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Message-ID: <xr93k4s9ygnk.fsf@ninji.mtv.corp.google.com>
Date: Wed, 14 Apr 2010 12:31:59 -0700
From: Greg Thelen <gthelen@...gle.com>
To: Vivek Goyal <vgoyal@...hat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
balbir@...ux.vnet.ibm.com, Andrea Righi <arighi@...eler.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
Peter Zijlstra <peterz@...radead.org>,
Trond Myklebust <trond.myklebust@....uio.no>,
Suleiman Souhlal <suleiman@...gle.com>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Andrew Morton <akpm@...ux-foundation.org>,
containers@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock
Vivek Goyal <vgoyal@...hat.com> writes:
> On Wed, Apr 14, 2010 at 06:29:04PM +0900, KAMEZAWA Hiroyuki wrote:
>> On Tue, 13 Apr 2010 23:55:12 -0700
>> Greg Thelen <gthelen@...gle.com> wrote:
>>
>> > On Thu, Mar 18, 2010 at 8:00 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
>> > > On Fri, 19 Mar 2010 08:10:39 +0530
>> > > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>> > >
>> > >> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-03-19 10:23:32]:
>> > >>
>> > >> > On Thu, 18 Mar 2010 21:58:55 +0530
>> > >> > Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
>> > >> >
>> > >> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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()) {
>> > if (! trylock_page_cgroup(pc))
>> > return;
>> > } else
>> > lock_page_cgroup(pc);
>> >
>>
>> I prefer trylock_page_cgroup() always.
>>
>> I have another idea fixing this up _later_. (But I want to start from simple one.)
>>
>> My rough idea is following. Similar to your idea which you gave me before.
>>
>> ==
>> DEFINE_PERCPU(account_move_ongoing);
>> DEFINE_MUTEX(move_account_mutex):
>>
>> void memcg_start_account_move(void)
>> {
>> mutex_lock(&move_account_mutex);
>> for_each_online_cpu(cpu)
>> per_cpu(cpu, account_move_ongoing) += 1;
>> mutex_unlock(&move_account_mutex);
>> /* Wait until there are no lockless update */
>> synchronize_rcu();
>> return;
>> }
>>
>> void memcg_end_account_move(void)
>> {
>> mutex_lock(&move_account_mutex);
>> for_each_online_cpu(cpu)
>> per_cpu(cpu, account_move_ongoing) -= 1;
>> mutex_unlock(&move_account_mutex);
>> }
>>
>> /* return 1 when we took lock, return 0 if lockess OPs is guarantedd to be safe */
>> int memcg_start_filecache_accounting(struct page_cgroup *pc)
>> {
>> rcu_read_lock();
>> smp_rmb();
>> if (!this_cpu_read(move_account_ongoing))
>> return 0; /* no move account is ongoing */
>> lock_page_cgroup(pc);
>> return 1;
>> }
>>
>> void memcg_end_filecache_accounting(struct page_cgroup *pc, int unlock)
>> {
>> if (unlock)
>> unlock_page_cgroup(pc);
>>
>> rcu_read_unlock();
>> }
>>
>> and call memcg_start_account_move()/end_account_move() in the start/end of
>> migrainting chunk of pages.
>
> Hi Kame-san,
>
> May be I am missing something but how does it solve the issue of making sure
> lock_page_cgroup() is not held in interrupt context? IIUC, above code will
> make sure that for file cache accouting, lock_page_cgroup() is taken only
> if task migration is on. But say task migration is on, and then some IO
> completes and we update WRITEBACK stat (i think this is the one which can
> be called from interrupt context), then we will still take the
> lock_page_cgroup() and again run into the issue of deadlocks?
>
> Thanks
> Vivek
I agree. I think the lock/unlock_page_cgrpoup() calls suggested by
Kame-san should also include local_irq_save/restore() calls to prevent
the interrupt context deadlock Vivek describes. These new
local_irq_save/restore() calls would only be used if
move_account_ongoing is set. They behave just like the optional calls
to lock/unlock_page_cgroup().
--
Greg
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