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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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