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    SubjectRe: 103 sec. latency: sync_page() with TASK_UNINTERRUPTIBLE (?) (bisected)
    > Cause                                                Maximum     Percentage
    > sync_page __lock_page handle_mm_fault do_page_faul103659.0 msec 59.8 %
    > get_request_wait __make_request generic_make_reque1992.0 msec 10.2 %
    > get_request_wait __make_request generic_make_reque1620.7 msec 4.6 %
    > sync_page __lock_page find_lock_page filemap_fault399.3 msec 3.4 %
    > sync_buffer __wait_on_buffer __bread ext3_get_bran292.9 msec 1.3 %
    > sync_page __lock_page handle_mm_fault do_page_faul200.5 msec 0.3 %
    > Scheduler: waiting for cpu 155.9 msec 18.9 %
    > sync_page __lock_page handle_mm_fault do_page_faul117.5 msec 0.1 %
    > congestion_wait try_to_free_pages __alloc_pages re103.5 msec 0.2 %
    > r_code
    >
    >
    > Process X (2910)
    > sync_page __lock_page handle_mm_fault do_page_faul103659.0 msec 98.5 %
    > Scheduler: waiting for cpu 64.6 msec 1.3 %
    > sync_page __lock_page find_lock_page filemap_fault 51.4 msec 0.2 %lt do_page_fault error_code
    > sync_buffer __wait_on_buffer __bread ext3_get_bran 17.8 msec 0.0 % ext3_get_block do_mpage_readpage mpage_readpa
    > sync_page __lock_page handle_mm_fault do_page_faul 9.1 msec 0.0 %ig ault
    >
    > The experience goes like this:
    >
    > 1) Boot with 2.6.25-rc6-00243-g028011e
    > 2) Log into X
    > 3) Open a 380MB file with xjed
    >
    > It takes more than 6 minutes to load the file, and meanwhile
    > I experience very bad desktop interactivity (like 1 minute to
    > the result of pressing F12 in Window Maker to appear in the
    > screen).
    >
    > If, after step 2), I start using firefox, thunderbird, play some
    > music etc and then close all these apps and go to step 3),
    > the loading finishes in about 2 minutes (and I have very
    > good interactivity meanwhile).
    >
    > So I noticed that when xjed is opening the file it is in
    > 'D' state (reported by ps), and while greping the kernel
    > source code for 'sync_page' I've found this comment
    > in mm/filemap.c:
    >
    > /**
    > * __lock_page - get a lock on the page, assuming we need to sleep to get it
    > * @page: the page to lock
    > *
    > * Ugly. Running sync_page() in state TASK_UNINTERRUPTIBLE is scary. If some
    > * random driver's requestfn sets TASK_RUNNING, we could busywait. However
    > * chances are that on the second loop, the block layer's plug list is empty,
    > * so sync_page() will then return in state TASK_UNINTERRUPTIBLE.
    > */
    >
    > "Ugly. Running sync_page() in state TASK_UNINTERRUPTIBLE is scary."
    >
    > It appears that my problem with xjed in 'D' state while loading,
    > and sync_page() appearing in latencytop with 103 secs of latency
    > may be related through the "ugliness" described above.
    >
    > So I was wondering if there is a way to fix this. Note that
    > this issue does not happen if I load the file after using the
    > computer for a while, so it is not impossible to have good
    > interactivity while loading that big file.


    After lots of tests, it turned out to be a regression from
    2.6.24.

    I bisected it down to f1a9ee758de7de1e040de849fdef46e6802ea117
    ("kswapd should only wait on IO if there is IO").

    If a revert the above commit from the latest git kernel
    (v2.6.25-rc6-243-g028011e), then I get an very good interactivity
    while xjed is loading the 380MB file (meaning that I can do other things
    during the load). Otherwise it takes almost 3 times longer to load,
    and interactivity is very bad in the meanwhile. I doubled checked it.

    I hope this information will be useful, and I can try any patches
    if necessary.



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