inc(&sig->count);
+}
+
+void do_thread_group_cputime_update(struct work_struct *work)
+{
+ struct task_cputime *totals;
+ struct signal_struct *sig;
+ struct task_struct *t, *n;
+ unsigned long flags;
+
+ cputime_t utime = cputime_zero, stime = cputime_zero;
+ u64 runtime = 0;
+
+ sig = container_of(work, struct signal_struct, cputime.work.work);
+
+ rcu_read_lock();
+ n = t = sig->curr_target;
+ if (n) do {
+ /*
+ * Dead tasks come later.
+ */
+ if (unlikely(n->exit_state))
+ goto next;
+
+ /*
+ * Add time for each task.
+ */
+ stime = cputime_add(stime, n->stime);
+ utime = cputime_add(utime, n->utime);
+ runtime += n->se.sum_exec_runtime;
+
+next:
+ n = next_thread(n);
+ } while (n != t);
+
+ /*
+ * Add accumulated time of dead tasks.
+ *
+ * There is a risk we missed a task which was dying but hasn't
+ * been added to the accumulated time yet, too bad, better luck
+ * next time we try.
+ */
+ stime = cputime_add(stime, sig->cstime);
+ utime = cputime_add(utime, sig->cutime);
+ runtime += sig->cruntime;
+
+ /*
+ * Ensure time goes forward.
+ */
+ totals = &sig->cputime.totals;
+ spin_lock_irqsave(&totals->lock, flags);
+ if (cputime_gt(stime, totals->stime))
+ totals->stime = stime;
+ if (cputime_gt(utime, totals->utime))
+ totals->utime = utime;
+ if (runtime > totals->sum_exec_runtime)
+ totals->sum_exec_runtime = runtime;
+ spin_unlock_irqrestore(&totals->lock, flags);
+ rcu_read_unlock();
+
+ put_signal(sig);
+}
+
+/*
* Account a single tick of cpu time.
* @p: the process that the cpu time gets accounted to
* @user_tick: indicates if the tick is a user or a system tick
@@ -4459,6 +4558,8 @@ void account_process_tick(struct task_struct *p, int user_tick)
one_jiffy_scaled);
else
account_idle_time(one_jiffy);
+
+ __thread_group_cputime_update(p);
}

/*
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index bc1563e..5fe3b3d 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -499,7 +499,6 @@ static void update_curr(struct cfs_rq *cfs_rq)
struct task_struct *curtask = task_of(curr);

cpuacct_charge(curtask, delta_exec);
- account_group_exec_runtime(curtask, delta_exec);
}
}

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 299d012..607951e 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -581,7 +581,6 @@ static void update_curr_rt(struct rq *rq)
schedstat_set(curr->se.exec_max, max(curr->se.exec_max, delta_exec));

curr->se.sum_exec_runtime += delta_exec;
- account_group_exec_runtime(curr, delta_exec);

curr->se.exec_start = rq->clock;
cpuacct_charge(curr, delta_exec);
diff --git a/kernel/sched_stats.h b/kernel/sched_stats.h
index 8ab0cef..9831d8c 100644
--- a/kernel/sched_stats.h
+++ b/kernel/sched_stats.h
@@ -276,94 +276,3 @@ sched_info_switch(struct task_struct *prev, struct task_struct *next)
#define sched_info_dequeued(t) do { } while (0)
#define sched_info_switch(t, next) do { } while (0)
#endif /* CONFIG_SCHEDSTATS || CONFIG_TASK_DELAY_ACCT */
-
-/*
- * The following are functions that support scheduler-internal time accounting.
- * These functions are generally called at the timer tick. None of this depends
- * on CONFIG_SCHEDSTATS.
- */
-
-/**
- * account_group_user_time - Maintain utime for a thread group.
- *
- * @tsk: Pointer to task structure.
- * @cputime: Time value by which to increment the utime field of the
- * thread_group_cputime structure.
- *
- * If thread group time is being maintained, get the structure for the
- * running CPU and update the utime field there.
- */
-static inline void account_group_user_time(struct task_struct *tsk,
- cputime_t cputime)
-{
- struct task_cputime *times;
- struct signal_struct *sig;
-
- /* tsk == current, ensure it is safe to use ->signal */
- if (unlikely(tsk->exit_state))
- return;
-
- sig = tsk->signal;
- times = &sig->cputime.totals;
-
- spin_lock(&times->lock);
- times->utime = cputime_add(times->utime, cputime);
- spin_unlock(&times->lock);
-}
-
-/**
- * account_group_system_time - Maintain stime for a thread group.
- *
- * @tsk: Pointer to task structure.
- * @cputime: Time value by which to increment the stime field of the
- * thread_group_cputime structure.
- *
- * If thread group time is being maintained, get the structure for the
- * running CPU and update the stime field there.
- */
-static inline void account_group_system_time(struct task_struct *tsk,
- cputime_t cputime)
-{
- struct task_cputime *times;
- struct signal_struct *sig;
-
- /* tsk == current, ensure it is safe to use ->signal */
- if (unlikely(tsk->exit_state))
- return;
-
- sig = tsk->signal;
- times = &sig->cputime.totals;
-
- spin_lock(&times->lock);
- times->stime = cputime_add(times->stime, cputime);
- spin_unlock(&times->lock);
-}
-
-/**
- * account_group_exec_runtime - Maintain exec runtime for a thread group.
- *
- * @tsk: Pointer to task structure.
- * @ns: Time value by which to increment the sum_exec_runtime field
- * of the thread_group_cputime structure.
- *
- * If thread group time is being maintained, get the structure for the
- * running CPU and update the sum_exec_runtime field there.
- */
-static inline void account_group_exec_runtime(struct task_struct *tsk,
- unsigned long long ns)
-{
- struct task_cputime *times;
- struct signal_struct *sig;
-
- sig = tsk->signal;
- /* see __exit_signal()->task_rq_unlock_wait() */
- barrier();
- if (unlikely(!sig))
- return;
-
- times = &sig->cputime.totals;
-
- spin_lock(&times->lock);
- times->sum_exec_runtime += ns;
- spin_unlock(&times->lock);
-}



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