On Fri, 8 Mar 2013, Joonsoo Kim wrote: > On Thu, Mar 07, 2013 at 10:54:15AM -0800, Hugh Dickins wrote: > > On Thu, 7 Mar 2013, Joonsoo Kim wrote: > > > > > When we found that the flag has a bit of PAGE_FLAGS_CHECK_AT_PREP, > > > we reset the flag. If we always reset the flag, we can reduce one > > > branch operation. So remove it. > > > > > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > > > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > > > > I don't object to this patch. But certainly I would have written it > > that way in order not to dirty a cacheline unnecessarily. It may be > > obvious to you that the cacheline in question is almost always already > > dirty, and the branch almost always more expensive. But I'll leave that > > to you, and to those who know more about these subtle costs than I do. > > Yes. I already think about that. I thought that even if a cacheline is > not dirty at this time, we always touch the 'struct page' in > set_freepage_migratetype() a little later, so dirtying is not the problem. I expect that a very high proportion of user pages have PG_uptodate to be cleared here; and there's also the recently added page_nid_reset_last(), which will dirty the flags or a nearby field when CONFIG_NUMA_BALANCING. Those argue in favour of your patch. > > But, now, I re-think this and decide to drop this patch. > The reason is that 'struct page' of 'compound pages' may not be dirty > at this time and will not be dirty at later time. Actual compound pages would have PG_head or PG_tail or PG_compound to be cleared there, I believe (check if I'm right on that). The questionable case is the ordinary order>0 case without __GFP_COMP (and page_nid_reset_last() is applied to each subpage of those). > So this patch is bad idea. I'm not so sure. I doubt your patch will make a giant improvement in kernel performance! But it might make a little - maybe you just need to give some numbers from perf to justify it (but I'm easily dazzled by numbers - don't expect me to judge the result). Hugh > > Is there any comments? > > Thanks. > > > Hugh > > > > > > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > > > index 8fcced7..778f2a9 100644 > > > --- a/mm/page_alloc.c > > > +++ b/mm/page_alloc.c > > > @@ -614,8 +614,7 @@ static inline int free_pages_check(struct page *page) > > > return 1; > > > } > > > page_nid_reset_last(page); > > > - if (page->flags & PAGE_FLAGS_CHECK_AT_PREP) > > > - page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP; > > > + page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP; > > > return 0; > > > } > > > > > > > -- > > To unsubscribe, send a message with 'unsubscribe linux-mm' in > > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > > see: http://www.linux-mm.org/ . > > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>