Re: [PATCH 0/4] Convert khugepaged to a task_work function

[Alex Thorlton]

On Mon, Nov 10, 2014 at 11:03:57AM +0000, Mel Gorman wrote:
> On Tue, Oct 28, 2014 at 08:58:42AM -0400, Rik van Riel wrote:
> > On 10/28/2014 08:12 AM, Andi Kleen wrote:
> > > Alex Thorlton <athorlton@xxxxxxx> writes:
> > > 
> > >> Last week, while discussing possible fixes for some unexpected/unwanted behavior
> > >> from khugepaged (see: https://lkml.org/lkml/2014/10/8/515) several people
> > >> mentioned possibly changing changing khugepaged to work as a task_work function
> > >> instead of a kernel thread.  This will give us finer grained control over the
> > >> page collapse scans, eliminate some unnecessary scans since tasks that are
> > >> relatively inactive will not be scanned often, and eliminate the unwanted
> > >> behavior described in the email thread I mentioned.
> > > 
> > > With your change, what would happen in a single threaded case?
> > > 
> > > Previously one core would scan and another would run the workload.
> > > With your change both scanning and running would be on the same
> > > core.
> > > 
> > > Would seem like a step backwards to me.
> > 
> 
> Only in the single-threaded, one process for the whole system case.
> khugepaged can only scan one address space at a time and if processes
> fail to allocate a huge page on fault then they must wait until
> khugepaged gets to scan them. The wait time is not unbounded, but it
> could be considerable.
> 
> As pointed out elsewhere, scanning from task-work context allows the
> scan rate to adapt due to different inputs -- runtime on CPU probably
> being the most relevant. Another scan factor could be NUMA sharing within
> THP-boundaries in which case we don't want to either collapse or continue
> scanning at the same rate.
> 
> > It's not just scanning, either.
> > 
> > Memory compaction can spend a lot of time waiting on
> > locks. Not consuming CPU or anything, but just waiting.
> > 
> 
> I did not pick apart the implementation closely as it's still RFC but
> there is no requirement for the reclaim/compaction to take place from
> task work context. That would likely cause user-visible stalls in any
> number of situations can trigger bug reports.

Not much to pick apart, really.  Basically just turns off khugepaged and
calls into the mm scan code from a task work function.  I didn't
consider the problems with possibly hitting compaction code.

> One possibility would be to try allocate a THP GFP_ATOMIC from task_work
> context and only start the scan if that allocation succeeds. Scan the
> address space for a THP to collapse. If a collapse target it found and
> the allocated THP is on the correct node then great -- use it. If not,
> the first page should be freed and a second GFP_ATOMIC allocation
> attempt made. 

I like this idea, especially because it wouldn't be all that complicated
to implement.  I'll take a look at doing something like this.

> If a THP allocation fails then wake we need something to try allocate the
> page on the processes behalf. khugepaged could be repurposed to do the
> reclaim/compaction step or kswapd could be woken up.

I think keeping khugepaged around to do the reclaim/compaction stuff
sounds like a good idea as well.  khugepaged could continue to scan in
the same way that it does now, but the only work it would do would be to
handle compaction/reclaim.  I don't know that code path very well, so
I'm not really sure if that makes a lot of sense, but I'll look into it.

> > I am not convinced that moving all that waiting to task
> > context is a good idea.
> > 
> 
> It allows the scanning of page tables to be parallelised, moves the
> work into the task context where it can be accounted for and the scan
> rate can be adapted to prevent useless work. I think those are desirable
> characteristics although there is no data on the expected gains of doing
> something like this. It's the proper deferral of THP allocations that is
> likely to cause the most headaches.

I might have missed something in the code, but isn't it possible to run
into compaction if a high-order allocation fails from task context
already?  It would likely be rare to hit, but I don't see anything that
prevents it.  That being said, the way the scans work in this code,
I'm adding in an extra opportunity to run into compaction code, which
probably isn't desirable.

- Alex
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