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Message-ID: <8c8e96d0-226b-de46-d498-0d8a6947b4c9@huawei.com> Date: Mon, 15 Apr 2019 23:50:49 +0800 From: "chengjian (D)" <cj.chengjian@...wei.com> To: Peter Zijlstra <peterz@...radead.org> CC: <huawei.libin@...wei.com>, <xiexiuqi@...wei.com>, <yangyingliang@...wei.com>, <mingo@...hat.com>, <linux-kernel@...r.kernel.org>, "chengjian (D)" <cj.chengjian@...wei.com> Subject: Re: [PATCH] sched/fair: Use 'unsigned long' for group_shares,group_runnable 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
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