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    SubjectRe: kswapd0: excessive CPU usage
    Dne 15.10.2012 13:09, Mel Gorman napsal(a):
    > On Mon, Oct 15, 2012 at 11:54:13AM +0200, Jiri Slaby wrote:
    >> On 10/12/2012 03:57 PM, Mel Gorman wrote:
    >>> mm: vmscan: scale number of pages reclaimed by reclaim/compaction only in direct reclaim
    >>>
    >>> Jiri Slaby reported the following:
    >>>
    >>> (It's an effective revert of "mm: vmscan: scale number of pages
    >>> reclaimed by reclaim/compaction based on failures".)
    >>> Given kswapd had hours of runtime in ps/top output yesterday in the
    >>> morning and after the revert it's now 2 minutes in sum for the last 24h,
    >>> I would say, it's gone.
    >>>
    >>> The intention of the patch in question was to compensate for the loss of
    >>> lumpy reclaim. Part of the reason lumpy reclaim worked is because it
    >>> aggressively reclaimed pages and this patch was meant to be a
    >>> sane compromise.
    >>>
    >>> When compaction fails, it gets deferred and both compaction and
    >>> reclaim/compaction is deferred avoid excessive reclaim. However, since
    >>> commit c6543459 (mm: remove __GFP_NO_KSWAPD), kswapd is woken up each time
    >>> and continues reclaiming which was not taken into account when the patch
    >>> was developed.
    >>>
    >>> As it is not taking deferred compaction into account in this path it scans
    >>> aggressively before falling out and making the compaction_deferred check in
    >>> compaction_ready. This patch avoids kswapd scaling pages for reclaim and
    >>> leaves the aggressive reclaim to the process attempting the THP
    >>> allocation.
    >>>
    >>> Signed-off-by: Mel Gorman <mgorman@suse.de>
    >>> ---
    >>> mm/vmscan.c | 10 ++++++++--
    >>> 1 file changed, 8 insertions(+), 2 deletions(-)
    >>>
    >>> diff --git a/mm/vmscan.c b/mm/vmscan.c
    >>> index 2624edc..2b7edfa 100644
    >>> --- a/mm/vmscan.c
    >>> +++ b/mm/vmscan.c
    >>> @@ -1763,14 +1763,20 @@ static bool in_reclaim_compaction(struct scan_control *sc)
    >>> #ifdef CONFIG_COMPACTION
    >>> /*
    >>> * If compaction is deferred for sc->order then scale the number of pages
    >>> - * reclaimed based on the number of consecutive allocation failures
    >>> + * reclaimed based on the number of consecutive allocation failures. This
    >>> + * scaling only happens for direct reclaim as it is about to attempt
    >>> + * compaction. If compaction fails, future allocations will be deferred
    >>> + * and reclaim avoided. On the other hand, kswapd does not take compaction
    >>> + * deferral into account so if it scaled, it could scan excessively even
    >>> + * though allocations are temporarily not being attempted.
    >>> */
    >>> static unsigned long scale_for_compaction(unsigned long pages_for_compaction,
    >>> struct lruvec *lruvec, struct scan_control *sc)
    >>> {
    >>> struct zone *zone = lruvec_zone(lruvec);
    >>>
    >>> - if (zone->compact_order_failed <= sc->order)
    >>> + if (zone->compact_order_failed <= sc->order &&
    >>> + !current_is_kswapd())
    >>> pages_for_compaction <<= zone->compact_defer_shift;
    >>> return pages_for_compaction;
    >>> }
    >>
    >> Yes, applying this instead of the revert fixes the issue as well.
    >>
    >


    I've applied this patch on 3.7.0-rc3 kernel - and I still see excessive CPU
    usage - mainly after suspend/resume

    Here is just simple kswapd backtrace from running kernel:

    kswapd0 R running task 0 30 2 0x00000000
    ffff8801331ddae8 0000000000000082 ffff880135b8a340 0000000000000008
    ffff880135b8a340 ffff8801331ddfd8 ffff8801331ddfd8 ffff8801331ddfd8
    ffff880071db8000 ffff880135b8a340 0000000000000286 ffff8801331dc000
    Call Trace:
    [<ffffffff81555cd2>] preempt_schedule+0x42/0x60
    [<ffffffff81557b75>] _raw_spin_unlock+0x55/0x60
    [<ffffffff811929d1>] put_super+0x31/0x40
    [<ffffffff81192aa2>] drop_super+0x22/0x30
    [<ffffffff81193be9>] prune_super+0x149/0x1b0
    [<ffffffff81141e2a>] shrink_slab+0xba/0x510
    [<ffffffff81185baa>] ? mem_cgroup_iter+0x17a/0x2e0
    [<ffffffff81185afa>] ? mem_cgroup_iter+0xca/0x2e0
    [<ffffffff811450f9>] balance_pgdat+0x629/0x7f0
    [<ffffffff81145434>] kswapd+0x174/0x620
    [<ffffffff8106fd20>] ? __init_waitqueue_head+0x60/0x60
    [<ffffffff811452c0>] ? balance_pgdat+0x7f0/0x7f0
    [<ffffffff8106f50b>] kthread+0xdb/0xe0
    [<ffffffff8106f430>] ? kthread_create_on_node+0x140/0x140
    [<ffffffff8155fb1c>] ret_from_fork+0x7c/0xb0
    [<ffffffff8106f430>] ? kthread_create_on_node+0x140/0x140


    Zdenek




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