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Subjectscheduling problem?
Hi,

I am seeing (what I believe is;) severe process CPU starvation in
2.4.0-prerelease. At first, I attributed it to semaphore troubles
as when I enable semaphore deadlock detection in IKD and set it to
5 seconds, it triggers 100% of the time on nscd when I do sequential
I/O (iozone eg). In the meantime, I've done a slew of tracing, and
I think the holder of the semaphore I'm timing out on just flat isn't
being scheduled so it can release it. In the usual case of nscd, I
_think_ it's another nscd holding the semaphore. In no trace can I
go back far enough to catch the taker of the semaphore or any user
task other than iozone running between __down() time and timeout 5
seconds later. (trace buffer covers ~8 seconds of kernel time)

I think the snippet below captures the gist of the problem.

c012f32e nr_free_pages +<e/4c> (0.16) pid(256)
c012f37a nr_inactive_clean_pages +<e/44> (0.22) pid(256)
c01377f2 wakeup_bdflush +<12/a0> (0.14) pid(256)
c011620a wake_up_process +<e/58> (0.29) pid(256)
c012eea4 __alloc_pages_limit +<10/b8> (0.28) pid(256)
c012eea4 __alloc_pages_limit +<10/b8> (0.30) pid(256)
c012e3fa wakeup_kswapd +<12/d4> (0.25) pid(256)
c0115613 __wake_up +<13/130> (0.41) pid(256)
c011527b schedule +<13/398> (0.66) pid(256->6)
c01077db __switch_to +<13/d0> (0.70) pid(6)
c01893c6 generic_unplug_device +<e/38> (0.25) pid(6)
c011527b schedule +<13/398> (0.50) pid(6->256)
c01077db __switch_to +<13/d0> (0.29) pid(256)
c012eea4 __alloc_pages_limit +<10/b8> (0.22) pid(256)
c012d267 reclaim_page +<13/408> (0.54) pid(256)
c012679e __remove_inode_page +<e/74> (0.54) pid(256)
c0126fe0 add_to_page_cache_unique +<10/e4> (0.23) pid(256)
c0126751 add_page_to_hash_queue +<d/4c> (0.16) pid(256)
c012c9fa lru_cache_add +<e/f4> (0.29) pid(256)
c0153ac5 ext2_prepare_write +<d/28> (0.15) pid(256)
c013697f block_prepare_write +<f/4c> (0.15) pid(256)
c0136233 __block_prepare_write +<13/214> (0.17) pid(256)
c0135fd4 create_empty_buffers +<10/7c> (0.17) pid(256)
c0135d13 create_buffers +<13/1bc> (0.14) pid(256)
(repeats zillion times)

Despite wakeup_kswapd(0), we never schedule kswapd until much
later when we hit a wakeup_kswapd(1).. in the interrum, we
bounce back and forth betweem bdflush and iozone.. no other
task is scheduled through very many schedules.
(Per kdb, I had a few tasks which would have liked some CPU ;-)

In addition, kswapd is quite the CPU piggie when it's doing
page_launder() as this profiled snippet of one of kswapd's
running periods shows. (60ms with no schedule)

0.1093% 66.06 11.01 6 c010d9c2 timer_interrupt
0.1462% 88.39 0.28 313 c0134ea2 __remove_from_lru_list
0.1489% 90.02 11.25 8 c01b2077 do_rw_disk
0.1599% 96.66 32.22 3 c018a9e7 elevator_linus_merge
0.1624% 98.18 0.30 325 c0115613 __wake_up
0.2016% 121.88 0.42 290 c012c01f kmem_cache_free
0.2075% 125.43 0.66 189 c0189f1d end_buffer_io_sync
0.2089% 126.32 15.79 8 c01ad533 ide_build_sglist
34.8258% 21054.55 0.53 39486 c0137667 try_to_free_buffers
62.4743% 37769.90 0.95 39796 c012f2b9 __free_pages
Total entries: 81970 Total usecs: 60456.69 Idle: 0.00%

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