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    SubjectRe: [TESTCASE] Clean pages clogging the VM
    On Thu, Aug 19, 2010 at 12:07:31AM +0800, Wu, Fengguang wrote:
    > On Thu, Aug 19, 2010 at 12:06:13AM +0800, Wu Fengguang wrote:
    > > On Wed, Aug 18, 2010 at 04:13:08PM +0200, Johannes Weiner wrote:
    > > > Hi Matthew,
    > > >
    > > > On Tue, Aug 17, 2010 at 03:50:01PM -0400, Matthew Wilcox wrote:
    > > > >
    > > > > No comment on this? Was it just that I posted it during the VM summit?
    > > >
    > > > I have not forgotten about it. I just have a hard time reproducing
    > > > those extreme stalls you observed.
    > > >
    > > > Running that test on a 2.5GHz machine with 2G of memory gives me
    > > > stalls of up to half a second. The patchset I am experimenting with
    > > > gets me down to peaks of 70ms, but it needs further work.
    > > >
    > > > Mapped file pages get two rounds on the LRU list, so once the VM
    > > > starts scanning, it has to go through all of them twice and can only
    > > > reclaim them on the second encounter.
    > > >
    > > > At that point, since we scan without making progress, we start waiting
    > > > for IO, which is not happening in this case, so we sit there until a
    > > > timeout expires.
    > >
    > > Right, this could lead to some 1s stall. Shaohua and me also noticed
    > > this when investigating the responsiveness issues. And we are wondering
    > > if it makes sense to do congestion_wait() only when the bdi is really
    > > congested? There are no IO underway anyway in this case.
    > >
    > > > This stupid-waiting can be improved, and I am working on that. But
    > >
    > > Yeah, stupid waiting :)
    How about this one?


    Subject: mm: check device is really congested before sleep in direct page reclaim

    congestion_wait() blindly sleep without checking if device is really congested.
    In a workload without any write, it can cause direct page reclaim sleep 100ms
    and hasn't any help for page reclaim.
    There might be other places calling congestion_wait() and need check if
    device is really congested, but I can't audit all, so this just changes the
    direct page reclaim code path. The new congestion_wait_check() will make sure
    at least one device is congested before going into sleep.

    Signed-off-by: Shaohua Li <shaohua.li@intel.com>

    ---
    include/linux/backing-dev.h | 1 +
    mm/backing-dev.c | 14 ++++++++++++--
    mm/vmscan.c | 2 +-
    3 files changed, 14 insertions(+), 3 deletions(-)

    Index: linux/mm/backing-dev.c
    ===================================================================
    --- linux.orig/mm/backing-dev.c 2010-08-18 16:41:04.000000000 +0800
    +++ linux/mm/backing-dev.c 2010-08-19 08:59:14.000000000 +0800
    @@ -725,13 +725,16 @@ static wait_queue_head_t congestion_wqh[
    __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[1])
    };

    +static atomic_t nr_congested_bdi[2];
    +
    void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
    {
    enum bdi_state bit;
    wait_queue_head_t *wqh = &congestion_wqh[sync];

    bit = sync ? BDI_sync_congested : BDI_async_congested;
    - clear_bit(bit, &bdi->state);
    + if (test_and_clear_bit(bit, &bdi->state))
    + atomic_dec(&nr_congested_bdi[sync]);
    smp_mb__after_clear_bit();
    if (waitqueue_active(wqh))
    wake_up(wqh);
    @@ -743,7 +746,8 @@ void set_bdi_congested(struct backing_de
    enum bdi_state bit;

    bit = sync ? BDI_sync_congested : BDI_async_congested;
    - set_bit(bit, &bdi->state);
    + if (!test_and_set_bit(bit, &bdi->state))
    + atomic_inc(&nr_congested_bdi[sync]);
    }
    EXPORT_SYMBOL(set_bdi_congested);

    @@ -769,3 +773,9 @@ long congestion_wait(int sync, long time
    }
    EXPORT_SYMBOL(congestion_wait);

    +long congestion_wait_check(int sync, long timeout)
    +{
    + if (atomic_read(&nr_congested_bdi[sync]) == 0)
    + return 0;
    + return congestion_wait(sync, timeout);
    +}
    Index: linux/include/linux/backing-dev.h
    ===================================================================
    --- linux.orig/include/linux/backing-dev.h 2010-08-18 16:41:04.000000000 +0800
    +++ linux/include/linux/backing-dev.h 2010-08-18 16:41:23.000000000 +0800
    @@ -285,6 +285,7 @@ enum {
    void clear_bdi_congested(struct backing_dev_info *bdi, int sync);
    void set_bdi_congested(struct backing_dev_info *bdi, int sync);
    long congestion_wait(int sync, long timeout);
    +long congestion_wait_check(int sync, long timeout);


    static inline bool bdi_cap_writeback_dirty(struct backing_dev_info *bdi)
    Index: linux/mm/vmscan.c
    ===================================================================
    --- linux.orig/mm/vmscan.c 2010-08-18 16:41:04.000000000 +0800
    +++ linux/mm/vmscan.c 2010-08-18 16:41:23.000000000 +0800
    @@ -1910,7 +1910,7 @@ static unsigned long do_try_to_free_page
    /* Take a nap, wait for some writeback to complete */
    if (!sc->hibernation_mode && sc->nr_scanned &&
    priority < DEF_PRIORITY - 2)
    - congestion_wait(BLK_RW_ASYNC, HZ/10);
    + congestion_wait_check(BLK_RW_ASYNC, HZ/10);
    }

    out:

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