[Devel] Re: [PATCH -mmotm 1/5] memcg: disable irq at page cgroup lock

[Greg Thelen]

Vivek Goyal <vgoyal at redhat.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 at google.com> wrote:
>> 
>> > On Thu, Mar 18, 2010 at 8:00 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
>> > > On Fri, 19 Mar 2010 08:10:39 +0530
>> > > Balbir Singh <balbir at linux.vnet.ibm.com> wrote:
>> > >
>> > >> * KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> [2010-03-19 10:23:32]:
>> > >>
>> > >> > On Thu, 18 Mar 2010 21:58:55 +0530
>> > >> > Balbir Singh <balbir at linux.vnet.ibm.com> wrote:
>> > >> >
>> > >> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu at 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()) {
>> > 		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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