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From: Greg Thelen <gthelen@google.com>
To: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Vivek Goyal <vgoyal@redhat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
	balbir@linux.vnet.ibm.com, Andrea Righi <arighi@develer.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Trond Myklebust <trond.myklebust@fys.uio.no>,
	Suleiman Souhlal <suleiman@google.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Andrew Morton <akpm@linux-foundation.org>,
	containers@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock
Date: Wed, 14 Apr 2010 23:38:30 -0700	[thread overview]
Message-ID: <i2k49b004811004142338je898a741q78a12c902c52724a@mail.gmail.com> (raw)
In-Reply-To: <20100415152104.62593f37.nishimura@mxp.nes.nec.co.jp>

On Wed, Apr 14, 2010 at 11:21 PM, Daisuke Nishimura
<nishimura@mxp.nes.nec.co.jp> wrote:
> On Wed, 14 Apr 2010 21:48:25 -0700, Greg Thelen <gthelen@google.com> wrote:
>> On Wed, Apr 14, 2010 at 7:40 PM, Daisuke Nishimura
>> <nishimura@mxp.nes.nec.co.jp> wrote:
>> > On Wed, 14 Apr 2010 13:14:07 -0700, Greg Thelen <gthelen@google.com> wrote:
>> >> Vivek Goyal <vgoyal@redhat.com> 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 <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> >> >> > On Fri, 19 Mar 2010 08:10:39 +0530
>> >> >> > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>> >> >> >
>> >> >> >> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2010-03-19 10:23:32]:
>> >> >> >>
>> >> >> >> > On Thu, 18 Mar 2010 21:58:55 +0530
>> >> >> >> > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>> >> >> >> >
>> >> >> >> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [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

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  reply	other threads:[~2010-04-15  6:39 UTC|newest]

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-03-14 23:26 [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v7) Andrea Righi
2010-03-14 23:26 ` [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock Andrea Righi
2010-03-15  0:06   ` KAMEZAWA Hiroyuki
2010-03-15 10:00     ` Andrea Righi
2010-03-17  7:04   ` Balbir Singh
2010-03-17 11:58   ` Balbir Singh
2010-03-17 23:54     ` KAMEZAWA Hiroyuki
2010-03-18  0:45       ` KAMEZAWA Hiroyuki
2010-03-18  2:16         ` Daisuke Nishimura
2010-03-18  2:58           ` KAMEZAWA Hiroyuki
2010-03-18  5:12           ` Balbir Singh
2010-03-18  4:19       ` Balbir Singh
2010-03-18  4:21         ` KAMEZAWA Hiroyuki
2010-03-18  6:25           ` Balbir Singh
2010-03-18  4:35         ` KAMEZAWA Hiroyuki
2010-03-18 16:28           ` Balbir Singh
2010-03-19  1:23             ` KAMEZAWA Hiroyuki
2010-03-19  2:40               ` Balbir Singh
2010-03-19  3:00                 ` KAMEZAWA Hiroyuki
     [not found]                   ` <xr93hbnepmj6.fsf@ninji.mtv.corp.google.com>
2010-04-14  6:55                     ` Greg Thelen
2010-04-14  9:29                       ` KAMEZAWA Hiroyuki
2010-04-14 14:04                         ` Vivek Goyal
2010-04-14 19:31                           ` Greg Thelen
2010-04-15  0:14                           ` KAMEZAWA Hiroyuki
2010-04-14 16:22                         ` Greg Thelen
2010-04-15  0:22                           ` KAMEZAWA Hiroyuki
2010-04-14 14:05                       ` Vivek Goyal
2010-04-14 20:14                         ` Greg Thelen
2010-04-15  2:40                           ` Daisuke Nishimura
2010-04-15  4:48                             ` Greg Thelen
2010-04-15  6:21                               ` Daisuke Nishimura
2010-04-15  6:38                                 ` Greg Thelen [this message]
2010-04-15  6:54                                 ` KAMEZAWA Hiroyuki
2010-04-23 20:17                                   ` Greg Thelen
2010-04-23 20:54                                     ` Peter Zijlstra
2010-04-24 15:53                                       ` Greg Thelen
2010-04-23 20:57                                     ` Peter Zijlstra
2010-04-24  2:22                                       ` KAMEZAWA Hiroyuki
2010-04-23 21:19                                     ` Peter Zijlstra
2010-04-24  2:19                                     ` KAMEZAWA Hiroyuki
2010-04-14 14:44                       ` Balbir Singh
2010-03-14 23:26 ` [PATCH -mmotm 2/5] memcg: dirty memory documentation Andrea Righi
2010-03-16  7:41   ` Daisuke Nishimura
2010-03-17 17:48     ` Greg Thelen
2010-03-17 19:02       ` Balbir Singh
2010-03-17 22:43     ` Andrea Righi
2010-03-14 23:26 ` [PATCH -mmotm 3/5] page_cgroup: introduce file cache flags Andrea Righi
2010-03-14 23:26 ` [PATCH -mmotm 4/5] memcg: dirty pages accounting and limiting infrastructure Andrea Righi
2010-03-15  2:26   ` KAMEZAWA Hiroyuki
2010-03-16  2:32   ` Daisuke Nishimura
2010-03-16 14:11     ` Vivek Goyal
2010-03-16 15:09       ` Daisuke Nishimura
2010-03-17 22:37       ` Andrea Righi
2010-03-17 22:52     ` Andrea Righi
2010-03-18  6:48   ` Greg Thelen
2010-03-14 23:26 ` [PATCH -mmotm 5/5] memcg: dirty pages instrumentation Andrea Righi
2010-03-15  2:31   ` KAMEZAWA Hiroyuki
2010-03-15  2:36 ` [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v7) KAMEZAWA Hiroyuki
2010-03-15 10:02   ` Andrea Righi
2010-03-15 17:12 ` Vivek Goyal
2010-03-15 17:19   ` Vivek Goyal
2010-03-17 11:54     ` Balbir Singh
2010-03-17 13:34       ` Vivek Goyal
2010-03-17 18:53         ` Balbir Singh
2010-03-17 19:15           ` Vivek Goyal
2010-03-17 19:17         ` Balbir Singh
2010-03-17 19:48           ` Vivek Goyal
2010-03-17  6:44 ` Balbir Singh
  -- strict thread matches above, loose matches on Subject: below --
2010-03-09 23:00 [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v6) Andrea Righi
2010-03-09 23:00 ` [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock Andrea Righi

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