Re: [PATCH v3 17/17] mm, vmscan: make compaction_ready() more accurate and readable

[Vlastimil Babka <vbabka@suse.cz>]

On 07/06/2016 07:55 AM, Joonsoo Kim wrote:
> On Fri, Jun 24, 2016 at 11:54:37AM +0200, Vlastimil Babka wrote:
>> The compaction_ready() is used during direct reclaim for costly order
>> allocations to skip reclaim for zones where compaction should be attempted
>> instead. It's combining the standard compaction_suitable() check with its own
>> watermark check based on high watermark with extra gap, and the result is
>> confusing at best.
>>
>> This patch attempts to better structure and document the checks involved.
>> First, compaction_suitable() can determine that the allocation should either
>> succeed already, or that compaction doesn't have enough free pages to proceed.
>> The third possibility is that compaction has enough free pages, but we still
>> decide to reclaim first - unless we are already above the high watermark with
>> gap.  This does not mean that the reclaim will actually reach this watermark
>> during single attempt, this is rather an over-reclaim protection. So document
>> the code as such. The check for compaction_deferred() is removed completely, as
>> it in fact had no proper role here.
>>
>> The result after this patch is mainly a less confusing code. We also skip some
>> over-reclaim in cases where the allocation should already succed.
>>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> Acked-by: Michal Hocko <mhocko@suse.com>
>> ---
>>  mm/vmscan.c | 43 ++++++++++++++++++++-----------------------
>>  1 file changed, 20 insertions(+), 23 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 484ff05d5a8f..724131661f0c 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -2462,40 +2462,37 @@ static bool shrink_zone(struct zone *zone, struct scan_control *sc,
>>  }
>>
>>  /*
>> - * Returns true if compaction should go ahead for a high-order request, or
>> - * the high-order allocation would succeed without compaction.
>> + * Returns true if compaction should go ahead for a costly-order request, or
>> + * the allocation would already succeed without compaction. Return false if we
>> + * should reclaim first.
>>   */
>>  static inline bool compaction_ready(struct zone *zone, int order, int classzone_idx)
>>  {
>>  	unsigned long balance_gap, watermark;
>> -	bool watermark_ok;
>> +	enum compact_result suitable;
>> +
>> +	suitable = compaction_suitable(zone, order, 0, classzone_idx);
>> +	if (suitable == COMPACT_PARTIAL)
>> +		/* Allocation should succeed already. Don't reclaim. */
>> +		return true;
>> +	if (suitable == COMPACT_SKIPPED)
>> +		/* Compaction cannot yet proceed. Do reclaim. */
>> +		return false;
>>
>>  	/*
>> -	 * Compaction takes time to run and there are potentially other
>> -	 * callers using the pages just freed. Continue reclaiming until
>> -	 * there is a buffer of free pages available to give compaction
>> -	 * a reasonable chance of completing and allocating the page
>> +	 * Compaction is already possible, but it takes time to run and there
>> +	 * are potentially other callers using the pages just freed. So proceed
>> +	 * with reclaim to make a buffer of free pages available to give
>> +	 * compaction a reasonable chance of completing and allocating the page.
>> +	 * Note that we won't actually reclaim the whole buffer in one attempt
>> +	 * as the target watermark in should_continue_reclaim() is lower. But if
>> +	 * we are already above the high+gap watermark, don't reclaim at all.
>>  	 */
>>  	balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
>>  			zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
>>  	watermark = high_wmark_pages(zone) + balance_gap + compact_gap(order);
>> -	watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, classzone_idx);
>
> Hmm... it doesn't explain why both high_wmark_pages and balance_gap
> are needed. If we want to make a buffer, one of them would work.

Mel's series removed KSWAPD_ZONE_BALANCE_GAP_RATIO meanwhile, so that 
should be fine.

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