sched/fair: Fix cfs_rq avg tracking underflow

As per commit:

  b7fa30c9cc48 ("sched/fair: Fix post_init_entity_util_avg() serialization")

> the code generated from update_cfs_rq_load_avg():
>
> 	if (atomic_long_read(&cfs_rq->removed_load_avg)) {
> 		s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
> 		sa->load_avg = max_t(long, sa->load_avg - r, 0);
> 		sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0);
> 		removed_load = 1;
> 	}
>
> turns into:
>
> ffffffff81087064:       49 8b 85 98 00 00 00    mov    0x98(%r13),%rax
> ffffffff8108706b:       48 85 c0                test   %rax,%rax
> ffffffff8108706e:       74 40                   je     ffffffff810870b0 <update_blocked_averages+0xc0>
> ffffffff81087070:       4c 89 f8                mov    %r15,%rax
> ffffffff81087073:       49 87 85 98 00 00 00    xchg   %rax,0x98(%r13)
> ffffffff8108707a:       49 29 45 70             sub    %rax,0x70(%r13)
> ffffffff8108707e:       4c 89 f9                mov    %r15,%rcx
> ffffffff81087081:       bb 01 00 00 00          mov    $0x1,%ebx
> ffffffff81087086:       49 83 7d 70 00          cmpq   $0x0,0x70(%r13)
> ffffffff8108708b:       49 0f 49 4d 70          cmovns 0x70(%r13),%rcx
>
> Which you'll note ends up with sa->load_avg -= r in memory at
> ffffffff8108707a.

So I _should_ have looked at other unserialized users of ->load_avg,
but alas. Luckily nikbor reported a similar /0 from task_h_load() which
instantly triggered recollection of this here problem.

Aside from the intermediate value hitting memory and causing problems,
there's another problem: the underflow detection relies on the signed
bit. This reduces the effective width of the variables, IOW its
effectively the same as having these variables be of signed type.

This patch changes to a different means of unsigned underflow
detection to not rely on the signed bit. This allows the variables to
use the 'full' unsigned range. And it does so with explicit LOAD -
STORE to ensure any intermediate value will never be visible in
memory, allowing these unserialized loads.

Note: GCC generates crap code for this, might warrant a look later.

Note2: I say 'full' above, if we end up at U*_MAX we'll still explode;
       maybe we should do clamping on add too.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yuyang Du <yuyang.du@intel.com>
Cc: bsegall@google.com
Cc: kernel@kyup.com
Cc: morten.rasmussen@arm.com
Cc: pjt@google.com
Cc: steve.muckle@linaro.org
Fixes: 9d89c257dfb9 ("sched/fair: Rewrite runnable load and utilization average tracking")
Link: http://lkml.kernel.org/r/20160617091948.GJ30927@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 8974189222159154c55f24ddad33e3613960521a)

Conflicts:

	kernel/sched/fair.c

    Orabug: 27026563

Signed-off-by: Gayatri Vasudevan <gayatri.vasudevan@oracle.com>
Reviewed-by: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d826ff7ef3491aeee99c1ee04e8e6f82361e78ee..daca92d9cbb5243ffc6eb09e651bac23a1bfd5e5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2643,6 +2643,23 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq, int force) {}
 
 static inline u64 cfs_rq_clock_task(struct cfs_rq *cfs_rq);
 
+/*
+ * Unsigned subtract and clamp on underflow.
+ *
+ * Explicitly do a load-store to ensure the intermediate value never hits
+ * memory. This allows lockless observations without ever seeing the negative
+ * values.
+ */
+#define sub_positive(_ptr, _val) do {                          \
+       typeof(_ptr) ptr = (_ptr);                              \
+       typeof(*ptr) val = (_val);                              \
+       typeof(*ptr) res, var = READ_ONCE(*ptr);                \
+       res = var - val;                                        \
+       if (res > var)                                          \
+               res = 0;                                        \
+       WRITE_ONCE(*ptr, res);                                  \
+} while (0)
+
 /* Group cfs_rq's load_avg is used for task_h_load and update_cfs_share */
 static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
 {
@@ -2651,15 +2668,15 @@ static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
 
        if (atomic_long_read(&cfs_rq->removed_load_avg)) {
                long r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
-               sa->load_avg = max_t(long, sa->load_avg - r, 0);
-               sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0);
+               sub_positive(&sa->load_avg, r);
+               sub_positive(&sa->load_sum, r * LOAD_AVG_MAX);
        }
 
        if (atomic_long_read(&cfs_rq->removed_util_avg)) {
                long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0);
-               sa->util_avg = max_t(long, sa->util_avg - r, 0);
-               sa->util_sum = max_t(s32, sa->util_sum -
-                       ((r * LOAD_AVG_MAX) >> SCHED_LOAD_SHIFT), 0);
+               sub_positive(&sa->util_avg, r);
+               sub_positive(&sa->util_sum,
+                   ((r * LOAD_AVG_MAX) >> SCHED_LOAD_SHIFT));
        }
 
        decayed = __update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
@@ -8081,14 +8098,10 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
        __update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq), &se->avg,
                se->on_rq * scale_load_down(se->load.weight), cfs_rq->curr == se, NULL);
 
-       cfs_rq->avg.load_avg =
-               max_t(long, cfs_rq->avg.load_avg - se->avg.load_avg, 0);
-       cfs_rq->avg.load_sum =
-               max_t(s64, cfs_rq->avg.load_sum - se->avg.load_sum, 0);
-       cfs_rq->avg.util_avg =
-               max_t(long, cfs_rq->avg.util_avg - se->avg.util_avg, 0);
-       cfs_rq->avg.util_sum =
-               max_t(s32, cfs_rq->avg.util_sum - se->avg.util_sum, 0);
+       sub_positive(&cfs_rq->avg.load_avg, se->avg.load_avg);
+       sub_positive(&cfs_rq->avg.load_sum, se->avg.load_sum);
+       sub_positive(&cfs_rq->avg.util_avg, se->avg.util_avg);
+       sub_positive(&cfs_rq->avg.util_sum, se->avg.util_sum);
 #endif
 }