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Date: Tue, 25 Sep 2012 16:05:17 +0900
From: Minchan Kim <minchan@...nel.org>
To: Mel Gorman <mgorman@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Richard Davies <richard@...chsys.com>,
Shaohua Li <shli@...nel.org>, Rik van Riel <riel@...hat.com>,
Avi Kivity <avi@...hat.com>,
QEMU-devel <qemu-devel@...gnu.org>, KVM <kvm@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/9] mm: compaction: Acquire the zone->lru_lock as late
as possible
Hi Mel,
I have a question below.
On Fri, Sep 21, 2012 at 11:46:19AM +0100, Mel Gorman wrote:
> Compactions migrate scanner acquires the zone->lru_lock when scanning a range
> of pages looking for LRU pages to acquire. It does this even if there are
> no LRU pages in the range. If multiple processes are compacting then this
> can cause severe locking contention. To make matters worse commit b2eef8c0
> (mm: compaction: minimise the time IRQs are disabled while isolating pages
> for migration) releases the lru_lock every SWAP_CLUSTER_MAX pages that are
> scanned.
>
> This patch makes two changes to how the migrate scanner acquires the LRU
> lock. First, it only releases the LRU lock every SWAP_CLUSTER_MAX pages if
> the lock is contended. This reduces the number of times it unnecessarily
> disables and re-enables IRQs. The second is that it defers acquiring the
> LRU lock for as long as possible. If there are no LRU pages or the only
> LRU pages are transhuge then the LRU lock will not be acquired at all
> which reduces contention on zone->lru_lock.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> Acked-by: Rik van Riel <riel@...hat.com>
> ---
> mm/compaction.c | 63 +++++++++++++++++++++++++++++++++++++------------------
> 1 file changed, 43 insertions(+), 20 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 6b55491..a6068ff 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -50,6 +50,11 @@ static inline bool migrate_async_suitable(int migratetype)
> return is_migrate_cma(migratetype) || migratetype == MIGRATE_MOVABLE;
> }
>
> +static inline bool should_release_lock(spinlock_t *lock)
> +{
> + return need_resched() || spin_is_contended(lock);
> +}
> +
> /*
> * Compaction requires the taking of some coarse locks that are potentially
> * very heavily contended. Check if the process needs to be scheduled or
> @@ -62,7 +67,7 @@ static inline bool migrate_async_suitable(int migratetype)
> static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
> bool locked, struct compact_control *cc)
> {
> - if (need_resched() || spin_is_contended(lock)) {
> + if (should_release_lock(lock)) {
> if (locked) {
> spin_unlock_irqrestore(lock, *flags);
> locked = false;
> @@ -327,7 +332,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> isolate_mode_t mode = 0;
> struct lruvec *lruvec;
> unsigned long flags;
> - bool locked;
> + bool locked = false;
>
> /*
> * Ensure that there are not too many pages isolated from the LRU
> @@ -347,23 +352,17 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>
> /* Time to isolate some pages for migration */
> cond_resched();
> - spin_lock_irqsave(&zone->lru_lock, flags);
> - locked = true;
> for (; low_pfn < end_pfn; low_pfn++) {
> struct page *page;
>
> /* give a chance to irqs before checking need_resched() */
> - if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
> - spin_unlock_irqrestore(&zone->lru_lock, flags);
> - locked = false;
> + if (locked && !((low_pfn+1) % SWAP_CLUSTER_MAX)) {
> + if (should_release_lock(&zone->lru_lock)) {
> + spin_unlock_irqrestore(&zone->lru_lock, flags);
> + locked = false;
> + }
> }
>
> - /* Check if it is ok to still hold the lock */
> - locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
> - locked, cc);
> - if (!locked || fatal_signal_pending(current))
> - break;
> -
> /*
> * migrate_pfn does not necessarily start aligned to a
> * pageblock. Ensure that pfn_valid is called when moving
> @@ -403,21 +402,38 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> pageblock_nr = low_pfn >> pageblock_order;
> if (!cc->sync && last_pageblock_nr != pageblock_nr &&
> !migrate_async_suitable(get_pageblock_migratetype(page))) {
> - low_pfn += pageblock_nr_pages;
> - low_pfn = ALIGN(low_pfn, pageblock_nr_pages) - 1;
> - last_pageblock_nr = pageblock_nr;
> - continue;
> + goto next_pageblock;
> }
>
> + /* Check may be lockless but that's ok as we recheck later */
> if (!PageLRU(page))
> continue;
>
> /*
> - * PageLRU is set, and lru_lock excludes isolation,
> - * splitting and collapsing (collapsing has already
> - * happened if PageLRU is set).
> + * PageLRU is set. lru_lock normally excludes isolation
> + * splitting and collapsing (collapsing has already happened
> + * if PageLRU is set) but the lock is not necessarily taken
> + * here and it is wasteful to take it just to check transhuge.
> + * Check transhuge without lock and skip if it's either a
> + * transhuge or hugetlbfs page.
> */
> if (PageTransHuge(page)) {
> + if (!locked)
> + goto next_pageblock;
Why skip all pages in a pageblock if !locked?
Shouldn't we add some comment?
> + low_pfn += (1 << compound_order(page)) - 1;
> + continue;
> + }
> +
> + /* Check if it is ok to still hold the lock */
> + locked = compact_checklock_irqsave(&zone->lru_lock, &flags,
> + locked, cc);
> + if (!locked || fatal_signal_pending(current))
> + break;
> +
> + /* Recheck PageLRU and PageTransHuge under lock */
> + if (!PageLRU(page))
> + continue;
> + if (PageTransHuge(page)) {
> low_pfn += (1 << compound_order(page)) - 1;
> continue;
> }
> @@ -444,6 +460,13 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> ++low_pfn;
> break;
> }
> +
> + continue;
> +
> +next_pageblock:
> + low_pfn += pageblock_nr_pages;
> + low_pfn = ALIGN(low_pfn, pageblock_nr_pages) - 1;
> + last_pageblock_nr = pageblock_nr;
> }
>
> acct_isolated(zone, locked, cc);
> --
> 1.7.9.2
>
> --
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--
Kind regards,
Minchan Kim
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