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Subject[PATCH v2 2/3] time,signal: protect resource use statistics with seqlock
On Sat, 16 Aug 2014 16:11:59 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> Rik, I do not understand why did you silently ignore my comments about
> this change twice ;)

Here is the version I have actually been running my latest
tests with. This one addresses the forward progress issue
you pointed out.

---8<---

Subject: time,signal: protect resource use statistics with seqlock

Both times() and clock_gettime(CLOCK_PROCESS_CPUTIME_ID) have scalability
issues on large systems, due to both functions being serialized with a
lock.

The lock protects against reporting a wrong value, due to a thread in the
task group exiting, its statistics reporting up to the signal struct, and
that exited task's statistics being counted twice (or not at all).

Protecting that with a lock results in times and clock_gettime being
completely serialized on large systems.

This can be fixed by using a seqlock around the events that gather and
propagate statistics. As an additional benefit, the protection code can
be moved into thread_group_cputime, slightly simplifying the calling
functions.

In the case of posix_cpu_clock_get_task things can be simplified a
lot, because the calling function already ensures tsk sticks around,
and the rest is now taken care of in thread_group_cputime.

This way the statistics reporting code can run lockless.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
include/linux/sched.h | 1 +
kernel/exit.c | 4 ++++
kernel/fork.c | 1 +
kernel/sched/cputime.c | 33 ++++++++++++++++++++-------------
kernel/sys.c | 2 --
kernel/time/posix-cpu-timers.c | 14 --------------
6 files changed, 26 insertions(+), 29 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 857ba40..91f9209 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -646,6 +646,7 @@ struct signal_struct {
* Live threads maintain their own counters and add to these
* in __exit_signal, except for the group leader.
*/
+ seqlock_t stats_lock;
cputime_t utime, stime, cutime, cstime;
cputime_t gtime;
cputime_t cgtime;
diff --git a/kernel/exit.c b/kernel/exit.c
index b93d46d..fa09b86 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -127,6 +127,7 @@ static void __exit_signal(struct task_struct *tsk)
* the signal_struct.
*/
task_cputime(tsk, &utime, &stime);
+ write_seqlock(&sig->stats_lock);
sig->utime += utime;
sig->stime += stime;
sig->gtime += task_gtime(tsk);
@@ -140,6 +141,7 @@ static void __exit_signal(struct task_struct *tsk)
sig->sum_sched_runtime += tsk->se.sum_exec_runtime;
sig->nr_threads--;
__unhash_process(tsk, group_dead);
+ write_sequnlock(&sig->stats_lock);

/*
* Do this under ->siglock, we can race with another thread
@@ -1042,6 +1044,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
spin_lock_irq(&p->real_parent->sighand->siglock);
psig = p->real_parent->signal;
sig = p->signal;
+ write_seqlock(&psig->stats_lock);
psig->cutime += tgutime + sig->cutime;
psig->cstime += tgstime + sig->cstime;
psig->cgtime += task_gtime(p) + sig->gtime + sig->cgtime;
@@ -1064,6 +1067,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
psig->cmaxrss = maxrss;
task_io_accounting_add(&psig->ioac, &p->ioac);
task_io_accounting_add(&psig->ioac, &sig->ioac);
+ write_sequnlock(&psig->stats_lock);
spin_unlock_irq(&p->real_parent->sighand->siglock);
}

diff --git a/kernel/fork.c b/kernel/fork.c
index 1380d8a..5d7cf2b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1068,6 +1068,7 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
sig->curr_target = tsk;
init_sigpending(&sig->shared_pending);
INIT_LIST_HEAD(&sig->posix_timers);
+ seqlock_init(&sig->stats_lock);

hrtimer_init(&sig->real_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
sig->real_timer.function = it_real_fn;
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 3e52836..49b7cfe 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -288,18 +288,28 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
struct signal_struct *sig = tsk->signal;
cputime_t utime, stime;
struct task_struct *t;
-
- times->utime = sig->utime;
- times->stime = sig->stime;
- times->sum_exec_runtime = sig->sum_sched_runtime;
+ unsigned int seq, nextseq;

rcu_read_lock();
- for_each_thread(tsk, t) {
- task_cputime(t, &utime, &stime);
- times->utime += utime;
- times->stime += stime;
- times->sum_exec_runtime += task_sched_runtime(t);
- }
+ /* Attempt a lockless read on the first round. */
+ nextseq = 0;
+ do {
+ seq = nextseq;
+ read_seqbegin_or_lock(&sig->stats_lock, &seq);
+ times->utime = sig->utime;
+ times->stime = sig->stime;
+ times->sum_exec_runtime = sig->sum_sched_runtime;
+
+ for_each_thread(tsk, t) {
+ task_cputime(t, &utime, &stime);
+ times->utime += utime;
+ times->stime += stime;
+ times->sum_exec_runtime += task_sched_runtime(t);
+ }
+ /* If lockless access failed, take the lock. */
+ nextseq = 1;
+ } while (need_seqretry(&sig->stats_lock, seq));
+ done_seqretry(&sig->stats_lock, seq);
rcu_read_unlock();
}

@@ -611,9 +621,6 @@ void task_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime_t *st)
cputime_adjust(&cputime, &p->prev_cputime, ut, st);
}

-/*
- * Must be called with siglock held.
- */
void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime_t *st)
{
struct task_cputime cputime;
diff --git a/kernel/sys.c b/kernel/sys.c
index ce81291..b663664 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -862,11 +862,9 @@ void do_sys_times(struct tms *tms)
{
cputime_t tgutime, tgstime, cutime, cstime;

- spin_lock_irq(&current->sighand->siglock);
thread_group_cputime_adjusted(current, &tgutime, &tgstime);
cutime = current->signal->cutime;
cstime = current->signal->cstime;
- spin_unlock_irq(&current->sighand->siglock);
tms->tms_utime = cputime_to_clock_t(tgutime);
tms->tms_stime = cputime_to_clock_t(tgstime);
tms->tms_cutime = cputime_to_clock_t(cutime);
diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 3b89464..492b986 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -272,22 +272,8 @@ static int posix_cpu_clock_get_task(struct task_struct *tsk,
if (same_thread_group(tsk, current))
err = cpu_clock_sample(which_clock, tsk, &rtn);
} else {
- unsigned long flags;
- struct sighand_struct *sighand;
-
- /*
- * while_each_thread() is not yet entirely RCU safe,
- * keep locking the group while sampling process
- * clock for now.
- */
- sighand = lock_task_sighand(tsk, &flags);
- if (!sighand)
- return err;
-
if (tsk == current || thread_group_leader(tsk))
err = cpu_clock_sample_group(which_clock, tsk, &rtn);
-
- unlock_task_sighand(tsk, &flags);
}

if (!err)

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