Re: [uml-devel] kernel stalls in balance_dirty_pages_ratelimited()

[Thomas Meyer <thomas@m3y3r.de>]

Am Dienstag, den 14.10.2014, 07:43 +0100 schrieb Anton Ivanov:
> On 14/10/14 06:38, Anton Ivanov wrote:
> > How does the stall manifest itself?
> >
> > Do you have the journal thread (and sometimes a couple of other threads)
> > sitting in D state?
> 
> Sorry, should not be asking questions at 6 am before the 3rd double 
> espresso.
> 
> I think it is the same bug I am chasing - a stall in ubd, you hit it on 
> swap while I hit it in normal operation on a swapless system. I see a 
> stall in the journal instead of a backing dev stall.
> 
> If you apply the ubd patches out of my patchsets, you can trigger this 
> one with ease. In theory, all they do is to make UBD faster so they 
> should not by themselves introduce new races. They may however make the 
> older ones more pronounced.
> 
> My working hypothesis is a race somewhere in the vm subsystem. I have 
> been unable to nail it though.

Hi Anton,

I see this bug on a 3.17 uml kernel with the sync fix patch from
Thorsten Knabe applied.

The stall has to do with the writepage ratelimit mechanism, as the
mechanism seems to reach a state where it tries to write out page, per
page:

Breakpoint 1, balance_dirty_pages (pages_dirtied=1, mapping=<optimized out>) at	mm/page-writeback.c:1338
(gdb) bt
#0  balance_dirty_pages	(pages_dirtied=1, mapping=<optimized out>) at mm/page-writeback.c:1338

pages_dirtied = 1 !!

#0  try_to_grab_pending (work=0x7fa2a348, is_dwork=true, flags=0x72ff5ab8) at kernel/workqueue.c:1159
#1  0x0000000060051feb in mod_delayed_work_on (cpu=2141365064, wq=0x1, dwork=0x72ff5ab8, delay=<optimized out>) at kernel/workqueue.c:1510
#2  0x00000000600f382c in mod_delayed_work (delay=<optimized out>, dwork=<optimized out>, wq=<optimized out>) at include/linux/workqueue.h:504
#3  bdi_wakeup_thread (bdi=<optimized out>) at fs/fs-writeback.c:98
#4  0x00000000600f4aca in bdi_start_background_writeback (bdi=<optimized out>) at fs/fs-writeback.c:179
#5  0x000000006042d4c0 in balance_dirty_pages (pages_dirtied=<optimized out>, mapping=<optimized out>) at mm/page-writeback.c:1408
#6  0x00000000600a6e1a in balance_dirty_pages_ratelimited (mapping=<optimized out>) at mm/page-writeback.c:1627
#7  0x00000000600ba54f in do_wp_page (mm=<optimized out>, vma=<optimized out>, address=<optimized out>, page_table=<optimized out>, pmd=<optimized out>, orig_pte=..., ptl=<optimized out>) at mm/memory.c:2178
#8  0x00000000600bc986 in handle_pte_fault (flags=<optimized out>, pmd=<optimized out>, pte=<optimized out>, address=<optimized out>, vma=<optimized out>, mm=<optimized out>) at mm/memory.c:3230
#9  __handle_mm_fault (flags=<optimized out>, address=<optimized out>, vma=<optimized out>, mm=<optimized out>) at mm/memory.c:3335
#10 handle_mm_fault (mm=<optimized out>, vma=0x7f653228, address=1472490776, flags=<optimized out>) at mm/memory.c:3364
#11 0x0000000060028cec in handle_page_fault (address=1472490776, ip=<optimized out>, is_write=<optimized out>, is_user=0, code_out=<optimized out>) at arch/um/kernel/trap.c:75
#12 0x00000000600290d7 in segv (fi=..., ip=1228924391, is_user=<optimized out>, regs=0x73eb8de8) at arch/um/kernel/trap.c:222
#13 0x0000000060029395 in segv_handler (sig=<optimized out>, unused_si=<optimized out>, regs=<optimized out>) at arch/um/kernel/trap.c:191
#14 0x0000000060039c0f in userspace (regs=0x73eb8de8) at arch/um/os-Linux/skas/process.c:429
#15 0x0000000060026a8c in fork_handler () at arch/um/kernel/process.c:149
#16 0x000000000070b620 in ?? ()
#17 0x0000000000000000 in ?? ()

I'm not sure if this is the same error you encounter.

This is on an ubd device with a cow image attached to it.

The original ubd file and the cow file are spares ones, and do also
contain a swap partition.

I hope to get tracepoints/perf working, now as there is stacktrace
support in uml. An interessting tracepoint would be 
TRACE_EVENT(bdi_dirty_ratelimit) or TRACE_EVENT(balance_dirty_pages)


> 
> A.
> 
> >
> > A.
> >
> > On 13/10/14 22:48, Thomas Meyer wrote:
> >> #0  balance_dirty_pages_ratelimited (mapping=0x792cc618) at mm/page-writeback.c:1587
> >> #1  0x00000000600ba54f in do_wp_page (mm=<optimized out>, vma=<optimized out>, address=<optimized out>, page_table=<optimized out>, pmd
> >> =<optimized out>, orig_pte=..., ptl=<optimized out>) at mm/memory.c:2178
> >> #2  0x00000000600bc986 in handle_pte_fault (flags=<optimized out>, pmd=<optimized out>, pte=<optimized out>, address=<optimized out>, v
> >> ma=<optimized out>, mm=<optimized out>) at mm/memory.c:3230
> >> #3  __handle_mm_fault (flags=<optimized out>, address=<optimized out>, vma=<optimized out>, mm=<optimized out>) at mm/memory.c:3335
> >> #4  handle_mm_fault (mm=<optimized out>, vma=0x78008e88, address=1462695424, flags=<optimized out>) at mm/memory.c:3364
> >> #5  0x0000000060028cec in handle_page_fault (address=1462695424, ip=<optimized out>, is_write=<optimized out>, is_user=0, code_out=<opt
> >> imized out>) at arch/um/kernel/trap.c:75
> >> #6  0x00000000600290d7 in segv (fi=..., ip=1228924391, is_user=<optimized out>, regs=0x624f5728) at arch/um/kernel/trap.c:222
> >> #7  0x0000000060029395 in segv_handler (sig=<optimized out>, unused_si=<optimized out>, regs=<optimized out>) at arch/um/kernel/trap.c:
> >> 191
> >> #8  0x0000000060039c0f in userspace (regs=0x624f5728) at arch/um/os-Linux/skas/process.c:429
> >> #9  0x0000000060026a8c in fork_handler () at arch/um/kernel/process.c:149
> >> #10 0x0000000000000000 in ?? ()
> >>
> >> backing_dev_info:
> >> p *mapping->backing_dev_info
> >> $2 = {bdi_list = {next = 0x605901a0 <bdi_list>, prev = 0x80a42890}, ra_pages = 32, state = 8, capabilities = 4, congested_fn = 0x0, con
> >> gested_data = 0x0, name = 0x604fb827 "block", bdi_stat = {{count = 4}, {count = 0}, {count = 318691}, {count = 314567}}, bw_time_stamp
> >> = 4339445229, dirtied_stamp = 318686, written_stamp = 314564, write_bandwidth = 166, avg_write_bandwidth = 164, dirty_ratelimit = 1, ba
> >> lanced_dirty_ratelimit = 1, completions = {events = {count = 3}, period = 4481, lock = {raw_lock = {<No data fields>}}}, dirty_exceeded
> >>   = 0, min_ratio = 0, max_ratio = 100, max_prop_frac = 1024, wb = {bdi = 0x80a42278, nr = 0, last_old_flush = 4339445229, dwork = {work
> >> = {data = {counter = 65}, entry = {next = 0x80a42350, prev = 0x80a42350}, func = 0x600f4b25 <bdi_writeback_workfn>}, timer = {entry = {
> >> next = 0x606801a0 <boot_tvec_bases+4896>, prev = 0x803db650}, expires = 4339445730, base = 0x6067ee82 <boot_tvec_bases+2>, function = 0
> >> x60051dbb <delayed_work_timer_fn>, data = 2158240584, slack = -1}, wq = 0x808d9c00, cpu = 1}, b_dirty = {next = 0x7a4ce1f8, prev = 0x80
> >> 6ad9a8}, b_io = {next = 0x80a423c0, prev = 0x80a423c0}, b_more_io = {next = 0x80a423d0, prev = 0x80a423d0}, list_lock = {{rlock = {raw_
> >> lock = {<No data fields>}}}}}, wb_lock = {{rlock = {raw_lock = {<No data fields>}}}}, work_list = {next = 0x80a423e0, prev = 0x80a423e0
> >> }, dev = 0x80b68e00, laptop_mode_wb_timer = {entry = {next = 0x0, prev = 0x0}, expires = 0, base = 0x6067ee80 <boot_tvec_bases>, functi
> >> on = 0x600a6efd <laptop_mode_timer_fn>, data = 2158240008, slack = -1}, debug_dir = 0x80419e58, debug_stats = 0x80419d98}
> >>
> >> when i set the cap_dirty from the backing-dev ( capabilities = 5 ) the system comes back to normal.
> >>
> >> any ideas what's going on here?
> >>
> >> with kind regards
> >> thomas
> >>
> >>
> >>
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> 
> 
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