Re: [PATCH 2/2] mm: compaction: Minimise the time IRQs are disabledwhile isolating pages for migration

From: KAMEZAWA Hiroyuki
Date: Mon Feb 28 2011 - 04:48:58 EST

On Mon, 28 Feb 2011 09:28:14 +0000
Mel Gorman <mel@xxxxxxxxx> wrote:

> On Mon, Feb 28, 2011 at 02:54:02PM +0900, KAMEZAWA Hiroyuki wrote:
> > On Mon, 28 Feb 2011 06:48:18 +0100
> > Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
> >
> > > On Mon, Feb 28, 2011 at 11:17:46AM +0900, KAMEZAWA Hiroyuki wrote:
> > > > BTW, I forget why we always take zone->lru_lock with IRQ disabled....
> > >
> > > To decrease lock contention in SMP to deliver overall better
> > > performance (not sure how much it helps though). It was supposed to be
> > > hold for a very short time (PAGEVEC_SIZE) to avoid giving irq latency
> > > problems.
> > >
> >
> > memory hotplug uses MIGRATE_ISOLATED migrate types for scanning pfn range
> > without lru_lock. I wonder whether we can make use of it (the function
> > which memory hotplug may need rework for the compaction but migrate_type can
> > be used, I think).
> >
> I don't see how migrate_type would be of any benefit here particularly
> as compaction does not directly affect the migratetype of a pageblock. I
> have not checked closely which part of hotplug you are on about but if
> you're talking about when pages actually get offlined, the zone lock is
> not necessary there because the pages are not on the LRU. In compactions
> case, they are. Did I misunderstand?

memory offline code doesn't take big lru_lock (and call isolate_lru_page())
at picking up migration target pages from LRU. While this, allocation from
the zone is allowed. memory offline is done by mem_section unit.

memory offline does.

1. making a whole section as MIGRATETYPE_ISOLATED.
2. scan pfn within section.
3. find a page on LRU
4. isolate_lru_page() -> take/release lru_lock. ----(*)
5. migrate it.
6. making all pages in the range as RESERVED.

During this, by marking the pageblock as MIGRATETYPE_ISOLATED,

- new allocation will never picks up a page in the range.
- newly freed pages in the range will never be allocated and never in pcp.
- page type of the range will never change.

then, memory offline success.

If (*) seems too heavy anyway and will be no help even if with some batching
as isolate_lru_page_pagevec() or some, okay please forget offlining.

BTW, can't we drop disable_irq() from all lru_lock related codes ?


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