Re: Bisected: s2disk (uswsusp only) hangs just before poweroff

Alan Jenkins
2009-12-02T11:49:46Z

Rafael J. Wysocki wrote:
> On Tuesday 01 December 2009, Mel Gorman wrote:
>
>> On Tue, Dec 01, 2009 at 07:59:40PM +0000, Alan Jenkins wrote:
>>
>>> Hi
>>>
>>> Suspend to disk is (sometimes) hanging for me in 2.6.32-rc. I finally
>>> got around to bisecting it, which blamed the following commit by Mel:
>>>
>>> 5f8dcc2 "page-allocator: split per-cpu list into one-list-per-migrate-type"
>>>
>>> I was able to confirm this by reverting the commit, which fixed the
>>> hang. I had to revert one other commit first to avoid a conflict:
>>>
>>> a6f9edd "page-allocator: maintain rolling count of pages to free from
>>> the PCP"
>>>
>>>
>> Which RC kernel? Specifically, are the commits
>>
>> cc4a6851466039a8a688c843962a05689059ff3b always wake kswapd when restarting an allocation attempt
>> 9d0ed60fe9cd1fbf57f755cd27a23ae9114d7210 Do not allow interrupts to use ALLOC_HARDER
>>
>> applied?
>>
>> The latter one in particular might make a difference if s2disk is
>> pushing the system far below the watermarks. I don't suppose you know
>> where it's hanging? i.e. is it hanging in the allocator itself?
>>
>> If those patches are applied, then one difference that 5f8dcc2 makes is
>> that pages on the PCP lists but not of the right migratetype are not
>> used. Prior to that commit, an allocation might succeed even if the
>> buddy lists were empty because one of the other PCP page types would be
>> used.
>>
>>
>>> -- detail --
>>>
>>> When I suspend my EeePc 701 to disk, it sometimes hangs after writing
>>> out the hibernation image. The system is still able to resume from this
>>> image (after working around the hang by pressing the power button).
>>>
>>> This is specific to s2disk from the uswsusp package (which is now
>>> installed by default on debian unstable). It doesn't happen if I
>>> uninstall uswsusp and use the in-kernel suspend instead.
>>>
>>>
>> This leads me to believe that uswsusp is able to push available pages
>> far below what is expected. It's a total guess though, I have no idea
>> how uswsusp is implemented or how it differs from what is in kernel.
>>
>
> It doesn't differ at all in that respect. Actually, it uses the same code, but
> the distro configuration may be such that it leaves fewer available pages
> than the default in-kernel hibernation.
>
> Thanks,
> Rafael
>

It seems unintuitive that lack of memory is a problem _after we've
written out the hibernation image_. The backtrace I captured shows the
hang happens within hibernation_platform_enter()...

Hmm. Doesn't the in-kernel suspend free the in-memory image before
powering off?

int hibernate(void)
...
pr_debug("PM: writing image.\n");
error = swsusp_write(flags);
swsusp_free();
if (!error)
power_down();

Would that explain why only uswsusp is affected? Do we want to fix
snapshot_read() in user.c, so that it calls swsusp_free() once all the
data has been read?

Regards
Alan