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

[Greg Thelen]

Vivek Goyal <vgoyal at 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 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()) {
>             ^^^^^^^^^
> Or may be in_interrupt()?

Good catch.  I will replace irqs_disabled() with in_interrupt().

Thank you.

--
Greg
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