Message-ID: <585A0DBE.2090404@redhat.com>
Date:   Wed, 21 Dec 2016 13:06:06 +0800
From:   Xunlei Pang <xpang@...hat.com>
To:     Baoquan He <bhe@...hat.com>, xlpang@...hat.com
Cc:     kexec@...ts.infradead.org, akpm@...ux-foundation.org,
        zhong jiang <zhongjiang@...wei.com>, ebiederm@...ssion.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kexec: add cond_resched into
 kimage_alloc_crash_control_pages

On 12/19/2016 at 11:23 AM, Baoquan He wrote:
> On 12/09/16 at 03:16pm, Xunlei Pang wrote:
>> On 12/09/2016 at 01:13 PM, zhong jiang wrote:
>>> 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>
>>>>>
>>>>> A soft lookup will occur when I run trinity in syscall kexec_load.
>>>>> the corresponding stack information is as follows.
>>>>>
>>>>> [  237.235937] BUG: soft lockup - CPU#6 stuck for 22s! [trinity-c6:13859]
>>>>> [  237.242699] Kernel panic - not syncing: softlockup: hung tasks
>>>>> [  237.248573] CPU: 6 PID: 13859 Comm: trinity-c6 Tainted: G           O L ----V-------   3.10.0-327.28.3.35.zhongjiang.x86_64 #1
>>>>> [  237.259984] Hardware name: Huawei Technologies Co., Ltd. Tecal BH622 V2/BC01SRSA0, BIOS RMIBV386 06/30/2014
>>>>> [  237.269752]  ffffffff8187626b 0000000018cfde31 ffff88184c803e18 ffffffff81638f16
>>>>> [  237.277471]  ffff88184c803e98 ffffffff8163278f 0000000000000008 ffff88184c803ea8
>>>>> [  237.285190]  ffff88184c803e48 0000000018cfde31 ffff88184c803e67 0000000000000000
>>>>> [  237.292909] Call Trace:
>>>>> [  237.295404]  <IRQ>  [<ffffffff81638f16>] dump_stack+0x19/0x1b
>>>>> [  237.301352]  [<ffffffff8163278f>] panic+0xd8/0x214
>>>>> [  237.306196]  [<ffffffff8111d6fc>] watchdog_timer_fn+0x1cc/0x1e0
>>>>> [  237.312157]  [<ffffffff8111d530>] ? watchdog_enable+0xc0/0xc0
>>>>> [  237.317955]  [<ffffffff810aa182>] __hrtimer_run_queues+0xd2/0x260
>>>>> [  237.324087]  [<ffffffff810aa720>] hrtimer_interrupt+0xb0/0x1e0
>>>>> [  237.329963]  [<ffffffff8164ae5c>] ? call_softirq+0x1c/0x30
>>>>> [  237.335500]  [<ffffffff81049a77>] local_apic_timer_interrupt+0x37/0x60
>>>>> [  237.342228]  [<ffffffff8164bacf>] smp_apic_timer_interrupt+0x3f/0x60
>>>>> [  237.348771]  [<ffffffff8164a11d>] apic_timer_interrupt+0x6d/0x80
>>>>> [  237.354967]  <EOI>  [<ffffffff810f3a00>] ? kimage_alloc_control_pages+0x80/0x270
>>>>> [  237.362875]  [<ffffffff811c3ebe>] ? kmem_cache_alloc_trace+0x1ce/0x1f0
>>>>> [  237.369592]  [<ffffffff810f362f>] ? do_kimage_alloc_init+0x1f/0x90
>>>>> [  237.375992]  [<ffffffff810f3d1a>] kimage_alloc_init+0x12a/0x180
>>>>> [  237.382103]  [<ffffffff810f3f9a>] SyS_kexec_load+0x20a/0x260
>>>>> [  237.387957]  [<ffffffff816494c9>] system_call_fastpath+0x16/0x1b
>>>>>
>>>>> the first time allocate control pages may take too much time because
>>>>> crash_res.end can be set to a higher value. we need to add cond_resched
>>>>> to avoid the issue.
>>>>>
>>>>> The patch have been tested and above issue is not appear.
>>>>>
>>>>> Signed-off-by: zhong jiang <zhongjiang@...wei.com>
>>>>> ---
>>>>>  kernel/kexec_core.c | 2 ++
>>>>>  1 file changed, 2 insertions(+)
>>>>>
>>>>> 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....
>> How does "loop million times" happen? See my inlined comments prefixed with "pxl".
>>
>> kimage_alloc_crash_control_pages():
>>     while (hole_end <= crashk_res.end) {
>>         unsigned long i;
>>
>>         if (hole_end > KEXEC_CRASH_CONTROL_MEMORY_LIMIT)
>>             break;
>>         /* See if I overlap any of the segments */
>>         for (i = 0; i < image->nr_segments; i++) {  // pxl: max 16 loops, all existent segments are not overlapped, though may not sorted.
>>             unsigned long mstart, mend;
>>
>>             mstart = image->segment[i].mem;
>>             mend   = mstart + image->segment[i].memsz - 1;
>>             if ((hole_end >= mstart) && (hole_start <= mend)) {
>>                 /* Advance the hole to the end of the segment */
>>                 hole_start = (mend + (size - 1)) & ~(size - 1);
>>                 hole_end   = hole_start + size - 1;
>>                 break;  // pxl: If overlap was found, break for loop, @hole_end starts after the overlapped segment area, and will while loop again
>>             }
>>         }
>>         /* If I don't overlap any segments I have found my hole! */
>>         if (i == image->nr_segments) {
>>             pages = pfn_to_page(hole_start >> PAGE_SHIFT);
>>             image->control_page = hole_end;
>>             break;   // pxl: no overlap with all the segments, get the result and break the while loop. END.
>>         }   
>>     }
>>
>> So, the worst "while" loops in theory would be (image->nr_segments + 1), no?
> It's very interesting. I got the different result by mental arithmatic.

Hi Baoquan,

I meant the "while" loops excluding the "for" loops considering it's always limited to 16 each while loop.
So basically we have the same view :-)

Regards,
Xunlei

> Assume nr_segments is 16, and they are placed in continuous physical
> memory, then in the first while loop, it will failed in image->segment[0]
> and adjust hole_start and hole_end. Then it failed in 2nd while loop
> after comparing with image->segment[0] and image->segment[1]. Finally it
> will get a new position after image->segment[15] after 16 comparision in
> the 16th while loop. So the amount should be (1+2+3+3+16) which is
> (1+16)*8, 136 times.
>
> Not sure if the counting is right, I am wondering how it will loop
> millions of times even though crashk_res.end will exceed to 4G. The
> times should not be related to how much memory resreved, only the
> nr_segments, maybe I am wrong.
>
> Thanks
> Baoquan
>
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