[Devel] Re: [PATCH v2] memcg: reduce lock time at move charge (Was Re: [PATCH 04/10] memcg: disable local interrupts in lock_page_cgroup()

[Daisuke Nishimura]

On Thu, 7 Oct 2010 16:14:54 -0700
Andrew Morton <akpm at linux-foundation.org> wrote:

> On Thu, 7 Oct 2010 17:04:05 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> 
> > Now, at task migration among cgroup, memory cgroup scans page table and moving
> > account if flags are properly set.
> > 
> > The core code, mem_cgroup_move_charge_pte_range() does
> > 
> >  	pte_offset_map_lock();
> > 	for all ptes in a page table:
> > 		1. look into page table, find_and_get a page
> > 		2. remove it from LRU.
> > 		3. move charge.
> > 		4. putback to LRU. put_page()
> > 	pte_offset_map_unlock();
> > 
> > for pte entries on a 3rd level? page table.
> > 
> > This pte_offset_map_lock seems a bit long. This patch modifies a rountine as
> > 
> > 	for 32 pages: pte_offset_map_lock()
> > 		      find_and_get a page
> > 		      record it
> > 		      pte_offset_map_unlock()
> > 	for all recorded pages
> > 		      isolate it from LRU.
> > 		      move charge
> > 		      putback to LRU
> > 	for all recorded pages
> > 		      put_page()
> 
> The patch makes the code larger, more complex and slower!
> 
Before this patch:
   text    data     bss     dec     hex filename
  27163   11782    4100   43045    a825 mm/memcontrol.o

After this patch:
   text    data     bss     dec     hex filename
  27307   12294    4100   43701    aab5 mm/memcontrol.o

hmm, allocating mc.target[] statically might be bad, but I'm now wondering
whether I could allocate mc itself dynamically(I'll try).

> I do think we're owed a more complete description of its benefits than
> "seems a bit long".  Have problems been observed?  Any measurements
> taken?
> 
IIUC, this patch is necessary for "[PATCH] memcg: lock-free clear page writeback"
later, but I agree we should describe it.

Thanks,
Daisuke Nishimura.
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