--- /dev/null
+From 8974189222159154c55f24ddad33e3613960521a Mon Sep 17 00:00:00 2001
+From: Peter Zijlstra <peterz@infradead.org>
+Date: Thu, 16 Jun 2016 10:50:40 +0200
+Subject: sched/fair: Fix cfs_rq avg tracking underflow
+
+From: Peter Zijlstra <peterz@infradead.org>
+
+commit 8974189222159154c55f24ddad33e3613960521a upstream.
+
+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>
+Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+
+
+---
+ kernel/sched/fair.c | 33 +++++++++++++++++++++++++--------
+ 1 file changed, 25 insertions(+), 8 deletions(-)
+
+--- a/kernel/sched/fair.c
++++ b/kernel/sched/fair.c
+@@ -2821,6 +2821,23 @@ static inline void update_tg_load_avg(st
+
+ 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)
+ {
+@@ -2829,15 +2846,15 @@ static inline int 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);
++ sub_positive(&sa->load_avg, r);
++ sub_positive(&sa->load_sum, r * LOAD_AVG_MAX);
+ removed = 1;
+ }
+
+ 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, 0);
++ sub_positive(&sa->util_avg, r);
++ sub_positive(&sa->util_sum, r * LOAD_AVG_MAX);
+ }
+
+ decayed = __update_load_avg(now, cpu_of(rq_of(cfs_rq)), sa,
+@@ -2927,10 +2944,10 @@ static void detach_entity_load_avg(struc
+ &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);
+ }
+
+ /* Add the load generated by se into cfs_rq's load average */
projects / thirdparty / kernel / stable-queue.git / commitdiff
