Re: [PATCH] Add tracepoints to track pagecache transition

[KOSAKI Motohiro]

Hi


In my 1st impression, this patch description is a bit strange.

> The below patch adds instrumentation for pagecache.
> 
> I thought it would be useful to trace pagecache behavior for problem
> analysis (performance bottlenecks, behavior differences between stable
> time and trouble time).
> 
> By using those tracepoints, we can describe and visualize pagecache
> transition (file-by-file basis) in kernel and  pagecache
> consumes most of the memory in running system and pagecache hit rate
> and writeback behavior will influence system load and performance.

Why do you think this tracepoint describe pagecache hit rate?
and, why describe writeback behavior?

> 
> I attached an example which is visualization of pagecache status using
> SystemTap. 

it seems no attached. and SystemTap isn't used kernel developer at all.
I don't think it's enough explanation.
Can you make seekwatcher liked completed comsumer program?
(if you don't know seekwatcher, see http://oss.oracle.com/~mason/seekwatcher/)

> That graph describes pagecache transition of File A and File B
> on a file-by-file basis with the situation where regular I/O to File A
> is delayed because of other I/O to File B. 

If you want to see I/O activity, you need to add tracepoint into block layer.

> We visually understand
> pagecache for File A is narrowed down due to I/O pressure from File B.

confused. Can we assume the number of anon pages/files pages ratio don't chage?


> Peter Zijlstra wrote:
> > On Tue, 2009-02-17 at 18:00 +0900, Atsushi Tsuji wrote:
> > 
> >> The below patch adds instrumentation for pagecache.
> > 
> > And somehow you forgot to CC any of the mm people.. ;-)
> 
> Hi Peter,
> 
> Ah, sorry.
> Thank you for adding to CC list.
> 
> >> +DECLARE_TRACE(filemap_add_to_page_cache,
> >> +	TPPROTO(struct address_space *mapping, pgoff_t offset),
> >> +	TPARGS(mapping, offset));
> >> +DECLARE_TRACE(filemap_remove_from_page_cache,
> >> +	TPPROTO(struct address_space *mapping),
> >> +	TPARGS(mapping));
> > 
> > This is rather asymmetric, why don't we care about the offset for the
> > removed page?
> > 
> 
> Indeed.
> I added the offset to the argument for the removed page and resend fixed patch.
> 
> Signed-off-by: Atsushi Tsuji <a-tsuji@xxxxxxxxxxxxx>
> ---
> diff --git a/include/trace/filemap.h b/include/trace/filemap.h

please add diffstat.


> new file mode 100644
> index 0000000..a17dc92
> --- /dev/null
> +++ b/include/trace/filemap.h
> @@ -0,0 +1,13 @@
> +#ifndef _TRACE_FILEMAP_H
> +#define _TRACE_FILEMAP_H
> +
> +#include <linux/tracepoint.h>
> +
> +DECLARE_TRACE(filemap_add_to_page_cache,
> +	TPPROTO(struct address_space *mapping, pgoff_t offset),
> +	TPARGS(mapping, offset));
> +DECLARE_TRACE(filemap_remove_from_page_cache,
> +	TPPROTO(struct address_space *mapping, pgoff_t offset),
> +	TPARGS(mapping, offset));
> +
> +#endif
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 23acefe..23f75d2 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -34,6 +34,7 @@
>  #include <linux/hardirq.h> /* for BUG_ON(!in_atomic()) only */
>  #include <linux/memcontrol.h>
>  #include <linux/mm_inline.h> /* for page_is_file_cache() */
> +#include <trace/filemap.h>
>  #include "internal.h"
>  
>  /*
> @@ -43,6 +44,8 @@
>  
>  #include <asm/mman.h>
>  
> +DEFINE_TRACE(filemap_add_to_page_cache);
> +DEFINE_TRACE(filemap_remove_from_page_cache);
>  
>  /*
>   * Shared mappings implemented 30.11.1994. It's not fully working yet,
> @@ -120,6 +123,7 @@ void __remove_from_page_cache(struct page *page)
>  	page->mapping = NULL;
>  	mapping->nrpages--;
>  	__dec_zone_page_state(page, NR_FILE_PAGES);
> +	trace_filemap_remove_from_page_cache(mapping, page->index);

__remove_from_page_cache() is passed struct page.
Why don't you use struct page

and, this mean
  - the page have been removed from mapping.
  - vmstate have been decremented.
  - but, the page haven't been uncharged from memcg.

Why?


>  	BUG_ON(page_mapped(page));
>  	mem_cgroup_uncharge_cache_page(page);
>  
> @@ -475,6 +479,7 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
>  		if (likely(!error)) {
>  			mapping->nrpages++;
>  			__inc_zone_page_state(page, NR_FILE_PAGES);
> +			trace_filemap_add_to_page_cache(mapping, offset);

Why do you select this line?
In general, trace point calling under spin lock grabbing is a bit problematic.


>  		} else {
>  			page->mapping = NULL;
>  			mem_cgroup_uncharge_cache_page(page);
> 

And, both function is freqentlly called one.
I worry about performance issue. can you prove no degression?


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