On 2019/4/15 23:25, Peter Zijlstra wrote:
On Mon, Apr 15, 2019 at 11:20:31PM +0800, chengjian (D) wrote:
Hi, Peter
On 2019/4/15 20:46, Peter Zijlstra wrote:
I write a demo about this, which I described it as overflow.
I'm not saying there's no overflow, I'm saying there's nothing UB about
it.
Yeah. I got it.
This may not be an undefined behavior,
but rather a bug or logic error caused by overflow.
```cpp
static long calc_group_shares(struct cfs_rq *cfs_rq)
{
// ......
shares = (tg_shares * load); // 1048576 * 9144968455305 =
-8857549630719655936 (OVERFLOW)
if (tg_weight) // assume tg_weight is 3
shares /= tg_weight; // 0xd7068dd8c2daaaab shoule be
0x2c5be32e18300000
}
```
It will cause `se->runnable_weight` to have an incorrect value in
reweight_entity().
Thanks.
CHENG Jian
