lkml.org 
[lkml]   [2011]   [Feb]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH 1/2] mm: compaction: Minimise the time IRQs are disabled while isolating free pages
    Date
    compaction_alloc() isolates free pages to be used as migration targets.
    While its scanning, IRQs are disabled on the mistaken assumption the scanning
    should be short. Analysis showed that IRQs were in fact being disabled for
    substantial time. A simple test was run using large anonymous mappings with
    transparent hugepage support enabled to trigger frequent compactions. A
    monitor sampled what the worst IRQ-off latencies were and a post-processing
    tool found the following;

    Total sampled time IRQs off (not real total time): 22355
    Event compaction_alloc..compaction_alloc 8409 us count 1
    Event compaction_alloc..compaction_alloc 7341 us count 1
    Event compaction_alloc..compaction_alloc 2463 us count 1
    Event compaction_alloc..compaction_alloc 2054 us count 1
    Event shrink_inactive_list..shrink_zone 1864 us count 1
    Event shrink_inactive_list..shrink_zone 88 us count 1
    Event save_args..call_softirq 36 us count 1
    Event save_args..call_softirq 35 us count 2
    Event __make_request..__blk_run_queue 24 us count 1
    Event __alloc_pages_nodemask..__alloc_pages_nodemask 6 us count 1

    i.e. compaction is disabled IRQs for a prolonged period of time - 8ms in
    one instance. The full report generated by the tool can be found at
    http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-vanilla-micro.report .
    This patch reduces the time IRQs are disabled by simply disabling IRQs
    at the last possible minute. An updated IRQs-off summary report then
    looks 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

    A full report is again available at
    http://www.csn.ul.ie/~mel/postings/minfree-20110225/irqsoff-minimiseirq-free-v1r4-micro.report .
    . As should be obvious, IRQ disabled latencies due to compaction are
    almost elimimnated for this particular test.

    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    ---
    mm/compaction.c | 17 ++++++++++++-----
    1 files changed, 12 insertions(+), 5 deletions(-)

    diff --git a/mm/compaction.c b/mm/compaction.c
    index 8be430b8..f47de94 100644
    --- a/mm/compaction.c
    +++ b/mm/compaction.c
    @@ -155,7 +155,6 @@ static void isolate_freepages(struct zone *zone,
    * pages on cc->migratepages. We stop searching if the migrate
    * and free page scanners meet or enough free pages are isolated.
    */
    - spin_lock_irqsave(&zone->lock, flags);
    for (; pfn > low_pfn && cc->nr_migratepages > nr_freepages;
    pfn -= pageblock_nr_pages) {
    unsigned long isolated;
    @@ -178,9 +177,18 @@ static void isolate_freepages(struct zone *zone,
    if (!suitable_migration_target(page))
    continue;

    - /* Found a block suitable for isolating free pages from */
    - isolated = isolate_freepages_block(zone, pfn, freelist);
    - nr_freepages += isolated;
    + /*
    + * Found a block suitable for isolating free pages from. Now
    + * we disabled interrupts, double check things are ok and
    + * isolate the pages. This is to minimise the time IRQs
    + * are disabled
    + */
    + spin_lock_irqsave(&zone->lock, flags);
    + if (suitable_migration_target(page)) {
    + isolated = isolate_freepages_block(zone, pfn, freelist);
    + nr_freepages += isolated;
    + }
    + spin_unlock_irqrestore(&zone->lock, flags);

    /*
    * Record the highest PFN we isolated pages from. When next
    @@ -190,7 +198,6 @@ static void isolate_freepages(struct zone *zone,
    if (isolated)
    high_pfn = max(high_pfn, pfn);
    }
    - spin_unlock_irqrestore(&zone->lock, flags);

    /* split_free_page does not map the pages */
    list_for_each_entry(page, freelist, lru) {
    --
    1.7.2.3


    \
     
     \ /
      Last update: 2011-02-25 19:03    [W:4.310 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site