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

[Alex Thorlton]

Please ignore!  Screwed up my git send-email...  Sending the proper
version here in a bit...

On Wed, Oct 22, 2014 at 09:35:02PM -0500, Alex Thorlton wrote:
> Hey everyone,
> 
> 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.
> 
> This initial patch is fully functional, but there are quite a few areas that
> will need to be polished up before it's ready to be considered for a merge.  I
> wanted to get this initial version out with some basic test results quickly, so
> that people can give their opinions and let me know if there's anything they'd
> like to see done differently (and there probably is :).  I'll give details on
> the code in the individual patches.
> 
> I gathered some pretty rudimentary test data using a 48-thread NAMD simulation
> pinned to a cpuset with 8 cpus and about 60g of memory.  I'm checking to see if
> I'm allowed to publish the input data so that others can replicate the test.  In
> the meantime, if somebody knows of a publicly available benchmark that stresses
> khugepaged, that would be helpful.
> 
> The only data point I gathered was the number of pages collapsed, sampled every
> ten seconds, for the lifetime of the job.  This one statistic gives a pretty
> decent illustration of the difference in behavior between the two kernels, but I
> intend to add some other counters to measure fully completed scans, failed
> allocations, and possibly scans skipped due to timer constraints.
> 
> The data for the standard kernel (with a very small patch to add the stat
> counter that I used to the task_struct) is available here:
> 
> http://oss.sgi.com/projects/memtests/pgcollapse/output-khpd
> 
> This was a fairly recent kernel (last Tuesday).  Commit ID:
> 2d65a9f48fcdf7866aab6457bc707ca233e0c791.  I'll send the patches I used for that
> kernel as a reply to this message shortly.
> 
> The output from the modified kernel is stored here:
> 
> http://oss.sgi.com/projects/memtests/pgcollapse/output-pgcollapse
> 
> The output is stored in a pretty dumb format (*really* wide).  Best viewed in a
> simple text editor with word wrap off, just fyi.
> 
> Quick summary of what I found:  Both kernels performed about the same when it
> comes to overall runtime, my kernel was 22 seconds faster with a total runtime
> of 4:13:07.  Not a significant difference, but important to note that there was
> no apparent performance degradation.  The most interesting result is that my
> kernel completed the majority of the necessary page collapses for this job in
> 2:04, whereas the mainline kernel took 29:05 to get to the same point.
> 
> Let me know what you think.  Any suggestions are appreciated!
> 
> - Alex
> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Bob Liu <lliubbo@xxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> 
> Alex Thorlton (4):
>   Disable khugepaged thread
>   Add pgcollapse controls to task_struct
>   Convert khugepaged scan functions to work with task_work
>   Add /proc files to expose per-mm pgcollapse stats
> 
>  fs/proc/base.c             |  23 +++++++
>  include/linux/khugepaged.h |  10 ++-
>  include/linux/sched.h      |  16 +++++
>  kernel/fork.c              |   7 ++
>  kernel/sched/fair.c        |  18 +++++
>  mm/huge_memory.c           | 162 +++++++++++++++------------------------------
>  6 files changed, 123 insertions(+), 113 deletions(-)
> 
> -- 
> 1.7.12.4
> 
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