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 that the task 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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alex Thorlton <athorlton@sgi.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Daeseok Youn <daeseok.youn@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Guillaume Morin <guillaume@morinfr.org>
Cc: Ionut Alexa <ionut.m.alexa@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Michal Schmidt <mschmidt@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: umgwanakikbuti@gmail.com
Cc: fweisbec@gmail.com
Cc: srao@redhat.com
Cc: lwoodman@redhat.com
Cc: atheurer@redhat.com
Link: http://lkml.kernel.org/r/20140816134010.26a9b572@annuminas.surriel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

5 files changed, 25 insertions, 29 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index b93d46dab6fc..fa09b86609db 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 0cf9cdb6e491..9387ae8ab048 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 3e52836359ba..49b7cfe98f7a 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 ce8129192a26..b6636643cbd1 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 3b8946416a5f..492b986195d5 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)