Re: [PATCH 4/8] mm, page_alloc: restructure direct compaction handling in slowpath

[Vlastimil Babka <vbabka@suse.cz>]

On 07/20/2016 12:50 AM, David Rientjes wrote:
> On Mon, 18 Jul 2016, Vlastimil Babka wrote:
> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 30443804f156..a04a67745927 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3510,7 +3510,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  	struct page *page = NULL;
>>  	unsigned int alloc_flags;
>>  	unsigned long did_some_progress;
>> -	enum migrate_mode migration_mode = MIGRATE_ASYNC;
>> +	enum migrate_mode migration_mode = MIGRATE_SYNC_LIGHT;
>>  	enum compact_result compact_result;
>>  	int compaction_retries = 0;
>>  	int no_progress_loops = 0;
>> @@ -3552,6 +3552,49 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>>  	if (page)
>>  		goto got_pg;
>>  
>> +	/*
>> +	 * For costly allocations, try direct compaction first, as it's likely
>> +	 * that we have enough base pages and don't need to reclaim.
>> +	 */
>> +	if (can_direct_reclaim && order > PAGE_ALLOC_COSTLY_ORDER) {
>> +		page = __alloc_pages_direct_compact(gfp_mask, order,
>> +						alloc_flags, ac,
>> +						MIGRATE_ASYNC,
>> +						&compact_result);
>> +		if (page)
>> +			goto got_pg;
>> +
>> +		/* Checks for THP-specific high-order allocations */
>> +		if (is_thp_gfp_mask(gfp_mask)) {
>> +			/*
>> +			 * 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;
>> +
>> +			/*
>> +			 * Compaction is contended so rather back off than cause
>> +			 * excessive stalls.
>> +			 */
>> +			if (compact_result == COMPACT_CONTENDED)
>> +				goto nopage;
>> +
>> +			/*
>> +			 * It can become very expensive to allocate transparent
>> +			 * hugepages at fault, so use asynchronous memory
>> +			 * compaction for THP unless it is khugepaged trying to
>> +			 * collapse. All other requests should tolerate at
>> +			 * least light sync migration.
>> +			 */
>> +			if (!(current->flags & PF_KTHREAD))
>> +				migration_mode = MIGRATE_ASYNC;
>> +		}
>> +	}
>>  
> 
> If gfp_pfmemalloc_allowed() == true, does this try to do compaction when 
> get_page_from_freelist() would have succeeded with no watermarks?

Yes, but the compaction will return immediately with COMPACT_SKIPPED, if
we are below min watermarks. So I don't think it's worth complicating
the code to avoid this?