Re: zram OOM behavior

[Minchan Kim <minchan@kernel.org>]

On Tue, Oct 30, 2012 at 06:06:56PM -0700, Luigi Semenzato wrote:
> On Tue, Oct 30, 2012 at 5:57 PM, Minchan Kim <minchan@kernel.org> wrote:
> > Hi Luigi,
> >
> > On Tue, Oct 30, 2012 at 12:12:02PM -0700, Luigi Semenzato wrote:
> >> On Mon, Oct 29, 2012 at 10:41 PM, David Rientjes <rientjes@google.com> wrote:
> >> > On Mon, 29 Oct 2012, Luigi Semenzato wrote:
> >> >
> >> >> However, now there is something that worries me more.  The trace of
> >> >> the thread with TIF_MEMDIE set shows that it has executed most of
> >> >> do_exit() and appears to be waiting to be reaped.  From my reading of
> >> >> the code, this implies that task->exit_state should be non-zero, which
> >> >> means that select_bad_process should have skipped that thread, which
> >> >> means that we cannot be in the deadlock situation, and my experiments
> >> >> are not consistent.
> >> >>
> >> >
> >> > Yeah, this is what I was referring to earlier, select_bad_process() will
> >> > not consider the thread for which you posted a stack trace for oom kill,
> >> > so it's not deferring because of it.  There are either other thread(s)
> >> > that have been oom killed and have not yet release their memory or the oom
> >> > killer is never being called.
> >>
> >> Thanks.  I now have better information on what's happening.
> >>
> >> The "culprit" is not the OOM-killed process (the one with TIF_MEMDIE
> >> set).  It's another process that's exiting for some other reason.
> >>
> >> select_bad_process() checks for thread->exit_state at the beginning,
> >> and skips processes that are exiting.  But later it checks for
> >> p->flags & PF_EXITING, and can return -1 in that case (and it does for
> >> me).
> >>
> >> It turns out that do_exit() does a lot of things between setting the
> >> thread->flags PF_EXITING bit (in exit_signals()) and setting
> >> thread->exit_state to non-zero (in exit_notify()).  Some of those
> >> things apparently need memory.  I caught one process responsible for
> >> the PTR_ERR(-1) while it was doing this:
> >>
> >> [  191.859358] VC manager      R running      0  2388   1108 0x00000104
> >> [  191.859377] err_ptr_count = 45623
> >> [  191.859384]  e0611b1c 00200086 f5608000 815ecd20 815ecd20 a0a9ebc3
> >> 0000002c f67cfd20
> >> [  191.859407]  f430a060 81191c34 e0611aec 81196d79 4168ef20 00000001
> >> e1302400 e130264c
> >> [  191.859428]  e1302400 e0611af4 813b71d5 e0611b00 810b42f1 e1302400
> >> e0611b0c 810b430e
> >> [  191.859450] Call Trace:
> >> [  191.859465]  [<81191c34>] ? __delay+0xe/0x10
> >> [  191.859478]  [<81196d79>] ? do_raw_spin_lock+0xa2/0xf3
> >> [  191.859491]  [<813b71d5>] ? _raw_spin_unlock+0xd/0xf
> >> [  191.859504]  [<810b42f1>] ? put_super+0x26/0x29
> >> [  191.859515]  [<810b430e>] ? drop_super+0x1a/0x1d
> >> [  191.859527]  [<8104512d>] __cond_resched+0x1b/0x2b
> >> [  191.859537]  [<813b67a7>] _cond_resched+0x18/0x21
> >> [  191.859549]  [<81093940>] shrink_slab+0x224/0x22f
> >> [  191.859562]  [<81095a96>] try_to_free_pages+0x1b7/0x2e6
> >> [  191.859574]  [<8108df2a>] __alloc_pages_nodemask+0x40a/0x61f
> >> [  191.859588]  [<810a9dbe>] read_swap_cache_async+0x4a/0xcf
> >> [  191.859600]  [<810a9ea4>] swapin_readahead+0x61/0x8d
> >> [  191.859612]  [<8109fff4>] handle_pte_fault+0x310/0x5fb
> >> [  191.859624]  [<810a0420>] handle_mm_fault+0xae/0xbd
> >> [  191.859637]  [<8101d0f9>] do_page_fault+0x265/0x284
> >> [  191.859648]  [<8104aa17>] ? dequeue_entity+0x236/0x252
> >> [  191.859660]  [<8101ce94>] ? vmalloc_sync_all+0xa/0xa
> >> [  191.859672]  [<813b7887>] error_code+0x67/0x6c
> >> [  191.859683]  [<81191d21>] ? __get_user_4+0x11/0x17
> >> [  191.859695]  [<81059f28>] ? exit_robust_list+0x30/0x105
> >> [  191.859707]  [<813b71b0>] ? _raw_spin_unlock_irq+0xd/0x10
> >> [  191.859718]  [<810446d5>] ? finish_task_switch+0x53/0x89
> >> [  191.859730]  [<8102351d>] mm_release+0x1d/0xc3
> >> [  191.859740]  [<81026ce9>] exit_mm+0x1d/0xe9
> >> [  191.859750]  [<81032b87>] ? exit_signals+0x57/0x10a
> >> [  191.859760]  [<81028082>] do_exit+0x19b/0x640
> >> [  191.859770]  [<81058598>] ? futex_wait_queue_me+0xaa/0xbe
> >> [  191.859781]  [<81030bbf>] ? recalc_sigpending_tsk+0x51/0x5c
> >> [  191.859793]  [<81030beb>] ? recalc_sigpending+0x17/0x3e
> >> [  191.859803]  [<81028752>] do_group_exit+0x63/0x86
> >> [  191.859813]  [<81032b19>] get_signal_to_deliver+0x434/0x44b
> >> [  191.859825]  [<81001e01>] do_signal+0x37/0x4fe
> >> [  191.859837]  [<81048eed>] ? set_next_entity+0x36/0x9d
> >> [  191.859850]  [<81050d8e>] ? timekeeping_get_ns+0x11/0x55
> >> [  191.859861]  [<8105a754>] ? sys_futex+0xcb/0xdb
> >> [  191.859871]  [<810024a7>] do_notify_resume+0x26/0x65
> >> [  191.859883]  [<813b73a5>] work_notifysig+0xa/0x11
> >> [  191.859893] Kernel panic - not syncing: too many ERR_PTR
> >>
> >> I don't know why mm_release() would page fault, but it looks like it does.
> >>
> >> So the OOM killer will not kill other processes because it thinks a
> >> process is exiting, which will free up memory.  But the exiting
> >> process needs memory to continue exiting --> deadlock.  Sounds
> >> plausible?
> >
> > It sounds right in your kernel but principal problem is min_filelist_kbytes patch.
> > If normal exited process in exit path requires a page and there is no free page
> > any more, it ends up going to OOM path after try to reclaim memory several time.
> > Then,
> > In select_bad_process,
> >
> >         if (task->flags & PF_EXITING) {
> >                if (task == current)             <== true
> >                         return OOM_SCAN_SELECT;
> > In oom_kill_process,
> >
> >         if (p->flags & PF_EXITING)
> >                 set_tsk_thread_flag(p, TIF_MEMDIE);
> >
> > At last, normal exited process would get a free page.
> >
> > But in your kernel, it seems not because I guess did_some_progress in
> > __alloc_pages_direct_reclaim is never 0. The why it is never 0 is
> > do_try_to_free_pages's all_unreclaimable can't do his role by your
> > min_filelist_kbytes. It makes __alloc_pages_slowpath's looping forever.
> >
> > Sounds plausible?
> 
> Thank you Minchan, it does sound plausible, but I have little
> experience with this and it will take some work to confirm.

No problem :)

> 
> I looked at the patch pretty carefully once, and I had the impression
> its effect could be fully analyzed by logical reasoning. I will check
> this again tomorrow, perhaps I can run some experiments.  I am adding
> Mandeep who wrote the patch.
> 
> However, we have worse problems if we don't use that patch.  Without
> the patch, and either with or without compressed swap, the same load
> causes horrible thrashing, with the system appearing to hang for
> minutes.  If we don't use that patch, do you have any suggestion on
> how to improve the code thrash situation?

As I said, the motivation of the patch is good for embedded system but
patch's implementation is kinda buggy. I will have a look and post if 
I'm luck to get a time.

BTW, a question.

How do you find proper value for min_filelist_kbytes?
Just experiment with several trial?

Thanks.

> 
> Thanks again!
> 
> >>
> >> OK, now someone is going to fix this, right? :-)
> >>
> >> --
> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> >> the body to majordomo@kvack.org.  For more info on Linux MM,
> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> >
> > --
> > Kind regards,
> > Minchan Kim
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>