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SubjectRe: [PATCH mmotm 2.5/4] memcg: disable irq at page cgroup lock (Re: [PATCH -mmotm 3/4] memcg: dirty pages accounting and limiting infrastructure)
On Thu, 11 Mar 2010 16:50:20 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> On Thu, 11 Mar 2010 15:15:11 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > On Thu, 11 Mar 2010 14:13:00 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >
> > > On Thu, 11 Mar 2010 13:58:47 +0900
> > > Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> > > > > I'll consider yet another fix for race in account migration if I can.
> > > > >
> > > > me too.
> > > >
> > >
> > > How about this ? Assume that the race is very rare.
> > >
> > > 1. use trylock when updating statistics.
> > > If trylock fails, don't account it.
> > >
> > > 2. add PCG_FLAG for all status as
> > >
> > > + PCG_ACCT_FILE_MAPPED, /* page is accounted as file rss*/
> > > + PCG_ACCT_DIRTY, /* page is dirty */
> > > + PCG_ACCT_WRITEBACK, /* page is being written back to disk */
> > > + PCG_ACCT_WRITEBACK_TEMP, /* page is used as temporary buffer for FUSE */
> > > + PCG_ACCT_UNSTABLE_NFS, /* NFS page not yet committed to the server */
> > >
> > > 3. At reducing counter, check PCG_xxx flags by
> > > TESTCLEARPCGFLAG()
> > >
> > > This is similar to an _used_ method of LRU accounting. And We can think this
> > > method's error-range never go too bad number.
> > >
> I agree with you. I've been thinking whether we can remove page cgroup lock
> in update_stat as we do in lru handling codes.
>
> > > I think this kind of fuzzy accounting is enough for writeback status.
> > > Does anyone need strict accounting ?
> > >
> >
> IMHO, we don't need strict accounting.
>
> > How this looks ?
> I agree to this direction. One concern is we re-introduce "trylock" again..
>
Yes, it's my concern, too.


> Some comments are inlined.

> > + 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;
> > + }
> Using !TestSetPageCgroupFileMapped(pc) or TestClearPageCgroupFileMapped(pc) is better ?
>

I used this style because we're under lock. (IOW, to show we're guarded by lock.)


> > + idx = MEM_CGROUP_STAT_FILE_MAPPED;
> > + break;
> > + default:
> > + BUG();
> > + break;
> > + }
> > /*
> > * Preemption is already disabled. We can use __this_cpu_xxx
> > */
> > - __this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED], val);
> > + __this_cpu_add(mem->stat->count[idx], val);
> > +}
> >
> > -done:
> > - unlock_page_cgroup(pc);
> > +void mem_cgroup_update_stat(struct page *page, int idx, bool charge)
> > +{
> > + struct page_cgroup *pc;
> > +
> > + pc = lookup_page_cgroup(page);
> > + if (unlikely(!pc))
> > + return;
> > +
> > + if (trylock_page_cgroup(pc)) {
> > + __mem_cgroup_update_stat(pc, idx, charge);
> > + unlock_page_cgroup(pc);
> > + }
> > + return;
> > +}
> > +
> > +static void mem_cgroup_migrate_stat(struct page_cgroup *pc,
> > + struct mem_cgroup *from, struct mem_cgroup *to)
> > +{
> > + preempt_disable();
> > + if (PageCgroupFileMapped(pc)) {
> > + __this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > + __this_cpu_inc(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > + }
> > + preempt_enable();
> > +}
> > +
> I think preemption is already disabled here too(by lock_page_cgroup()).
>
Ah, yes.


> > +static void
> > +__mem_cgroup_stat_fixup(struct page_cgroup *pc, struct mem_cgroup *mem)
> > +{
> > + /* We'are in uncharge() and lock_page_cgroup */
> > + if (PageCgroupFileMapped(pc)) {
> > + __this_cpu_dec(mem->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > + ClearPageCgroupFileMapped(pc);
> > + }
> > }
> >
> ditto.
>
ok.

> > /*
> > @@ -1810,13 +1859,7 @@ static void __mem_cgroup_move_account(st
> > VM_BUG_ON(pc->mem_cgroup != from);
> >
> > page = pc->page;
> > - if (page_mapped(page) && !PageAnon(page)) {
> > - /* Update mapped_file data for mem_cgroup */
> > - preempt_disable();
> > - __this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > - __this_cpu_inc(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> > - preempt_enable();
> > - }
> > + mem_cgroup_migrate_stat(pc, from, to);
> > mem_cgroup_charge_statistics(from, pc, false);
> > if (uncharge)
> > /* This is not "cancel", but cancel_charge does all we need. */
> I welcome this fixup. IIUC, we have stat leak in current implementation.
>

If necessary, I'd like to prepare fixed one as independent patch for mmotm.

Thanks,
-Kame



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