* Greg Thelen <gthelen@xxxxxxxxxx> [2010-04-13 23:55:12]:
> On Thu, Mar 18, 2010 at 8:00 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > On Fri, 19 Mar 2010 08:10:39 +0530
> > Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> >
> >> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-03-19 10:23:32]:
> >>
> >> > On Thu, 18 Mar 2010 21:58:55 +0530
> >> > Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> >> >
> >> > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [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;
Since this is just stats can we used deferred updates?
else
update a deferred structure
> } else
> lock_page_cgroup(pc);
>
> __mem_cgroup_update_page_stat(pc, idx, charge);
Do charging + any deferred charges pending in
__mem_cgroup_update_page_stat().
> unlock_page_cgroup(pc);
> }
>
> __mem_cgroup_update_page_stat() has a switch statement that updates all of the
> MEMCG_NR_FILE_{MAPPED,DIRTY,WRITEBACK,WRITEBACK_TEMP,UNSTABLE_NFS} counters
> using the following form:
> switch (idx) {
> case MEMCG_NR_FILE_MAPPED:
> if (charge) {
> if (!PageCgroupFileMapped(pc))
> SetPageCgroupFileMapped(pc);
> else
> val = 0;
> } else {
> if (PageCgroupFileMapped(pc))
> ClearPageCgroupFileMapped(pc);
> else
> val = 0;
> }
> idx = MEM_CGROUP_STAT_FILE_MAPPED;
> break;
>
> ...
> }
>
> /*
> * Preemption is already disabled. We can use __this_cpu_xxx
> */
> if (val > 0) {
> __this_cpu_inc(mem->stat->count[idx]);
> } else if (val < 0) {
> __this_cpu_dec(mem->stat->count[idx]);
> }
>
> In my current tree, irq is never saved/restored by cgroup locking code. To
> protect against interrupt reentrancy, trylock_page_cgroup() is used. As the
> comment indicates, this makes the new counters fuzzy.
>
--
Three Cheers,
Balbir
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