On 2/17/21 9:21 AM, Michal Hocko wrote:

[Cc linux-api]

On Tue 16-02-21 20:24:16, David Rientjes wrote:

quoted

Hi everybody,

Khugepaged is slow by default, it scans at most 4096 pages every 10s.  
That's normally fine as a system-wide setting, but some applications would 
benefit from a more aggressive approach (as long as they are willing to 
pay for it).

Instead of adding priorities for eligible ranges of memory to khugepaged, 
temporarily speeding khugepaged up for the whole system, or sharding its 
work for memory belonging to a certain process, one approach would be to 
allow userspace to induce hugepage collapse.

The benefit to this approach would be that this is done in process context 
so its cpu is charged to the process that is inducing the collapse.  
Khugepaged is not involved.

Yes, this makes a lot of sense to me.

quoted

Idea was to allow userspace to induce hugepage collapse through the new 
process_madvise() call.  This allows us to collapse hugepages on behalf of 
current or another process for a vectored set of ranges.

Yes, madvise sounds like a good fit for the purpose.

Agreed on both points.

quoted

This could be done through a new process_madvise() mode *or* it could be a 
flag to MADV_HUGEPAGE since process_madvise() allows for a flag parameter 
to be passed.  For example, MADV_F_SYNC.

Would this MADV_F_SYNC be applicable to other madvise modes? Most
existing madvise modes do not seem to make much sense. We can argue that
MADV_PAGEOUT would guarantee the range was indeed reclaimed but I am not
sure we want to provide such a strong semantic because it can limit
future reclaim optimizations.

To me MADV_HUGEPAGE_COLLAPSE sounds like the easiest way forward.

I guess in the old madvise(2) we could create a new combo of MADV_HUGEPAGE |
MADV_WILLNEED with this semantic? But you are probably more interested in
process_madvise() anyway. There the new flag would make more sense. But there's
also David H.'s proposal for MADV_POPULATE and there might be benefit in
considering both at the same time? Should e.g. MADV_POPULATE with MADV_HUGEPAGE
have the collapse semantics? But would MADV_POPULATE be added to
process_madvise() as well? Just thinking out loud so we don't end up with more
flags than necessary, it's already confusing enough as it is.