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SubjectRe: [PATCH 2/2] mm: compaction: Minimise the time IRQs are disabled while isolating pages for migration
On Fri, 25 Feb 2011 20:04:59 +0000
Mel Gorman <mel@csn.ul.ie> wrote:
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> compaction_alloc() isolates pages for migration in isolate_migratepages. While
> it's scanning, IRQs are disabled on the mistaken assumption the scanning
> should be short. Tests show this to be true for the most part but
> contention times on the LRU lock can be increased. Before this patch,
> the IRQ disabled times for a simple test looked like
>
> Total sampled time IRQs off (not real total time): 5493
> Event shrink_inactive_list..shrink_zone 1596 us count 1
> Event shrink_inactive_list..shrink_zone 1530 us count 1
> Event shrink_inactive_list..shrink_zone 956 us count 1
> Event shrink_inactive_list..shrink_zone 541 us count 1
> Event shrink_inactive_list..shrink_zone 531 us count 1
> Event split_huge_page..add_to_swap 232 us count 1
> Event save_args..call_softirq 36 us count 1
> Event save_args..call_softirq 35 us count 2
> Event __wake_up..__wake_up 1 us count 1
>
> This patch reduces the worst-case IRQs-disabled latencies by releasing the
> lock every SWAP_CLUSTER_MAX pages that are scanned and releasing the CPU if
> necessary. The cost of this is that the processing performing compaction will
> be slower but IRQs being disabled for too long a time has worse consequences
> as the following report shows;
>
> Total sampled time IRQs off (not real total time): 4367
> Event shrink_inactive_list..shrink_zone 881 us count 1
> Event shrink_inactive_list..shrink_zone 875 us count 1
> Event shrink_inactive_list..shrink_zone 868 us count 1
> Event shrink_inactive_list..shrink_zone 555 us count 1
> Event split_huge_page..add_to_swap 495 us count 1
> Event compact_zone..compact_zone_order 269 us count 1
> Event split_huge_page..add_to_swap 266 us count 1
> Event shrink_inactive_list..shrink_zone 85 us count 1
> Event save_args..call_softirq 36 us count 2
> Event __wake_up..__wake_up 1 us count 1
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
> mm/compaction.c | 18 ++++++++++++++++++
> 1 files changed, 18 insertions(+), 0 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 11d88a2..ec9eb0f 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -279,9 +279,27 @@ static unsigned long isolate_migratepages(struct zone *zone,
> }
>
> /* Time to isolate some pages for migration */
> + cond_resched();
> spin_lock_irq(&zone->lru_lock);
> for (; low_pfn < end_pfn; low_pfn++) {
> struct page *page;
> + bool unlocked = false;
> +
> + /* give a chance to irqs before checking need_resched() */
> + if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
> + spin_unlock_irq(&zone->lru_lock);
> + unlocked = true;
> + }
> + if (need_resched() || spin_is_contended(&zone->lru_lock)) {
> + if (!unlocked)
> + spin_unlock_irq(&zone->lru_lock);
> + cond_resched();
> + spin_lock_irq(&zone->lru_lock);
> + if (fatal_signal_pending(current))
> + break;
> + } else if (unlocked)
> + spin_lock_irq(&zone->lru_lock);
> +
> if (!pfn_valid_within(low_pfn))
> continue;
> nr_scanned++;

That was somewhat ghastly.

I think it can be made only 99% as ghastly by renaming "unlocked" to
"locked" and eliminating the double-negative logic.

--- a/mm/compaction.c~mm-compaction-minimise-the-time-irqs-are-disabled-while-isolating-pages-for-migration
+++ a/mm/compaction.c
@@ -279,9 +279,27 @@ static unsigned long isolate_migratepage
}

/* Time to isolate some pages for migration */
+ cond_resched();
spin_lock_irq(&zone->lru_lock);
for (; low_pfn < end_pfn; low_pfn++) {
struct page *page;
+ bool locked = true;
+
+ /* give a chance to irqs before checking need_resched() */
+ if (!((low_pfn+1) % SWAP_CLUSTER_MAX)) {
+ spin_unlock_irq(&zone->lru_lock);
+ locked = false;
+ }
+ if (need_resched() || spin_is_contended(&zone->lru_lock)) {
+ if (locked)
+ spin_unlock_irq(&zone->lru_lock);
+ cond_resched();
+ spin_lock_irq(&zone->lru_lock);
+ if (fatal_signal_pending(current))
+ break;
+ } else if (!locked)
+ spin_lock_irq(&zone->lru_lock);
+
if (!pfn_valid_within(low_pfn))
continue;
nr_scanned++;


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