From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail190.messagelabs.com (mail190.messagelabs.com [216.82.249.51]) by kanga.kvack.org (Postfix) with ESMTP id 457916B01EF for ; Thu, 15 Apr 2010 02:29:24 -0400 (EDT) Date: Thu, 15 Apr 2010 15:21:04 +0900 From: Daisuke Nishimura Subject: Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock Message-Id: <20100415152104.62593f37.nishimura@mxp.nes.nec.co.jp> In-Reply-To: References: <1268609202-15581-2-git-send-email-arighi@develer.com> <20100318133527.420b2f25.kamezawa.hiroyu@jp.fujitsu.com> <20100318162855.GG18054@balbir.in.ibm.com> <20100319102332.f1d81c8d.kamezawa.hiroyu@jp.fujitsu.com> <20100319024039.GH18054@balbir.in.ibm.com> <20100319120049.3dbf8440.kamezawa.hiroyu@jp.fujitsu.com> <20100414140523.GC13535@redhat.com> <20100415114022.ef01b704.nishimura@mxp.nes.nec.co.jp> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org To: Greg Thelen Cc: Vivek Goyal , KAMEZAWA Hiroyuki , balbir@linux.vnet.ibm.com, Andrea Righi , Peter Zijlstra , Trond Myklebust , Suleiman Souhlal , "Kirill A. Shutemov" , Andrew Morton , containers@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Daisuke Nishimura List-ID: On Wed, 14 Apr 2010 21:48:25 -0700, Greg Thelen wrote: > On Wed, Apr 14, 2010 at 7:40 PM, Daisuke Nishimura > wrote: > > On Wed, 14 Apr 2010 13:14:07 -0700, Greg Thelen wrote: > >> Vivek Goyal writes: > >> > >> > On Tue, Apr 13, 2010 at 11:55:12PM -0700, Greg Thelen wrote: > >> >> On Thu, Mar 18, 2010 at 8:00 PM, KAMEZAWA Hiroyuki wrote: > >> >> > On Fri, 19 Mar 2010 08:10:39 +0530 > >> >> > Balbir Singh wrote: > >> >> > > >> >> >> * KAMEZAWA Hiroyuki [2010-03-19 10:23:32]: > >> >> >> > >> >> >> > On Thu, 18 Mar 2010 21:58:55 +0530 > >> >> >> > Balbir Singh wrote: > >> >> >> > > >> >> >> > > * KAMEZAWA Hiroyuki [2010-03-18 13:35:27]: > >> >> >> > > >> >> >> > > > Then, no probelm. It's ok to add mem_cgroup_udpate_stat() indpendent from > >> >> >> > > > mem_cgroup_update_file_mapped(). The look may be messy but it's not your > >> >> >> > > > fault. But please write "why add new function" to patch description. > >> >> >> > > > > >> >> >> > > > I'm sorry for wasting your time. > >> >> >> > > > >> >> >> > > Do we need to go down this route? We could check the stat and do the > >> >> >> > > correct thing. In case of FILE_MAPPED, always grab page_cgroup_lock > >> >> >> > > and for others potentially look at trylock. It is OK for different > >> >> >> > > stats to be protected via different locks. > >> >> >> > > > >> >> >> > > >> >> >> > I _don't_ want to see a mixture of spinlock and trylock in a function. > >> >> >> > > >> >> >> > >> >> >> A well documented well written function can help. The other thing is to > >> >> >> of-course solve this correctly by introducing different locking around > >> >> >> the statistics. Are you suggesting the later? > >> >> >> > >> >> > > >> >> > No. As I wrote. > >> >> > A A A A - don't modify codes around FILE_MAPPED in this series. > >> >> > A A A A - add a new functions for new statistics > >> >> > Then, > >> >> > A A A A - think about clean up later, after we confirm all things work as expected. > >> >> > >> >> I have ported Andrea Righi's memcg dirty page accounting patches to latest > >> >> mmtom-2010-04-05-16-09. A In doing so I have to address this locking issue. A Does > >> >> the following look good? A I will (of course) submit the entire patch for review, > >> >> but I wanted make sure I was aiming in the right direction. > >> >> > >> >> void mem_cgroup_update_page_stat(struct page *page, > >> >> A A A A A A A A A A enum mem_cgroup_write_page_stat_item idx, bool charge) > >> >> { > >> >> A A static int seq; > >> >> A A struct page_cgroup *pc; > >> >> > >> >> A A if (mem_cgroup_disabled()) > >> >> A A A A A A return; > >> >> A A pc = lookup_page_cgroup(page); > >> >> A A if (!pc || mem_cgroup_is_root(pc->mem_cgroup)) > >> >> A A A A A A return; > >> >> > >> >> A A /* > >> >> A A * This routine does not disable irq when updating stats. A So it is > >> >> A A * possible that a stat update from within interrupt routine, could > >> >> A A * deadlock. A Use trylock_page_cgroup() to avoid such deadlock. A This > >> >> A A * makes the memcg counters fuzzy. A More complicated, or lower > >> >> A A * performing locking solutions avoid this fuzziness, but are not > >> >> A A * currently needed. > >> >> A A */ > >> >> A A if (irqs_disabled()) { > >> > A A A A A A ^^^^^^^^^ > >> > Or may be in_interrupt()? > >> > >> Good catch. A I will replace irqs_disabled() with in_interrupt(). > >> > > I think you should check both. __remove_from_page_cache(), which will update > > DIRTY, is called with irq disabled(iow, under mapping->tree_lock) but not in > > interrupt context. > > The only reason to use trylock in this case is to prevent deadlock > when running in a context that may have preempted or interrupted a > routine that already holds the bit locked. In the > __remove_from_page_cache() irqs are disabled, but that does not imply > that a routine holding the spinlock has been preempted. When the bit > is locked, preemption is disabled. The only way to interrupt a holder > of the bit for an interrupt to occur (I/O, timer, etc). So I think > that in_interrupt() is sufficient. Am I missing something? > IIUC, it's would be enough to prevent deadlock where one CPU tries to acquire the same page cgroup lock. But there is still some possibility where 2 CPUs can cause dead lock each other(please see the commit e767e056). IOW, my point is "don't call lock_page_cgroup() under mapping->tree_lock". Thanks, Daisuke Nishimura. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org