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

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Date:   Fri, 09 Dec 2016 18:19:37 +1300
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     zhong jiang <zhongjiang@...wei.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

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