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Date: Mon, 24 Jan 2011 09:14:41 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"hannes@...xchg.org" <hannes@...xchg.org>,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/7] memcg : fix charge function of THP allocation.
On Fri, 21 Jan 2011 17:48:18 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> wrote:
> On Fri, 21 Jan 2011 15:44:30 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
>
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> >
> > When THP is used, Hugepage size charge can happen. It's not handled
> > correctly in mem_cgroup_do_charge(). For example, THP can fallback
> > to small page allocation when HUGEPAGE allocation seems difficult
> > or busy, but memory cgroup doesn't understand it and continue to
> > try HUGEPAGE charging. And the worst thing is memory cgroup
> > believes 'memory reclaim succeeded' if limit - usage > PAGE_SIZE.
> >
> > By this, khugepaged etc...can goes into inifinite reclaim loop
> > if tasks in memcg are busy.
> >
> > After this patch
> > - Hugepage allocation will fail if 1st trial of page reclaim fails.
> > - distinguish THP allocaton from Bached allocation.
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> > mm/memcontrol.c | 51 +++++++++++++++++++++++++++++++++++----------------
> > 1 file changed, 35 insertions(+), 16 deletions(-)
> >
> > Index: mmotm-0107/mm/memcontrol.c
> > ===================================================================
> > --- mmotm-0107.orig/mm/memcontrol.c
> > +++ mmotm-0107/mm/memcontrol.c
> > @@ -1812,24 +1812,25 @@ enum {
> > CHARGE_OK, /* success */
> > CHARGE_RETRY, /* need to retry but retry is not bad */
> > CHARGE_NOMEM, /* we can't do more. return -ENOMEM */
> > + CHARGE_NEED_BREAK, /* big size allocation failure */
> > CHARGE_WOULDBLOCK, /* GFP_WAIT wasn't set and no enough res. */
> > CHARGE_OOM_DIE, /* the current is killed because of OOM */
> > };
> >
> > static int __mem_cgroup_do_charge(struct mem_cgroup *mem, gfp_t gfp_mask,
> > - int csize, bool oom_check)
> > + int page_size, bool do_reclaim, bool oom_check)
>
> I'm sorry, I can't understand why we need 'do_reclaim'. See below.
>
> > {
> > struct mem_cgroup *mem_over_limit;
> > struct res_counter *fail_res;
> > unsigned long flags = 0;
> > int ret;
> >
> > - ret = res_counter_charge(&mem->res, csize, &fail_res);
> > + ret = res_counter_charge(&mem->res, page_size, &fail_res);
> >
> > if (likely(!ret)) {
> > if (!do_swap_account)
> > return CHARGE_OK;
> > - ret = res_counter_charge(&mem->memsw, csize, &fail_res);
> > + ret = res_counter_charge(&mem->memsw, page_size, &fail_res);
> > if (likely(!ret))
> > return CHARGE_OK;
> >
> > @@ -1838,14 +1839,14 @@ static int __mem_cgroup_do_charge(struct
> > } else
> > mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
> >
> > - if (csize > PAGE_SIZE) /* change csize and retry */
> > + if (!do_reclaim)
> > return CHARGE_RETRY;
> >
>
> From the very beginning, do we need this "CHARGE_RETRY" ?
>
Reducing charge_size here in automatic and go back to the start of this function ?
I think returning here is better.
> > if (!(gfp_mask & __GFP_WAIT))
> > return CHARGE_WOULDBLOCK;
> >
> > ret = mem_cgroup_hierarchical_reclaim(mem_over_limit, NULL,
> > - gfp_mask, flags, csize);
> > + gfp_mask, flags, page_size);
> > /*
> > * try_to_free_mem_cgroup_pages() might not give us a full
> > * picture of reclaim. Some pages are reclaimed and might be
> > @@ -1853,19 +1854,28 @@ static int __mem_cgroup_do_charge(struct
> > * Check the limit again to see if the reclaim reduced the
> > * current usage of the cgroup before giving up
> > */
> > - if (ret || mem_cgroup_check_under_limit(mem_over_limit, csize))
> > + if (ret || mem_cgroup_check_under_limit(mem_over_limit, page_size))
> > return CHARGE_RETRY;
> >
> > /*
> > + * When page_size > PAGE_SIZE, THP calls this function and it's
> > + * ok to tell 'there are not enough pages for hugepage'. THP will
> > + * fallback into PAGE_SIZE allocation. If we do reclaim eagerly,
> > + * page splitting will occur and it seems much worse.
> > + */
> > + if (page_size > PAGE_SIZE)
> > + return CHARGE_NEED_BREAK;
> > +
> > + /*
> > * At task move, charge accounts can be doubly counted. So, it's
> > * better to wait until the end of task_move if something is going on.
> > */
> > if (mem_cgroup_wait_acct_move(mem_over_limit))
> > return CHARGE_RETRY;
> > -
> > /* If we don't need to call oom-killer at el, return immediately */
> > if (!oom_check)
> > return CHARGE_NOMEM;
> > +
> > /* check OOM */
> > if (!mem_cgroup_handle_oom(mem_over_limit, gfp_mask))
> > return CHARGE_OOM_DIE;
> > @@ -1885,7 +1895,7 @@ static int __mem_cgroup_try_charge(struc
> > int nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
> > struct mem_cgroup *mem = NULL;
> > int ret;
> > - int csize = max(CHARGE_SIZE, (unsigned long) page_size);
> > + bool use_pcp_cache = (page_size == PAGE_SIZE);
> >
> > /*
> > * Unlike gloval-vm's OOM-kill, we're not in memory shortage
> > @@ -1910,7 +1920,7 @@ again:
> > VM_BUG_ON(css_is_removed(&mem->css));
> > if (mem_cgroup_is_root(mem))
> > goto done;
> > - if (page_size == PAGE_SIZE && consume_stock(mem))
> > + if (use_pcp_cache && consume_stock(mem))
> > goto done;
> > css_get(&mem->css);
> > } else {
> > @@ -1933,7 +1943,7 @@ again:
> > rcu_read_unlock();
> > goto done;
> > }
> > - if (page_size == PAGE_SIZE && consume_stock(mem)) {
> > + if (use_pcp_cache && consume_stock(mem)) {
> > /*
> > * It seems dagerous to access memcg without css_get().
> > * But considering how consume_stok works, it's not
> > @@ -1967,17 +1977,26 @@ again:
> > oom_check = true;
> > nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
> > }
> > -
> > - ret = __mem_cgroup_do_charge(mem, gfp_mask, csize, oom_check);
> > + if (use_pcp_cache)
> > + ret = __mem_cgroup_do_charge(mem, gfp_mask,
> > + CHARGE_SIZE, false, oom_check);
> > + else
> > + ret = __mem_cgroup_do_charge(mem, gfp_mask,
> > + page_size, true, oom_check);
> >
>
> hmm, this confuses me. I think 'use_pcp_cache' will be used to decide
> whether we should do consume_stock() or not, but why we change charge size
> and reclaim behavior depending on it ? I think this code itself is right,
> but using 'use_pcp_cache' confused me.
>
Is it problem of function name ?
'do_batched_charge' or some ?
I'd like to use a 'xxxx_size' variable rather than 2 xxxx_size variable.
>
> > switch (ret) {
> > case CHARGE_OK:
> > break;
> > case CHARGE_RETRY: /* not in OOM situation but retry */
> > - csize = page_size;
> > + if (use_pcp_cache)/* need to reclaim pages */
> > + use_pcp_cache = false;
> > css_put(&mem->css);
> > mem = NULL;
> > goto again;
> > + case CHARGE_NEED_BREAK: /* page_size > PAGE_SIZE */
> > + css_put(&mem->css);
> > + /* returning faiulre doesn't mean OOM for hugepages */
> > + goto nomem;
>
> I like this change.
>
> > case CHARGE_WOULDBLOCK: /* !__GFP_WAIT */
> > css_put(&mem->css);
> > goto nomem;
> > @@ -1994,9 +2013,9 @@ again:
> > goto bypass;
> > }
> > } while (ret != CHARGE_OK);
> > -
> > - if (csize > page_size)
> > - refill_stock(mem, csize - page_size);
> > + /* This flag is cleared when we fail CHAEGE_SIZE charge. */
> > + if (use_pcp_cache)
> > + refill_stock(mem, CHARGE_SIZE - page_size);
>
> Ditto. can't we keep 'csize' and old code here ?
>
I remove csize. 2 'size' variable is confusing.
Thanks.
-Kame
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