Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression

[David Rientjes <rientjes@google.com>]

On Mon, 3 Dec 2018, Linus Torvalds wrote:

> Side note: I think maybe people should just look at that whole
> compaction logic for that block, because it doesn't make much sense to
> me:
> 
>                 /*
>                  * Checks for costly allocations with __GFP_NORETRY, which
>                  * includes THP page fault allocations
>                  */
>                 if (costly_order && (gfp_mask & __GFP_NORETRY)) {
>                         /*
>                          * If compaction is deferred for high-order allocations,
>                          * it is because sync compaction recently failed. If
>                          * this is the case and the caller requested a THP
>                          * allocation, we do not want to heavily disrupt the
>                          * system, so we fail the allocation instead of entering
>                          * direct reclaim.
>                          */
>                         if (compact_result == COMPACT_DEFERRED)
>                                 goto nopage;
> 
>                         /*
>                          * Looks like reclaim/compaction is worth trying, but
>                          * sync compaction could be very expensive, so keep
>                          * using async compaction.
>                          */
>                         compact_priority = INIT_COMPACT_PRIORITY;
>                 }
> 
> this is where David wants to add *his* odd test, and I think everybody
> looks at that added case
> 
> +                       if (order == pageblock_order &&
> +                                       !(current->flags & PF_KTHREAD))
> +                               goto nopage;
> 
> and just goes "Eww".
> 
> But I think the real problem is that it's the "goto nopage" thing that
> makes _sense_, and the current cases for "let's try compaction" that
> are the odd ones, and then David adds one new special case for the
> sensible behavior.
> 
> For example, why would COMPACT_DEFERRED mean "don't bother", but not
> all the other reasons it didn't really make sense?
> 
> So does it really make sense to fall through AT ALL to that "retry"
> case, when we explicitly already had (gfp_mask & __GFP_NORETRY)?
> 
> Maybe the real fix is to instead of adding yet another special case
> for "goto nopage", it should just be unconditional: simply don't try
> to compact large-pages if __GFP_NORETRY was set.
> 

I think what is intended, which may not be represented by the code, is 
that if compaction is not suitable (__compaction_suitable() returns 
COMPACT_SKIPPED because of failing watermarks) that for non-hugepage 
allocations reclaim may be useful.  We just want to reclaim memory so that 
memory compaction has pages available for migration targets.

Note the same caveat I keep bringing up still applies, though: if reclaim 
frees memory that is iterated over by the compaction migration scanner, it 
was pointless.  That is a memory compaction implementation detail and can 
lead to a lot of unnecessary reclaim (or even thrashing) if unmovable page 
fragmentation cause compaction to fail even after it has migrated 
everything it could.  I think the likelihood of that happening increases 
by the allocation order.