Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760657Ab2FUXMh (ORCPT ); Thu, 21 Jun 2012 19:12:37 -0400 Received: from fgwmail5.fujitsu.co.jp ([192.51.44.35]:34128 "EHLO fgwmail5.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754897Ab2FUXMf (ORCPT ); Thu, 21 Jun 2012 19:12:35 -0400 X-SecurityPolicyCheck: OK by SHieldMailChecker v1.7.4 Message-ID: <4FE3A998.3000606@jp.fujitsu.com> Date: Fri, 22 Jun 2012 08:09:12 +0900 From: Kamezawa Hiroyuki User-Agent: Mozilla/5.0 (Windows NT 6.0; rv:12.0) Gecko/20120428 Thunderbird/12.0.1 MIME-Version: 1.0 To: Greg Thelen CC: Sha Zhengju , linux-mm@kvack.org, cgroups@vger.kernel.org, yinghan@google.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, mhocko@suse.cz, Sha Zhengju Subject: Re: [PATCH 2/2] memcg: add per cgroup dirty pages accounting References: <1339761611-29033-1-git-send-email-handai.szj@taobao.com> <1339761717-29070-1-git-send-email-handai.szj@taobao.com> <4FDC28F0.8050805@jp.fujitsu.com> <4FE2D2F4.2020202@jp.fujitsu.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7155 Lines: 178 (2012/06/22 1:02), Greg Thelen wrote: > On Thu, Jun 21 2012, Kamezawa Hiroyuki wrote: > >> (2012/06/19 23:31), Sha Zhengju wrote: >>> On Sat, Jun 16, 2012 at 2:34 PM, Kamezawa Hiroyuki >>> wrote: >>>> (2012/06/16 0:32), Greg Thelen wrote: >>>>> >>>>> On Fri, Jun 15 2012, Sha Zhengju wrote: >>>>> >>>>>> This patch adds memcg routines to count dirty pages. I notice that >>>>>> the list has talked about per-cgroup dirty page limiting >>>>>> (http://lwn.net/Articles/455341/) before, but it did not get merged. >>>>> >>>>> >>>>> Good timing, I was just about to make another effort to get some of >>>>> these patches upstream. Like you, I was going to start with some basic >>>>> counters. >>>>> >>>>> Your approach is similar to what I have in mind. While it is good to >>>>> use the existing PageDirty flag, rather than introducing a new >>>>> page_cgroup flag, there are locking complications (see below) to handle >>>>> races between moving pages between memcg and the pages being {un}marked >>>>> dirty. >>>>> >>>>>> I've no idea how is this going now, but maybe we can add per cgroup >>>>>> dirty pages accounting first. This allows the memory controller to >>>>>> maintain an accurate view of the amount of its memory that is dirty >>>>>> and can provide some infomation while group's direct reclaim is working. >>>>>> >>>>>> After commit 89c06bd5 (memcg: use new logic for page stat accounting), >>>>>> we do not need per page_cgroup flag anymore and can directly use >>>>>> struct page flag. >>>>>> >>>>>> >>>>>> Signed-off-by: Sha Zhengju >>>>>> --- >>>>>> include/linux/memcontrol.h | 1 + >>>>>> mm/filemap.c | 1 + >>>>>> mm/memcontrol.c | 32 +++++++++++++++++++++++++------- >>>>>> mm/page-writeback.c | 2 ++ >>>>>> mm/truncate.c | 1 + >>>>>> 5 files changed, 30 insertions(+), 7 deletions(-) >>>>>> >>>>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h >>>>>> index a337c2e..8154ade 100644 >>>>>> --- a/include/linux/memcontrol.h >>>>>> +++ b/include/linux/memcontrol.h >>>>>> @@ -39,6 +39,7 @@ enum mem_cgroup_stat_index { >>>>>> MEM_CGROUP_STAT_FILE_MAPPED, /* # of pages charged as file rss */ >>>>>> MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */ >>>>>> MEM_CGROUP_STAT_DATA, /* end of data requires synchronization */ >>>>>> + MEM_CGROUP_STAT_FILE_DIRTY, /* # of dirty pages in page cache */ >>>>>> MEM_CGROUP_STAT_NSTATS, >>>>>> }; >>>>>> >>>>>> diff --git a/mm/filemap.c b/mm/filemap.c >>>>>> index 79c4b2b..5b5c121 100644 >>>>>> --- a/mm/filemap.c >>>>>> +++ b/mm/filemap.c >>>>>> @@ -141,6 +141,7 @@ void __delete_from_page_cache(struct page *page) >>>>>> * having removed the page entirely. >>>>>> */ >>>>>> if (PageDirty(page)&& mapping_cap_account_dirty(mapping)) { >>>>>> + mem_cgroup_dec_page_stat(page, >>>>>> MEM_CGROUP_STAT_FILE_DIRTY); >>>>> >>>>> >>>>> You need to use mem_cgroup_{begin,end}_update_page_stat around critical >>>>> sections that: >>>>> 1) check PageDirty >>>>> 2) update MEM_CGROUP_STAT_FILE_DIRTY counter >>>>> >>>>> This protects against the page from being moved between memcg while >>>>> accounting. Same comment applies to all of your new calls to >>>>> mem_cgroup_{dec,inc}_page_stat. For usage pattern, see >>>>> page_add_file_rmap. >>>>> >>>> >>>> If you feel some difficulty with mem_cgroup_{begin,end}_update_page_stat(), >>>> please let me know...I hope they should work enough.... >>>> >>> >>> Hi, Kame >>> >>> While digging into the bigger lock of mem_cgroup_{begin,end}_update_page_stat(), >>> I find the reality is more complex than I thought. Simply stated, >>> modifying page info >>> and update page stat may be wide apart and in different level (eg. >>> mm&fs), so if we >>> use the big lock it may lead to scalability and maintainability issues. >>> >>> For example: >>> mem_cgroup_begin_update_page_stat() >>> modify page information => TestSetPageDirty in　ceph_set_page_dirty() (fs/ceph/addr.c) >>> XXXXXX => other fs operations >>> mem_cgroup_update_page_stat() => account_page_dirtied() in　mm/page-writeback.c >>> mem_cgroup_end_update_page_stat(). >>> >>> We can choose to get lock in higher level meaning vfs set_page_dirty() >>> but this may span >>> too much and can also have some missing cases. >>> What's your opinion of this problem? >>> >> >> yes, that's sad....If set_page_dirty() is always called under lock_page(), the >> story will be easier (we'll take lock_page() in move side.) >> but the comment on set_page_dirty() says it's not true.....Now, I haven't found a magical >> way for avoiding the race. >> (*) If holding lock_page() in move_account() can be a generic solution, it will be good. >> A proposal from me is a small-start. You can start from adding hooks to a >> generic >> functions as set_page_dirty() and __set_page_dirty_nobuffers(), clear_page_dirty_for_io(). >> >> And see what happens. I guess we can add WARN_ONCE() against callers of update_page_stat() >> who don't take mem_cgroup_begin/end_update_page_stat() >> (by some new check, for example, checking !rcu_read_lock_held() in update_stat()) >> >> I think we can make TODO list and catch up remaining things one by one. >> >> Thanks, >> -Kame > > This might be a crazy idea. Synchronization of PageDirty with the > page->memcg->nr_dirty counter is a challenge because page->memcg can be > reassigned due to inter-memcg page moving. Yes. That's the heart of the problem. > Could we avoid moving dirty pages between memcg? How to detect it is the proebm here.... > Specifically, could we make them clean before moving. I considered that but a case CPU-A CPU-B wait_for_page_cleaned ..... SetPageDirty() account-memcg-nr_dirty is problematic. _If_ CPU-A lock_page() move_page_for_accounting() unlock_page() can help 99% of cases, I think this is a choice. But I haven't investigated how many callers of set_page_dirty() holds locks.... (I guess CleraPageDirty() callers are under lock_page() always...by quick look.) If most of callers calls lock_page() or mem_cgroup_begin/end_update....I think adding WARNING(!page_locked(page) || !rcu_read_locked()) to update_stat() will be a proof of concept and automatically shows what we should do more... > This problem feels similar to page migration. This would slow > down inter-memcg page movement, because it would require writeback. But > I'm suspect that this is an infrequent operation. I agree. But, IIUC, the reason page-migration waits for the end of I/O is that migrating pages under I/O (in being copied by devices) seems crazy. So, just lock_page() will be an enough help.... Thanks, -Kame -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/