Messages in this thread | | | Date | Mon, 07 Jan 2013 16:49:27 +0900 | From | Kamezawa Hiroyuki <> | Subject | Re: [PATCH V3 4/8] memcg: add per cgroup dirty pages accounting |
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(2013/01/07 5:02), Hugh Dickins wrote: > On Sat, 5 Jan 2013, Sha Zhengju wrote: >> On Wed, Jan 2, 2013 at 6:44 PM, Michal Hocko <mhocko@suse.cz> wrote: >>> >>> Maybe I have missed some other locking which would prevent this from >>> happening but the locking relations are really complicated in this area >>> so if mem_cgroup_{begin,end}_update_page_stat might be called >>> recursively then we need a fat comment which justifies that. >>> >> >> Ohhh...good catching! I didn't notice there is a recursive call of >> mem_cgroup_{begin,end}_update_page_stat in page_remove_rmap(). >> The mem_cgroup_{begin,end}_update_page_stat() design has depressed >> me a lot recently as the lock granularity is a little bigger than I thought. >> Not only the resource but also some code logic is in the range of locking >> which may be deadlock prone. The problem still exists if we are trying to >> add stat account of other memcg page later, may I make bold to suggest >> that we dig into the lock again... > > Forgive me, I must confess I'm no more than skimming this thread, > and don't like dumping unsigned-off patches on people; but thought > that on balance it might be more helpful than not if I offer you a > patch I worked on around 3.6-rc2 (but have updated to 3.8-rc2 below). > > I too was getting depressed by the constraints imposed by > mem_cgroup_{begin,end}_update_page_stat (good job though Kamezawa-san > did to minimize them), and wanted to replace by something freer, more > RCU-like. In the end it seemed more effort than it was worth to go > as far as I wanted, but I do think that this is some improvement over > what we currently have, and should deal with your recursion issue. > In what case does this improve performance ?
> But if this does appear useful to memcg people, then we really ought > to get it checked over by locking/barrier experts before going further. > I think myself that I've over-barriered it, and could use a little > lighter; but they (Paul McKenney, Peter Zijlstra, Oleg Nesterov come > to mind) will see more clearly, and may just hate the whole thing, > as yet another peculiar lockdep-avoiding hand-crafted locking scheme. > I've not wanted to waste their time on reviewing it, if it's not even > going to be useful to memcg people. > > It may be easier to understand if you just apply the patch and look > at the result in mm/memcontrol.c, where I tried to gather the pieces > together in one place and describe them ("These functions mediate..."). > > Hugh >
Hi, this patch seems interesting but...doesn't this make move_account() very slow if the number of cpus increases because of scanning all cpus per a page ? And this looks like reader-can-block-writer percpu rwlock..it's too heavy to writers if there are many readers.
Thanks, -Kame
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