Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

[Wu Fengguang]

On Tue, Apr 28, 2009 at 08:55:07AM +0200, Ingo Molnar wrote:
> 
> * Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> 
> > Export 9 page flags in /proc/kpageflags, and 8 more for kernel developers.
> > 
> > 1) for kernel hackers (on CONFIG_DEBUG_KERNEL)
> >    - all available page flags are exported, and
> >    - exported as is
> > 2) for admins and end users
> >    - only the more `well known' flags are exported:
> > 	11. KPF_MMAP		(pseudo flag) memory mapped page
> > 	12. KPF_ANON		(pseudo flag) memory mapped page (anonymous)
> > 	13. KPF_SWAPCACHE	page is in swap cache
> > 	14. KPF_SWAPBACKED	page is swap/RAM backed
> > 	15. KPF_COMPOUND_HEAD	(*)
> > 	16. KPF_COMPOUND_TAIL	(*)
> > 	17. KPF_UNEVICTABLE	page is in the unevictable LRU list
> > 	18. KPF_HWPOISON	hardware detected corruption
> > 	19. KPF_NOPAGE		(pseudo flag) no page frame at the address
> > 
> > 	(*) For compound pages, exporting _both_ head/tail info enables
> > 	    users to tell where a compound page starts/ends, and its order.
> > 
> >    - limit flags to their typical usage scenario, as indicated by KOSAKI:
> > 	- LRU pages: only export relevant flags
> > 		- PG_lru
> > 		- PG_unevictable
> > 		- PG_active
> > 		- PG_referenced
> > 		- page_mapped()
> > 		- PageAnon()
> > 		- PG_swapcache
> > 		- PG_swapbacked
> > 		- PG_reclaim
> > 	- no-IO pages: mask out irrelevant flags
> > 		- PG_dirty
> > 		- PG_uptodate
> > 		- PG_writeback
> > 	- SLAB pages: mask out overloaded flags:
> > 		- PG_error
> > 		- PG_active
> > 		- PG_private
> > 	- PG_reclaim: mask out the overloaded PG_readahead
> > 	- compound flags: only export huge/gigantic pages
> > 
> > Here are the admin/linus views of all page flags on a newly booted nfs-root system:
> > 
> > # ./page-types # for admin
> >          flags  page-count       MB  symbolic-flags                     long-symbolic-flags
> > 0x000000000000      491174     1918  ____________________________                
> > 0x000000000020           1        0  _____l______________________       lru      
> > 0x000000000028        2543        9  ___U_l______________________       uptodate,lru
> > 0x00000000002c        5288       20  __RU_l______________________       referenced,uptodate,lru
> > 0x000000004060           1        0  _____lA_______b_____________       lru,active,swapbacked
> 
> I think i have to NAK this kind of ad-hoc instrumentation of kernel 
> internals and statistics until we clear up why such instrumentation 
> measures are being accepted into the MM while other, more dynamic 
> and more flexible MM instrumentation are being resisted by Andrew.

An unexpected NAK - to throw away an orange because we are to have an apple? ;-)

Anyway here are the missing rationals.

1) FAST

It takes merely 0.2s to scan 4GB pages:

        ./page-types  0.02s user 0.20s system 99% cpu 0.216 total

2) SIMPLE

/proc/kpageflags will be a *long standing* hack we have to live with -
it was originally introduced by Matt to do shared memory accounting and
a facility to analyze applications' memory consumptions, with the hope
it will also help kernel developers someday.

So why not extend and embrace it, in a straightforward way?

3) USE CASES

I have/will take advantage of the above page-types command in a number ways:
- to help track down memory leak (the recent trace/ring_buffer.c case)
- to estimate the system wide readahead miss ratio
- Andi want to examine the major page types in different workloads
  (for the hwpoison work)
- Me too, for fun of learning: read/write/lock/whatever a lot of pages
  and examine their flags, to get an idea of some random kernel behaviors.
  (the dynamic tracing tools can be more helpful, as a different view)

4) COMPLEMENTARITY

In some cases the dynamic tracing tool is not enough (or too complex)
to rebuild the current status view.

I myself have a dynamic readahead tracing tool(very useful!).
At the same time I also use readahead accounting numbers, and the
/proc/filecache tool(frequently!), and the above page-types tool.
I simply need them all - they are handy for different cases.

Thanks,
Fengguang

> The above type of condensed information can be built out of dynamic 
> trace data too - and much more. Being able to track page state 
> transitions is very valuable when debugging VM problems. One such 
> 'view' of trace data would be a summary histogram like above.
> 
> ( done after a "echo 3 > /proc/sys/vm/drop_caches" to make sure all 
>   interesting pages have been re-established and their state is 
>   present in the trace. )
> 
> The SLAB code already has such a facility, kmemtrace: it's very 
> useful and successful in visualizing complex SLAB details, both 
> dynamically and statically.
> 
> I think the same general approach should be used for the page 
> allocator too (and for the page cache and some other struct page 
> based caches): the life-time of an object should be followed. If we 
> capture the important details we capture the big picture too. Pekka 
> already sent an RFC patch to extend kmemtrace in such a fashion. Why 
> is that more useful method not being pursued?
> 
> By extending upon the (existing) /proc/kpageflags hack a usecase is 
> taken away from the tracing based solution and a needless overlap is 
> created - and that's not particularly helpful IMHO. We now have all 
> the facilities upstream that allow us to do intelligent 
> instrumentation - we should make use of them.
> 
> 	Ingo
> 
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