On Thu, Jun 09, 2016 at 03:07:50PM +0200, Peter Zijlstra wrote:
> Which given the lack of serialization, and the code generated from
> update_cfs_rq_load_avg() is entirely possible.
>
> 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.
Surprised. I actually tweaked a little bit on this, but haven't had desirable
generated codes. Any compiler expert can shed some light on it?
> Ludicrous code generation if you ask me; I'd have expected something
> like (note, r15 holds 0):
>
> mov %r15, %rax
> xchg %rax, cfs_rq->removed_load_avg
> mov sa->load_avg, %rcx
> sub %rax, %rcx
> cmovs %r15, %rcx
> mov %rcx, sa->load_avg
>
> Adding the serialization (to _both_ call sites) should fix this.
Absolutely, both :)
I am going to remove the group entity post util initialization soon in the
flat hierarchical util implementation, it is not used anywhere.
