Re: [BUG] mm, thp: khugepaged can't allocate on requested node when confined to a cpuset

[Alex Thorlton]

On Tue, Oct 21, 2014 at 12:55:42PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 10, 2014 at 01:56:20PM -0500, Alex Thorlton wrote:
> > On Fri, Oct 10, 2014 at 11:20:52AM +0200, Peter Zijlstra wrote:
> > > So for the numa thing we do everything from the affected tasks context.
> > > There was a lot of arguments early on that that could never really work,
> > > but here we are.
> > >
> > > Should we convert khugepaged to the same? Drive the whole thing from
> > > task_work? That would make this issue naturally go away.
> > 
> > That seems like a reasonable idea to me, but that will change the way
> > that the compaction scans work right now, by quite a bit.  As I'm sure
> > you're aware, the way it works now is we tack our mm onto the
> > khugepagd_scan list in do_huge_pmd_anonymous_page (there might be some
> > other ways to get on there - I can't remember), then when khugepaged
> > wakes up it scans through each mm on the list until it hits the maximum
> > number of pages to scan on each pass.
> > 
> > If we move the compaction scan over to a task_work style function, we'll
> > only be able to scan the one task's mm at a time.  While the underlying
> > compaction infrastructure can function more or less the same, the timing
> > of when these scans occur, and exactly what the scans cover, will have
> > to change.  If we go for the most rudimentary approach, the scans will
> > occur each time a thread is about to return to userland after faulting
> > in a THP (we'll just replace the khugepaged_enter call with a
> > task_work_add), and will cover the mm for the current task.  A slightly
> > more advanced approach would involve a timer to ensure that scans don't
> > occur too often, as is currently handled by
> > khugepaged_scan_sleep_millisecs. In any case, I don't see a way around
> > the fact that we'll lose the multi-mm scanning functionality our
> > khugepaged_scan list provides, but maybe that's not a huge issue.
> 
> Right, can't see that being a problem, in fact its a bonus, because
> we'll stop scanning completely if there's no tasks running what so ever,
> so we get the power aware thing for free.

Good point.  Another plus to runninng the scans from the current task's
context.

> Also if you drive it like the numa scanning, you end up scanning tasks
> proportional to their runtime, there's no point scanning and collapsing
> pages for tasks than hardly ever run anyhow, that only sucks limited
> resources (large page allocations) for no win.

Yep - it's currently driven somewhat similar to numa scanning, but there
might be a bit more overhead to the way I'm doing it now vs. the numa
scans.  What currently happens is __khugepaged_enter makes a call to
task_work_add in the places where it used to add the current tasks mm to
the khugepaged_scan list/mm_slot hash, throttled by a timer similar to
alloc_sleep_millisecs.  The reason I say there may be more overhead is
because we potentially make the call to __khugepaged_enter and check the
timer once per page fault, which could definitely slow things down.

> > Before I run off and start writing patches, here's a brief summary of
> > what I think we could do here:
> > 
> > 1) Dissolve the khugepaged thread and related structs/timers (I'm
> >    expecting some backlash on this one).
> > 2) Replace khugepged_enter calls with calls to task_work_add(work,
> >    our_new_scan_function) - new scan function will look almost exactly
> >    like khugepaged_scan_mm_slot.
> > 3) Set up a timer similar to khugepaged_scan_sleep_millisecs that gets
> >    checked during/before our_new_scan_function to ensure that we're not
> >    scanning more often than necessary.  Also, set up progress markers to
> >    limit the number of pages scanned in a single pass.
> > 
> > By doing this, scans will get triggered each time a thread that has
> > faulted THPs is about to return to userland execution, throttled by our
> > new timer/progress indicators.  The major benefit here is that scans
> > will now occur in the desired task's context.
> > 
> > Let me know if you anybody sees any major flaws in this approach.
> 
> I would suggest you have a look at task_tick_numa(), maybe we can make
> something that works for both.

I originally looked at task_tick_numa when I started working on my new
code, but I think what's happening in there is fundamentally different
from what we're trying to achieve here.  In
task_tick_numa/task_numa_work, we're marking pages as inaccessible so
that they trigger a minor fault later on, i.e. generating some
information that will result in an action later on.  Whereas, from the
page collapse point of view, our task_work function is taking an action
based on some information that was generated during a page fault.  Sort
of an opposite action, right?  Not saying that there isn't a way that
they can work together, but the end goals of the two processes are
different enough that I wonder if it's appropriate to try to merge them
together.

I've got a rough draft of this code up and running now.  I'm gathering
some information on how it performs vs. the standard khugepaged.  Once
I've got all the information put together I'll post the code and the
test results so that we can compare, and start looking into areas that
can be improved.

Thanks, Peter!

- Alex
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