Re: [PATCH 3/5] sched/deadline: Fix wrong cpudl_find() in check_preempt_equal_dl()

Xunlei Pang
2015-01-28T21:27:56Z

Hi Peter,

On 28 January 2015 at 00:47, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Jan 19, 2015 at 04:49:38AM +0000, Xunlei Pang wrote:
>> In check_preempt_equal_dl(), cpudl_find() is called with a NULL
>> later_mask, thus cpudl_find() here doesn't check cpudl::free_cpus
>> at all.
>>
>> This patch takles this issue by always passing a non-NULL later_mask
>> to cpudl_find(), thereby fixing this issue.
>
> Fix what issue? Afaict this is an optimization not a fix.

Currently, check_preempt_equal_dl() invokes cpudl_find() with a NULL mask,
so cpudl_find() won't check cpudl::free_cpus. For example, 4 core
system, CPU0~CPU2
are all idle(free of deadline tasks), a deadline task is woken on CPU3
which already
has one running deadline task with the same deadline value, then
cpudl_find() will fail
causing CPU3 holding 2 deadline tasks while other cpus are idle,
obviously it should
be placed on one idle cpu.

Thanks,
Xunlei