linux-kernel - Re: [PATCH v2] kexec: add cond_resched into kimage_alloc_crash_control

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Message-ID: <584A47A2.1040001@huawei.com>
Date:   Fri, 9 Dec 2016 13:56:50 +0800
From:   zhong jiang <zhongjiang@...wei.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     <xlpang@...hat.com>, Xunlei Pang <xpang@...hat.com>,
        <akpm@...ux-foundation.org>, <kexec@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kexec: add cond_resched into kimage_alloc_crash_control_pages

On 2016/12/9 13:19, Eric W. Biederman wrote:
> zhong jiang <zhongjiang@...wei.com> writes:
>
>> On 2016/12/8 17:41, Xunlei Pang wrote:
>>> On 12/08/2016 at 10:37 AM, zhongjiang wrote:
>>>> From: zhong jiang <zhongjiang@...wei.com>
>>>>
> [snip]
>>>> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
>>>> index 5616755..bfc9621 100644
>>>> --- a/kernel/kexec_core.c
>>>> +++ b/kernel/kexec_core.c
>>>> @@ -441,6 +441,8 @@ static struct page *kimage_alloc_crash_control_pages(struct kimage *image,
>>>>  	while (hole_end <= crashk_res.end) {
>>>>  		unsigned long i;
>>>>  
>>>> +		cond_resched();
>>>> +
>>> I can't see why it would take a long time to loop inside, the job it does is simply to find a control area
>>> not overlapped with image->segment[], you can see the loop "for (i = 0; i < image->nr_segments; i++)",
>>> @hole_end will be advanced to the end of its next nearby segment once overlap was detected each loop,
>>> also there are limited (<=16) segments, so it won't take long to locate the right area.
>>>
>>> Am I missing something?
>>>
>>> Regards,
>>> Xunlei
>>   if the crashkernel = auto is set in cmdline.  it represent crashk_res.end will exceed to 4G, the first allocate control pages will
>>   loop  million times. if we set crashk_res.end to the higher value
>>   manually,  you can image....
> Or in short the cond_resched is about keeping things reasonable when the
> loop has worst case behavior.
>
> Eric
>
>
  Yes,   Thank you reply and comment.

  Regards,
  zhongjiang