Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751535Ab0DOGjG (ORCPT ); Thu, 15 Apr 2010 02:39:06 -0400 Received: from smtp-out.google.com ([74.125.121.35]:19107 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751325Ab0DOGjE convert rfc822-to-8bit (ORCPT ); Thu, 15 Apr 2010 02:39:04 -0400 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=mime-version:in-reply-to:references:from:date:message-id: subject:to:cc:content-type:content-transfer-encoding:x-system-of-record; b=m0+ToWr2xDE+/hS43uvSuRN4C4Vqg48RB6vEDviYcRsHXhwAYqhy4HfVjHsOZ0w+a bXqpT2fd18YVGHTx9pDCw== MIME-Version: 1.0 In-Reply-To: <20100415152104.62593f37.nishimura@mxp.nes.nec.co.jp> References: <1268609202-15581-2-git-send-email-arighi@develer.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> <20100415152104.62593f37.nishimura@mxp.nes.nec.co.jp> From: Greg Thelen Date: Wed, 14 Apr 2010 23:38:30 -0700 Message-ID: Subject: Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock To: Daisuke Nishimura 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 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5105 Lines: 104 On Wed, Apr 14, 2010 at 11:21 PM, Daisuke Nishimura wrote: > 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. >> >> >> > ? ? ? ?- don't modify codes around FILE_MAPPED in this series. >> >> >> > ? ? ? ?- add a new functions for new statistics >> >> >> > Then, >> >> >> > ? ? ? ?- 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. ?In doing so I have to address this locking issue. ?Does >> >> >> the following look good? ?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, >> >> >> ? ? ? ? ? ? ? ? ? ?enum mem_cgroup_write_page_stat_item idx, bool charge) >> >> >> { >> >> >> ? ?static int seq; >> >> >> ? ?struct page_cgroup *pc; >> >> >> >> >> >> ? ?if (mem_cgroup_disabled()) >> >> >> ? ? ? ? ? ?return; >> >> >> ? ?pc = lookup_page_cgroup(page); >> >> >> ? ?if (!pc || mem_cgroup_is_root(pc->mem_cgroup)) >> >> >> ? ? ? ? ? ?return; >> >> >> >> >> >> ? ?/* >> >> >> ? ? * This routine does not disable irq when updating stats. ?So it is >> >> >> ? ? * possible that a stat update from within interrupt routine, could >> >> >> ? ? * deadlock. ?Use trylock_page_cgroup() to avoid such deadlock. ?This >> >> >> ? ? * makes the memcg counters fuzzy. ?More complicated, or lower >> >> >> ? ? * performing locking solutions avoid this fuzziness, but are not >> >> >> ? ? * currently needed. >> >> >> ? ? */ >> >> >> ? ?if (irqs_disabled()) { >> >> > ? ? ? ? ? ? ^^^^^^^^^ >> >> > Or may be in_interrupt()? >> >> >> >> Good catch. ?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". I see your point. Thank you for explaining. -- Greg -- 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/