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Date: Thu, 5 Feb 2015 15:25:27 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Zefan Li <lizefan@huawei.com>
Cc: Ingo Molnar <mingo@kernel.org>, Mike Galbraith <umgwanakikbuti@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Stefan Bader <stefan.bader@canonical.com>
Subject: Re: [PATCH] sched, autogroup: Fix failure when writing to
 cpu.rt_runtime_us
Message-ID: <20150205142527.GI5029@twins.programming.kicks-ass.net>
References: <54D32AD4.1060003@huawei.com>
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On Thu, Feb 05, 2015 at 04:33:24PM +0800, Zefan Li wrote:
> This is how to reproduce the bug:
> 
> int main() {
>     struct sched_param param = {.sched_priority=1};
> 
>     if (fork() > 0)
>         exit(0);
> 
>     setsid();
> 
>     if (sched_setscheduler(0, SCHED_RR, &param) < 0){
>         perror("failed to sched_setscheduler()");
>         return -1;
>     }
> 
>     while(1)
>         ;
> }
> 
>   # ./test
>   # mount -t cgroup -o cpu xxx /cgroup
>   # cat /cgroup/cpu.rt_runtime_us
>   950000
>   # echo 940000 > /cgroup/cpu.rt_runtime_us
>   Device or Resource busy

That's -EBUSY, but you're changing an -EPERM condition. Neither your
patch nor explanation of the matter make sense.