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    SubjectRe: [PATCH 0/4] Memory controller soft limit patches (v3)
    On Mon, 2 Mar 2009 12:06:49 +0530
    Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

    > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-02 15:21:28]:
    >
    > > On Mon, 2 Mar 2009 11:35:19 +0530
    > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    > >
    > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-02 14:32:50]:
    > > >
    > > > > On Mon, 2 Mar 2009 10:10:43 +0530
    > > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    > > > >
    > > > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-02 09:24:04]:
    > > > > >
    > > > > > > On Sun, 01 Mar 2009 11:59:59 +0530
    > > > > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
    > > > > > >
    > > > > > > >
    > > > > > > > From: Balbir Singh <balbir@linux.vnet.ibm.com>
    > > > >
    > > > > > >
    > > > > > > At first, it's said "When cgroup people adds something, the kernel gets slow".
    > > > > > > This is my start point of reviewing. Below is comments to this version of patch.
    > > > > > >
    > > > > > > 1. I think it's bad to add more hooks to res_counter. It's enough slow to give up
    > > > > > > adding more fancy things..
    > > > > >
    > > > > > res_counters was desgined to be extensible, why is adding anything to
    > > > > > it going to make it slow, unless we turn on soft_limits?
    > > > > >
    > > > > You inserted new "if" logic in the core loop.
    > > > > (What I want to say here is not that this is definitely bad but that "isn't there
    > > > > any alternatives which is less overhead.)
    > > > >
    > > > >
    > > > > > >
    > > > > > > 2. please avoid to add hooks to hot-path. In your patch, especially a hook to
    > > > > > > mem_cgroup_uncharge_common() is annoying me.
    > > > > >
    > > > > > If soft limits are not enabled, the function does a small check and
    > > > > > leaves.
    > > > > >
    > > > > &soft_fail_res is passed always even if memory.soft_limit==ULONG_MAX
    > > > > res_counter_soft_limit_excess() adds one more function call and spinlock, and irq-off.
    > > > >
    > > >
    > > > OK, I see that overhead.. I'll figure out a way to work around it.
    > > >
    > > > > > >
    > > > > > > 3. please avoid to use global spinlock more.
    > > > > > > no lock is best. mutex is better, maybe.
    > > > > > >
    > > > > >
    > > > > > No lock to update a tree which is update concurrently?
    > > > > >
    > > > > Using tree/sort itself is nonsense, I believe.
    > > > >
    > > >
    > > > I tried using prio trees in the past, but they are not easy to update
    > > > either. I won't mind asking for suggestions for a data structure that
    > > > can scaled well, allow quick insert/delete and search.
    > > >
    > > Now, because the routine is called by kswapd() not by try_to_free.....
    > >
    > > It's not necessary to be very very fast. That's my point.
    > >
    >
    > OK, I get your point, but whay does that make RB-Tree data structure non-sense?
    >

    1. Until memory-shortage, rb-tree is kept to be updated and the users(kernel)
    has to pay its maintainace/check cost, whici is unnecessary.
    Considering trade-off, paying cost only when memory-shortage happens tend to
    be reasonable way.

    2. Current "exceed" just shows "How much we got over my soft limit" but doesn't
    tell any information per-node/zone. Considering this, this rb-tree
    information will not be able to help kswapd (on NUMA).
    But maintain per-node information uses too much resource.

    Considering above 2, it's not bad to find victim by proper logic
    from balance_pgdat() by using mem_cgroup_select_victim().
    like this:
    ==
    struct mem_cgroup *select_vicitim_at_soft_limit_via_balance_pgdat(int nid, int zid)
    {
    while (?) {
    vitcim = mem_cgroup_select_victim(init_mem_cgroup); #need some modification.
    if (victim is not over soft-limit)
    continue;
    /* Ok this is candidate */
    usage = mem_cgroup_nid_zid_usage(mem, nid, zid); #get sum of active/inactive
    if (usage_is_enough_big)
    return victim;
    }
    }
    balance_pgdat()
    ...... find target zone....
    ...
    mem = select_victime_at_soft_limit_via_balance_pgdat(nid, zid)
    if (mem)
    sc->mem = mem;
    shrink_zone();
    if (mem) {
    sc->mem = NULL;
    css_put(&mem->css);
    }
    ==

    We have to pay scan cost but it will not be too big(if there are not thousands of memcg.)
    Under above, round-robin rotation is used rather than sort.
    Maybe I can show you sample.....(but I'm a bit busy.)

    Thanks,
    -Kame



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