Re: [PATCH v2 2/3] CMA: aggressively allocate the pages on cma reserved memory when not used

[Gioh Kim]

I've not understand your code fully. Please let me ask some silly questions.

2014-05-28 오후 4:04, Joonsoo Kim 쓴 글:
> CMA is introduced to provide physically contiguous pages at runtime.
> For this purpose, it reserves memory at boot time. Although it reserve
> memory, this reserved memory can be used for movable memory allocation
> request. This usecase is beneficial to the system that needs this CMA
> reserved memory infrequently and it is one of main purpose of
> introducing CMA.
> 
> But, there is a problem in current implementation. The problem is that
> it works like as just reserved memory approach. The pages on cma reserved
> memory are hardly used for movable memory allocation. This is caused by
> combination of allocation and reclaim policy.
> 
> The pages on cma reserved memory are allocated if there is no movable
> memory, that is, as fallback allocation. So the time this fallback
> allocation is started is under heavy memory pressure. Although it is under
> memory pressure, movable allocation easily succeed, since there would be
> many pages on cma reserved memory. But this is not the case for unmovable
> and reclaimable allocation, because they can't use the pages on cma
> reserved memory. These allocations regard system's free memory as
> (free pages - free cma pages) on watermark checking, that is, free
> unmovable pages + free reclaimable pages + free movable pages. Because
> we already exhausted movable pages, only free pages we have are unmovable
> and reclaimable types and this would be really small amount. So watermark
> checking would be failed. It will wake up kswapd to make enough free
> memory for unmovable and reclaimable allocation and kswapd will do.
> So before we fully utilize pages on cma reserved memory, kswapd start to
> reclaim memory and try to make free memory over the high watermark. This
> watermark checking by kswapd doesn't take care free cma pages so many
> movable pages would be reclaimed. After then, we have a lot of movable
> pages again, so fallback allocation doesn't happen again. To conclude,
> amount of free memory on meminfo which includes free CMA pages is moving
> around 512 MB if I reserve 512 MB memory for CMA.
> 
> I found this problem on following experiment.
> 
> 4 CPUs, 1024 MB, VIRTUAL MACHINE
> make -j16
> 
> CMA reserve:            0 MB            512 MB
> Elapsed-time:           225.2           472.5
> Average-MemFree:        322490 KB       630839 KB
> 
> To solve this problem, I can think following 2 possible solutions.
> 1. allocate the pages on cma reserved memory first, and if they are
>     exhausted, allocate movable pages.
> 2. interleaved allocation: try to allocate specific amounts of memory
>     from cma reserved memory and then allocate from free movable memory.
> 
> I tested #1 approach and found the problem. Although free memory on
> meminfo can move around low watermark, there is large fluctuation on free
> memory, because too many pages are reclaimed when kswapd is invoked.
> Reason for this behaviour is that successive allocated CMA pages are
> on the LRU list in that order and kswapd reclaim them in same order.
> These memory doesn't help watermark checking from kwapd, so too many
> pages are reclaimed, I guess.
> 
> So, I implement #2 approach.
> One thing I should note is that we should not change allocation target
> (movable list or cma) on each allocation attempt, since this prevent
> allocated pages to be in physically succession, so some I/O devices can
> be hurt their performance. To solve this, I keep allocation target
> in at least pageblock_nr_pages attempts and make this number reflect
> ratio, free pages without free cma pages to free cma pages. With this
> approach, system works very smoothly and fully utilize the pages on
> cma reserved memory.
> 
> Following is the experimental result of this patch.
> 
> 4 CPUs, 1024 MB, VIRTUAL MACHINE
> make -j16
> 
> <Before>
> CMA reserve:            0 MB            512 MB
> Elapsed-time:           225.2           472.5
> Average-MemFree:        322490 KB       630839 KB
> nr_free_cma:            0               131068
> pswpin:                 0               261666
> pswpout:                75              1241363
> 
> <After>
> CMA reserve:            0 MB            512 MB
> Elapsed-time:           222.7           224
> Average-MemFree:        325595 KB       393033 KB
> nr_free_cma:            0               61001
> pswpin:                 0               6
> pswpout:                44              502
> 
> There is no difference if we don't have cma reserved memory (0 MB case).
> But, with cma reserved memory (512 MB case), we fully utilize these
> reserved memory through this patch and the system behaves like as
> it doesn't reserve any memory.
> 
> With this patch, we aggressively allocate the pages on cma reserved memory
> so latency of CMA can arise. Below is the experimental result about
> latency.
> 
> 4 CPUs, 1024 MB, VIRTUAL MACHINE
> CMA reserve: 512 MB
> Backgound Workload: make -jN
> Real Workload: 8 MB CMA allocation/free 20 times with 5 sec interval
> 
> N:                    1        4       8        16
> Elapsed-time(Before): 4309.75  9511.09 12276.1  77103.5
> Elapsed-time(After):  5391.69 16114.1  19380.3  34879.2
> 
> So generally we can see latency increase. Ratio of this increase
> is rather big - up to 70%. But, under the heavy workload, it shows
> latency decrease - up to 55%. This may be worst-case scenario, but
> reducing it would be important for some system, so, I can say that
> this patch have advantages and disadvantages in terms of latency.
> 
> Although I think that this patch is right direction for CMA, there is
> side-effect in following case. If there is small memory zone and CMA
> occupys most of them, LRU for this zone would have many CMA pages. When
> reclaim is started, these CMA pages would be reclaimed, but not counted
> for watermark checking, so too many CMA pages could be reclaimed
> unnecessarily. Until now, this can't happen because free CMA pages aren't
> used easily. But, with this patch, free CMA pages are used easily so
> this problem can be possible. I will handle it on another patchset
> after some investigating.
> 
> v2: In fastpath, just replenish counters. Calculation is done whenver
>      cma area is varied
> 
> Acked-by: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> diff --git a/arch/powerpc/kvm/book3s_hv_cma.c b/arch/powerpc/kvm/book3s_hv_cma.c
> index d9d3d85..84a7582 100644
> --- a/arch/powerpc/kvm/book3s_hv_cma.c
> +++ b/arch/powerpc/kvm/book3s_hv_cma.c
> @@ -132,6 +132,8 @@ struct page *kvm_alloc_cma(unsigned long nr_pages, unsigned long align_pages)
>   		if (ret == 0) {
>   			bitmap_set(cma->bitmap, pageno, nr_chunk);
>   			page = pfn_to_page(pfn);
> +			adjust_managed_cma_page_count(page_zone(page),
> +								nr_pages);

I think it should be -nr_pages to decrease the managed_cma_pages variable.
But it is not. I think there is a reason.
Why the managed_cma_pages is increased by allocation?

>   			memset(pfn_to_kaddr(pfn), 0, nr_pages << PAGE_SHIFT);
>   			break;
>   		} else if (ret != -EBUSY) {
> @@ -180,6 +182,7 @@ bool kvm_release_cma(struct page *pages, unsigned long nr_pages)
>   		     (pfn - cma->base_pfn) >> (KVM_CMA_CHUNK_ORDER - PAGE_SHIFT),
>   		     nr_chunk);
>   	free_contig_range(pfn, nr_pages);
> +	adjust_managed_cma_page_count(page_zone(pages), nr_pages);
>   	mutex_unlock(&kvm_cma_mutex);
>   
>   	return true;
> @@ -210,6 +213,8 @@ static int __init kvm_cma_activate_area(unsigned long base_pfn,
>   		}
>   		init_cma_reserved_pageblock(pfn_to_page(base_pfn));
>   	} while (--i);
> +	adjust_managed_cma_page_count(zone, count);
> +
>   	return 0;
>   }
>   
> diff --git a/drivers/base/dma-contiguous.c b/drivers/base/dma-contiguous.c
> index 165c2c2..c578d5a 100644
> --- a/drivers/base/dma-contiguous.c
> +++ b/drivers/base/dma-contiguous.c
> @@ -160,6 +160,7 @@ static int __init cma_activate_area(struct cma *cma)
>   		}
>   		init_cma_reserved_pageblock(pfn_to_page(base_pfn));
>   	} while (--i);
> +	adjust_managed_cma_page_count(zone, cma->count);
>   
>   	return 0;
>   }
> @@ -307,6 +308,7 @@ struct page *dma_alloc_from_contiguous(struct device *dev, int count,
>   		if (ret == 0) {
>   			bitmap_set(cma->bitmap, pageno, count);
>   			page = pfn_to_page(pfn);
> +			adjust_managed_cma_page_count(page_zone(page), count);

I think this also should be -count.

>   			break;
>   		} else if (ret != -EBUSY) {
>   			break;
> @@ -353,6 +355,7 @@ bool dma_release_from_contiguous(struct device *dev, struct page *pages,
>   	mutex_lock(&cma_mutex);
>   	bitmap_clear(cma->bitmap, pfn - cma->base_pfn, count);
>   	free_contig_range(pfn, count);
> +	adjust_managed_cma_page_count(page_zone(pages), count);
>   	mutex_unlock(&cma_mutex);
>   
>   	return true;
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 39b81dc..51cffc1 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -415,6 +415,7 @@ extern int alloc_contig_range(unsigned long start, unsigned long end,
>   extern void free_contig_range(unsigned long pfn, unsigned nr_pages);
>   
>   /* CMA stuff */
> +extern void adjust_managed_cma_page_count(struct zone *zone, long count);
>   extern void init_cma_reserved_pageblock(struct page *page);
>   
>   #endif
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index fac5509..f52cb96 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -389,6 +389,20 @@ struct zone {
>   	int			compact_order_failed;
>   #endif
>   
> +#ifdef CONFIG_CMA
> +	unsigned long managed_cma_pages;
> +	/*
> +	 * Number of allocation attempt on each movable/cma type
> +	 * without switching type. max_try(movable/cma) maintain
> +	 * predefined calculated counter and replenish nr_try_(movable/cma)
> +	 * with each of them whenever both of them are 0.
> +	 */
> +	int nr_try_movable;
> +	int nr_try_cma;
> +	int max_try_movable;
> +	int max_try_cma;
> +#endif
> +
>   	ZONE_PADDING(_pad1_)
>   
>   	/* Fields commonly accessed by the page reclaim scanner */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 674ade7..ca678b6 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -788,6 +788,56 @@ void __init __free_pages_bootmem(struct page *page, unsigned int order)
>   }
>   
>   #ifdef CONFIG_CMA
> +void adjust_managed_cma_page_count(struct zone *zone, long count)
> +{
> +	unsigned long flags;
> +	long total, cma, movable;
> +
> +	spin_lock_irqsave(&zone->lock, flags);
> +	zone->managed_cma_pages += count;
> +
> +	total = zone->managed_pages;
> +	cma = zone->managed_cma_pages;
> +	movable = total - cma - high_wmark_pages(zone);

If cma can be negative value, above calcuation increase movable value becuase -cma becomes positive value.
Does it need a sign check?



> +
> +	/* No cma pages, so do only movable allocation */
> +	if (cma <= 0) {
> +		zone->max_try_movable = pageblock_nr_pages;
> +		zone->max_try_cma = 0;
> +		goto out;
> +	}
> +
> +	/*
> +	 * We want to consume cma pages with well balanced ratio so that
> +	 * we have consumed enough cma pages before the reclaim. For this
> +	 * purpose, we can use the ratio, movable : cma. And we doesn't
> +	 * want to switch too frequently, because it prevent allocated pages
> +	 * from beging successive and it is bad for some sorts of devices.
> +	 * I choose pageblock_nr_pages for the minimum amount of successive
> +	 * allocation because it is the size of a huge page and fragmentation
> +	 * avoidance is implemented based on this size.
> +	 *
> +	 * To meet above criteria, I derive following equation.
> +	 *
> +	 * if (movable > cma) then; movable : cma = X : pageblock_nr_pages
> +	 * else (movable <= cma) then; movable : cma = pageblock_nr_pages : X
> +	 */
> +	if (movable > cma) {
> +		zone->max_try_movable =
> +			(movable * pageblock_nr_pages) / cma;

I think you assume that cma value cannot be negative. If cma can be negative, the resule of dividing by cma becomes negative. Right?


> +		zone->max_try_cma = pageblock_nr_pages;
> +	} else {
> +		zone->max_try_movable = pageblock_nr_pages;
> +		zone->max_try_cma = cma * pageblock_nr_pages / movable;
> +	}
> +
> +out:
> +	zone->nr_try_movable = zone->max_try_movable;
> +	zone->nr_try_cma = zone->max_try_cma;
> +
> +	spin_unlock_irqrestore(&zone->lock, flags);
> +}
> +
>   /* Free whole pageblock and set its migration type to MIGRATE_CMA. */
>   void __init init_cma_reserved_pageblock(struct page *page)
>   {
> @@ -1136,6 +1186,36 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>   	return NULL;
>   }
>   
> +#ifdef CONFIG_CMA
> +static struct page *__rmqueue_cma(struct zone *zone, unsigned int order)
> +{
> +	struct page *page;
> +
> +	if (zone->nr_try_movable > 0)
> +		goto alloc_movable;
> +
> +	if (zone->nr_try_cma > 0) {
> +		/* Okay. Now, we can try to allocate the page from cma region */
> +		zone->nr_try_cma -= 1 << order;
> +		page = __rmqueue_smallest(zone, order, MIGRATE_CMA);
> +
> +		/* CMA pages can vanish through CMA allocation */
> +		if (unlikely(!page && order == 0))
> +			zone->nr_try_cma = 0;
> +
> +		return page;
> +	}
> +
> +	/* Reset counter */
> +	zone->nr_try_movable = zone->max_try_movable;
> +	zone->nr_try_cma = zone->max_try_cma;
> +
> +alloc_movable:
> +	zone->nr_try_movable -= 1 << order;
> +	return NULL;
> +}
> +#endif
> +
>   /*
>    * Do the hard work of removing an element from the buddy allocator.
>    * Call me with the zone->lock already held.
> @@ -1143,10 +1223,15 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>   static struct page *__rmqueue(struct zone *zone, unsigned int order,
>   						int migratetype)
>   {
> -	struct page *page;
> +	struct page *page = NULL;
> +
> +	if (IS_ENABLED(CONFIG_CMA) &&

You might know that CONFIG_CMA is enabled and there is no CMA memory, because CONFIG_CMA_SIZE_MBYTES can be zero.
Is IS_ENABLED(CONFIG_CMA) alright in that case?

> +		migratetype == MIGRATE_MOVABLE && zone->managed_cma_pages)
> +		page = __rmqueue_cma(zone, order);
>   
>   retry_reserve:
> -	page = __rmqueue_smallest(zone, order, migratetype);
> +	if (!page)
> +		page = __rmqueue_smallest(zone, order, migratetype);
>   
>   	if (unlikely(!page) && migratetype != MIGRATE_RESERVE) {
>   		page = __rmqueue_fallback(zone, order, migratetype);
> @@ -4849,6 +4934,8 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
>   		zone_seqlock_init(zone);
>   		zone->zone_pgdat = pgdat;
>   		zone_pcp_init(zone);
> +		if (IS_ENABLED(CONFIG_CMA))
> +			zone->managed_cma_pages = 0;
>   
>   		/* For bootup, initialized properly in watermark setup */
>   		mod_zone_page_state(zone, NR_ALLOC_BATCH, zone->managed_pages);
> 
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