Re: [RFC] Hugepage collapse in process context

[Vlastimil Babka <vbabka@suse.cz>]

On 2/17/21 9:21 AM, Michal Hocko wrote:
> [Cc linux-api]
> 
> On Tue 16-02-21 20:24:16, David Rientjes wrote:
>> 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.
> 
>> 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.

>> 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.