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SubjectRe: [PATCH 23/25] mm, compaction: Be selective about what pageblocks to clear skip hints
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On 1/4/19 1:50 PM, Mel Gorman wrote:
> Pageblock hints are cleared when compaction restarts or kswapd makes enough
> progress that it can sleep but it's over-eager in that the bit is cleared
> for migration sources with no LRU pages and migration targets with no free
> pages. As pageblock skip hint flushes are relatively rare and out-of-band
> with respect to kswapd, this patch makes a few more expensive checks to
> see if it's appropriate to even clear the bit. Every pageblock that is
> not cleared will avoid 512 pages being scanned unnecessarily on x86-64.
>
> The impact is variable with different workloads showing small differences
> in latency, success rates and scan rates. This is expected as clearing
> the hints is not that common but doing a small amount of work out-of-band
> to avoid a large amount of work in-band later is generally a good thing.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Similar doubts to the previous patch wrt sampling. But if it works, ok.

> ---
> include/linux/mmzone.h | 2 +
> mm/compaction.c | 119 +++++++++++++++++++++++++++++++++++++++++--------
> 2 files changed, 102 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index cc4a507d7ca4..faa1e6523f49 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -480,6 +480,8 @@ struct zone {
> unsigned long compact_cached_free_pfn;
> /* pfn where async and sync compaction migration scanner should start */
> unsigned long compact_cached_migrate_pfn[2];
> + unsigned long compact_init_migrate_pfn;
> + unsigned long compact_init_free_pfn;
> #endif
>
> #ifdef CONFIG_COMPACTION
> diff --git a/mm/compaction.c b/mm/compaction.c
> index cc532e81a7b7..7f316e1a7275 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -231,6 +231,62 @@ static bool pageblock_skip_persistent(struct page *page)
> return false;
> }
>
> +static bool
> +__reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
> + bool check_target)
> +{
> + struct page *page = pfn_to_online_page(pfn);
> + struct page *end_page;
> +
> + if (!page)
> + return false;
> + if (zone != page_zone(page))
> + return false;
> + if (pageblock_skip_persistent(page))
> + return false;
> +
> + /*
> + * If skip is already cleared do no further checking once the
> + * restart points have been set.
> + */
> + if (check_source && check_target && !get_pageblock_skip(page))
> + return true;
> +
> + /*
> + * If clearing skip for the target scanner, do not select a
> + * non-movable pageblock as the starting point.
> + */
> + if (!check_source && check_target &&
> + get_pageblock_migratetype(page) != MIGRATE_MOVABLE)
> + return false;
> +
> + /*
> + * Only clear the hint if a sample indicates there is either a
> + * free page or an LRU page in the block. One or other condition
> + * is necessary for the block to be a migration source/target.
> + */
> + page = pfn_to_page(pageblock_start_pfn(pfn));
> + if (zone != page_zone(page))
> + return false;
> + end_page = page + pageblock_nr_pages;

Watch out for start pfn being invalid, and end_page being invalid or after zone end?

> +
> + do {
> + if (check_source && PageLRU(page)) {
> + clear_pageblock_skip(page);
> + return true;
> + }
> +
> + if (check_target && PageBuddy(page)) {
> + clear_pageblock_skip(page);
> + return true;
> + }
> +
> + page += (1 << PAGE_ALLOC_COSTLY_ORDER);

Also probably check pfn_valid_within() and page_zone?

> + } while (page < end_page);
> +
> + return false;
> +}
> +
> /*
> * This function is called to clear all cached information on pageblocks that
> * should be skipped for page isolation when the migrate and free page scanner

...

> @@ -1193,7 +1273,7 @@ fast_isolate_freepages(struct compact_control *cc)
> * If starting the scan, use a deeper search and use the highest
> * PFN found if a suitable one is not found.
> */
> - if (cc->free_pfn == pageblock_start_pfn(zone_end_pfn(cc->zone) - 1)) {
> + if (cc->free_pfn >= cc->zone->compact_init_free_pfn) {
> limit = pageblock_nr_pages >> 1;
> scan_start = true;
> }
> @@ -1338,7 +1418,6 @@ static void isolate_freepages(struct compact_control *cc)
> unsigned long isolate_start_pfn; /* exact pfn we start at */
> unsigned long block_end_pfn; /* end of current pageblock */
> unsigned long low_pfn; /* lowest pfn scanner is able to scan */
> - unsigned long nr_isolated;
> struct list_head *freelist = &cc->freepages;
> unsigned int stride;
>
> @@ -1374,6 +1453,8 @@ static void isolate_freepages(struct compact_control *cc)
> block_end_pfn = block_start_pfn,
> block_start_pfn -= pageblock_nr_pages,
> isolate_start_pfn = block_start_pfn) {
> + unsigned long nr_isolated;

Unrelated cleanup? Nevermind.

> /*
> * This can iterate a massively long zone without finding any
> * suitable migration targets, so periodically check resched.
> @@ -2020,7 +2101,7 @@ static enum compact_result compact_zone(struct compact_control *cc)
> cc->zone->compact_cached_migrate_pfn[1] = cc->migrate_pfn;
> }
>
> - if (cc->migrate_pfn == start_pfn)
> + if (cc->migrate_pfn <= cc->zone->compact_init_migrate_pfn)
> cc->whole_zone = true;
> }
>
>

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