RE: [PATCH 1/2] sched/wait: Break up long wake list walk

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> On Mon, Aug 14, 2017 at 5:52 PM, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> wrote:
> > We encountered workloads that have very long wake up list on large
> > systems. A waker takes a long time to traverse the entire wake list
> > and execute all the wake functions.
> >
> > We saw page wait list that are up to 3700+ entries long in tests of
> > large
> > 4 and 8 socket systems.  It took 0.8 sec to traverse such list during
> > wake up.  Any other CPU that contends for the list spin lock will spin
> > for a long time.  As page wait list is shared by many pages so it
> > could get very long on systems with large memory.
> 
> I really dislike this patch.
> 
> The patch seems a band-aid for really horrible kernel behavior, rather than
> fixing the underlying problem itself.
> 
> Now, it may well be that we do end up needing this band-aid in the end, so
> this isn't a NAK of the patch per se. But I'd *really* like to see if we can fix the
> underlying cause for what you see somehow..
> 
> In particular, if this is about the page wait table, maybe we can just make the
> wait table bigger. IOW, are people actually waiting on the
> *same* page, or are they mainly waiting on totally different pages, just
> hashing to the same wait queue?
> 
> Because right now that page wait table is a small fixed size, and the only
> reason it's a small fixed size is that nobody reported any issues with it -
> particularly since we now avoid the wait table entirely for the common cases
> by having that "contention" bit.
> 
> But it really is a *small* table. We literally have
> 
>    #define PAGE_WAIT_TABLE_BITS 8
> 
> so it's just 256 entries. We could easily it much bigger, if we are actually
> seeing a lot of collissions.
> 
> We *used* to have a very complex per-zone thing for bit-waitiqueues, but
> that was because we got lots and lots of contention issues, and everybody
> *always* touched the wait-queues whether they waited or not (so being per-
> zone was a big deal)
> 
> We got rid of all that per-zone complexity when the normal case didn't hit in
> the page wait queues at all, but we may have over-done the simplification a
> bit since nobody showed any issue.
> 
> In particular, we used to size the per-zone thing by amount of memory.
> We could easily re-introduce that for the new simpler page queues.
> 
> The page_waitiqueue() is a simple helper function inside mm/filemap.c, and
> thanks to the per-page "do we have actual waiters" bit that we have now, we
> can actually afford to make it bigger and more complex now if we want to.
> 
> What happens to your load if you just make that table bigger? You can
> literally test by just changing the constant from 8 to 16 or something, making
> us use twice as many bits for hashing. A "real"
> patch would size it by amount of memory, but just for testing the contention
> on your load, you can do the hacky one-liner.

Hi Linus,

We tried both 12 and 16 bit table and that didn't make a difference.
The long wake ups are mostly on the same page when we do instrumentation

Here is the wake_up_page_bit call stack when the workaround is running, which
is collected by perf record -g -a -e probe:wake_up_page_bit -- sleep 10

# To display the perf.data header info, please use --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 374  of event 'probe:wake_up_page_bit'
# Event count (approx.): 374
#
# Overhead  Trace output      
# ........  ..................
#
   100.00%  (ffffffffae1ad000)
            |
            ---wake_up_page_bit
               |          
               |--49.73%--migrate_misplaced_transhuge_page
               |          do_huge_pmd_numa_page
               |          __handle_mm_fault
               |          handle_mm_fault
               |          __do_page_fault
               |          do_page_fault
               |          page_fault
               |          |          
               |          |--28.07%--0x2b7b7
               |          |          |          
               |          |          |--13.64%--0x127a2
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |          |--13.37%--0x127d8
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |          |--0.53%--0x1280e
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |           --0.27%--0x12844
               |          |                     0x7fb5247eddc5
               |          |          
               |          |--18.18%--0x2b788
               |          |          |          
               |          |          |--14.97%--0x127a2
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |          |--1.34%--0x1287a
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |          |--0.53%--0x128b0
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |          |--0.53%--0x1280e
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |          |--0.53%--0x127d8
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |           --0.27%--0x12844
               |          |                     0x7fb5247eddc5
               |          |          
               |          |--1.07%--0x2b823
               |          |          |          
               |          |          |--0.53%--0x127a2
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |          |--0.27%--0x1287a
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |           --0.27%--0x127d8
               |          |                     0x7fb5247eddc5
               |          |          
               |          |--0.80%--0x2b88f
               |          |          |          
               |          |           --0.53%--0x127d8
               |          |                     0x7fb5247eddc5
               |          |          
               |          |--0.80%--0x2b7f4
               |          |          |          
               |          |          |--0.53%--0x127d8
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |           --0.27%--0x127a2
               |          |                     0x7fb5247eddc5
               |          |          
               |          |--0.53%--0x2b8fb
               |          |          0x127a2
               |          |          0x7fb5247eddc5
               |          |          
               |           --0.27%--0x2b8e9
               |                     0x127a2
               |                     0x7fb5247eddc5
               |          
               |--44.12%--__handle_mm_fault
               |          handle_mm_fault
               |          __do_page_fault
               |          do_page_fault
               |          page_fault
               |          |          
               |          |--30.75%--_dl_relocate_object
               |          |          dl_main
               |          |          _dl_sysdep_start
               |          |          0x40
               |          |          
               |           --13.37%--memset
               |                     _dl_map_object
               |                     |          
               |                     |--2.94%--_etext
               |                     |          
               |                     |--0.80%--0x7f34ea294b08
               |                     |          0
               |                     |          
               |                     |--0.80%--0x7f1d5fa64b08
               |                     |          0
               |                     |          
               |                     |--0.53%--0x7fd4c83dbb08
               |                     |          0
               |                     |          
               |                     |--0.53%--0x7efe3724cb08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7ff2cf0b69c0
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7fc9bc22cb08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7fc432971058
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7faf21ec2b08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7faf21ec2640
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f940f08e058
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f4b84122640
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f42c8fd7fd8
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f3f15778fd8
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f3f15776058
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f34ea27dfd8
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f34ea27b058
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f2a0409bb08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f2a04084fd8
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f2a04082058
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f1949633b08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f194961cfd8
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f1629f87b08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f1629f70fd8
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f1629f6e058
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f060696eb08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7f04ac14c9c0
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7efe8b4bbb08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7efe8b4a59c0
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7efe8b4a4fd8
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7efe8b4a2058
               |                     |          0
               |                     |          
               |                     |--0.27%--0x7efcd0c70b08
               |                     |          0
               |                     |          
               |                     |--0.27%--0x207ad8
               |                     |          0
               |                     |          
               |                      --0.27%--0x206b30
               |                                0
               |          
               |--2.14%--filemap_map_pages
               |          __handle_mm_fault
               |          handle_mm_fault
               |          __do_page_fault
               |          do_page_fault
               |          page_fault
               |          |          
               |          |--0.53%--_IO_vfscanf
               |          |          |          
               |          |          |--0.27%--0x6563697665442055
               |          |          |          
               |          |           --0.27%--_IO_vsscanf
               |          |                     0x6563697665442055
               |          |          
               |          |--0.53%--_dl_map_object_from_fd
               |          |          _dl_map_object
               |          |          |          
               |          |          |--0.27%--0x7faf21ec2640
               |          |          |          0
               |          |          |          
               |          |           --0.27%--_etext
               |          |          
               |          |--0.27%--__libc_enable_asynccancel
               |          |          __fopen_internal
               |          |          0x6d6f6f2f30373635
               |          |          
               |          |--0.27%--vfprintf
               |          |          _IO_vsprintf
               |          |          0x4
               |          |          
               |          |--0.27%--0x1fb40
               |          |          0x41d589495541f689
               |          |          
               |           --0.27%--memset@plt
               |          
               |--1.87%--do_huge_pmd_numa_page
               |          __handle_mm_fault
               |          handle_mm_fault
               |          __do_page_fault
               |          do_page_fault
               |          page_fault
               |          |          
               |          |--0.80%--0x2b7b7
               |          |          0x127d8
               |          |          0x7fb5247eddc5
               |          |          
               |          |--0.80%--0x2b788
               |          |          0x127a2
               |          |          0x7fb5247eddc5
               |          |          
               |           --0.27%--0x2b918
               |                     0x127d8
               |                     0x7fb5247eddc5
               |          
               |--1.87%--migrate_pages
               |          migrate_misplaced_page
               |          __handle_mm_fault
               |          handle_mm_fault
               |          __do_page_fault
               |          do_page_fault
               |          page_fault
               |          |          
               |          |--1.07%--0x2b7b7
               |          |          |          
               |          |          |--0.80%--0x127d8
               |          |          |          0x7fb5247eddc5
               |          |          |          
               |          |           --0.27%--0x1287a
               |          |                     0x7fb5247eddc5
               |          |          
               |           --0.80%--0x2b788
               |                     0x127a2
               |                     0x7fb5247eddc5
               |          
                --0.27%--do_wp_page
                          __handle_mm_fault
                          handle_mm_fault
                          __do_page_fault
                          do_page_fault
                          page_fault
                          _IO_link_in

Thanks,
Kan
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