Re: [RFC PATCH 1/3 V1] mm, page_alloc: use __rmqueue_smallest when borrow memory from MIGRATE_CMA

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Jul 05, 2012 at 09:36:14AM +0800, Lai Jiangshan wrote:
> On 07/04/2012 07:19 PM, Mel Gorman wrote:
> > On Wed, Jul 04, 2012 at 06:43:47PM +0800, Lai Jiangshan wrote:
> >> On 07/04/2012 06:17 PM, Mel Gorman wrote:
> >>> On Wed, Jul 04, 2012 at 03:26:16PM +0800, Lai Jiangshan wrote:
> >>>> The pages of MIGRATE_CMA can't not be changed to the other type,
> >>>> nor be moved to the other free list. 
> >>>>
> >>>> ==>
> >>>> So when we use __rmqueue_fallback() to borrow memory from MIGRATE_CMA,
> >>>> one of the highest order page is borrowed and it is split.
> >>>> But the free pages resulted by splitting can NOT
> >>>> be moved to MIGRATE_MOVABLE.
> >>>>
> >>>> ==>
> >>>> So in the next time of allocation, we NEED to borrow again,
> >>>> another one of the highest order page is borrowed from CMA and it is split.
> >>>> and results some other new split free pages.
> >>>>
> >>>
> >>> Then special case __rmqueue_fallback() to move pages stolen from
> >>> MIGRATE_CMA to the MIGRATE_MOVABLE lists but do not change the pageblock
> >>> type.
> >>
> >> Because unmovable-page-requirement can allocate page from
> >> MIGRATE_MOVABLE free list. So We can not move MIGRATE_CMA pages
> >> to the MIGRATE_MOVABLE free list.
> >>
> > 
> > Ok, good point.
> > 
> >> See here:
> >>
> >> MOVABLE list is empty
> >> UNMOVABLE list is empty
> >> movable-page-requirement
> >> 	borrow from CMA list
> >> 	split it, others are put into UNMOVABLE list
> >> unmovable-page-requiremnt
> >> 	borrow from UNMOVABLE list
> >> 	NOW, it is BUG, we use CMA pages for unmovable usage.
> >>
> > 
> > The patch still looks unnecessarily complex for what you are trying to
> 
> Which is complex in my code? __rmqueue_smallest()? __rmqueue_fallback()?
> 

It's the review that was confusing. It churned a lot of code, altered the
fallback lists, added a misleading name (MIGRATE_PRIME_TYPES because PRIME
in this context does not mean anything useful), added a warning that made
no sense and stopped __rmqueue_smallest from being inlined. In cases
like this it is better to split your patch into the part you want
followed by a cleanup patch if that is necessary.

> __rmqueue_smallest() ? I think it is required.
> 
> __rmqueue_fallback()?
> It is just a cleanup for __rmqueue_fallback(), CMA is removed out from
> __rmqueue_fallback(), so we can cleanup fallback(). I will remove/split
> the cleanup part of the patch in next round.
> 
> > achieve and as a result I'm not reviewing it as carefully as I should.
> > It looks like the entire patch boiled down to this hunk here
> > 
> > +#ifdef CONFIG_CMA
> > +	if (unlikely(!page) && migratetype == MIGRATE_MOVABLE)
> > +		page = __rmqueue_smallest(zone, order, MIGRATE_CMA);
> > +#endif
> > +
> > 
> > With that in place, this would would need to change from
> > 
> > [MIGRATE_MOVABLE]     = { MIGRATE_CMA, MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
> > 
> > to
> > 
> > [MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
> > 
> > because the fallback is already being handled as a special case. Leave
> > the other fallback logic as it is.
> > 
> > This is not tested at all and is only meant to illustrate why I think
> > your patch looks excessively complex for what you are trying to
> > achieve.
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 4beb7ae..0063e93 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -895,11 +895,9 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
> >  static int fallbacks[MIGRATE_TYPES][4] = {
> >  	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,     MIGRATE_RESERVE },
> >  	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,     MIGRATE_RESERVE },
> > +	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE,   MIGRATE_RESERVE },
> >  #ifdef CONFIG_CMA
> > -	[MIGRATE_MOVABLE]     = { MIGRATE_CMA,         MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_RESERVE },
> >  	[MIGRATE_CMA]         = { MIGRATE_RESERVE }, /* Never used */
> > -#else
> > -	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE,   MIGRATE_RESERVE },
> >  #endif
> >  	[MIGRATE_RESERVE]     = { MIGRATE_RESERVE }, /* Never used */
> >  	[MIGRATE_ISOLATE]     = { MIGRATE_RESERVE }, /* Never used */
> > @@ -1076,6 +1074,20 @@ static struct page *__rmqueue(struct zone *zone, unsigned int order,
> >  
> >  retry_reserve:
> >  	page = __rmqueue_smallest(zone, order, migratetype);
> > +#ifdef CONFIG_CMA
> > +	if (!unlikely(!page) && migratetype == MIGRATE_MOVABLE) {
> > +
> > +		/*
> > +		 * CMA is a special case where we want to use
> > +		 * the smallest available page instead of splitting
> > +		 * the largest chunks. We still must avoid the pages
> > +		 * moving to MIGRATE_MOVABLE where they might be
> > +		 * used for UNRECLAIMABLE or UNMOVABLE allocations
> > +		 */
> > +		migratetype = MIGRATE_CMA;
> > +		goto retry_reserve;
> > +	}
> > +#endif /* CONFIG_CMA */
> >  
> >  	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
> 
> 
> need to add
> 
> +		if (migratetype == MIGRATE_CMA)
> +			migratetype = MIGRATE_MOVABLE;
> 
> to restore the migratetype for fallback.
> 

True, but it'd still be easier to review and __rmqueue_smallest would
still be inlined.

> And your code are the same as mine in the view of CPU:
> __rmqueue_smallest(MIGRATE_MOVABLE)
> if failed: __rmqueue_smallest(MIGRATE_CMA)
> if failed: __rmqueue_fallback()
> if failed: __rmqueue_smallest(MIGRATE_RESERVE)
> 
> The differences are:
> you just use "goto" instead "if" for instruction control.
> your code are longer.
> the number of branch in your code = mine + 1
> 
> My code have better readability:
> Mine:
> 
> ========================================================================
> 	page = __rmqueue_smallest(zone, order, migratetype);
> 
> #ifdef CONFIG_CMA
> 	if (unlikely(!page) && migratetype == MIGRATE_MOVABLE)
> 		page = __rmqueue_smallest(zone, order, MIGRATE_CMA);
> #endif
> 
> 	if (unlikely(!page))
> 		page = __rmqueue_fallback(zone, order, migratetype);
> 
> 	if (unlikely(!page))
> 		page = __rmqueue_smallest(zone, order, MIGRATE_RESERVE);
> 
> 	trace_mm_page_alloc_zone_locked(page, order, migratetype);
> 	return page;
> =====================================================================
> 
> Yours:
> 
> ==================================================================
> retry_reserve:
> 	page = __rmqueue_smallest(zone, order, migratetype);
> 
> #ifdef CONFIG_CMA
> 	if (!unlikely(!page) && migratetype == MIGRATE_MOVABLE) {
> 
> 		/*
> 		 * CMA is a special case where we want to use
> 		 * the smallest available page instead of splitting
> 		 * the largest chunks. We still must avoid the pages
> 		 * moving to MIGRATE_MOVABLE where they might be
> 		 * used for UNRECLAIMABLE or UNMOVABLE allocations
> 		 */
> 		migratetype = MIGRATE_CMA;
> 		goto retry_reserve;
> 	}
> #endif /* CONFIG_CMA */
> 
> 
> 	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
> 		if (migratetype == MIGRATE_CMA)
> 			migratetype = MIGRATE_MOVABLE;
> 
> 		page = __rmqueue_fallback(zone, order, migratetype);
> 
> 		/*
> 		 * Use MIGRATE_RESERVE rather than fail an allocation. goto
> 		 * is used because __rmqueue_smallest is an inline function
> 		 * and we want just one call site
> 		 */
> 		if (!page) {
> 			migratetype = MIGRATE_RESERVE;
> 			goto retry_reserve;
> 		}
> 	}
> 
> 	trace_mm_page_alloc_zone_locked(page, order, migratetype);
> 	return page;
> ==========================================================================
> 
> How about this one? (just type it in the email client)
> 
> #define RMQUEUE_FALLBACK 1024

Use -1 to be clear this is an impossible index. Add a comment explaining
that

> int rmqueue_list[3][4] = {

Use MIGRATE_PCPTYPES and MIGRATE_PCPTYPES+1 to size the array.

> 	[MIGRATE_UNMOVABLE] = { MIGRATE_UNMOVABLE, RMQUEUE_FALLBACK, MIGRATE_RESERVE},
> 	[MIGRATE_RECLAIMABLE] = { MIGRATE_RECLAIMABLE, RMQUEUE_FALLBACK, MIGRATE_RESERVE},
> 	[MIGRATE_MOVABLE] = {MIGRATE_MOVABLE, MIGRATE_CMA, RMQUEUE_FALLBACK, MIGRATE_RESERVE},
> }
> 
> static struct page *__rmqueue(struct zone *zone, unsigned int order,
> 						int migratetype)
> {
> 	struct page *page;
> 	int i, mt;
> 
> 	for (i = 0; ; i++) {
> 		mt = rmqueue_list[migratetype][i];
> 		if (likely(mt != RMQUEUE_FALLBACK)
> 			page = __rmqueue_smallest(zone, order, mt);
> 		else
> 			page = __rmqueue_fallback(zone, order, migratetype);
> 
> 		/* MIGRATE_RESERVE is always the last one */
> 		if (likely(page) || (mt == MIGRATE_RESERVE))
> 			break;
> 	}
> 
> 	trace_mm_page_alloc_zone_locked(page, order, migratetype);
> 	return page;
> }

That would indeed churn the code a lot less and preserve the inlining of
__rmqueue_smallest. It comes at the cost of an additional static array
but the flow is clear at least.

-- 
Mel Gorman
SUSE Labs

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