| 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 |
| @@ -2682,6 +2682,23 @@ 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) |
| { |
| @@ -2690,15 +2707,15 @@ 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, |
| @@ -2764,10 +2781,10 @@ 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 */ |