Re: [patch] mm: memcontrol: support transparent huge pages under pressure

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Sep 22, 2014 at 10:52:50PM -0700, Greg Thelen wrote:
> 
> On Fri, Sep 19 2014, Johannes Weiner wrote:
> 
> > In a memcg with even just moderate cache pressure, success rates for
> > transparent huge page allocations drop to zero, wasting a lot of
> > effort that the allocator puts into assembling these pages.
> >
> > The reason for this is that the memcg reclaim code was never designed
> > for higher-order charges.  It reclaims in small batches until there is
> > room for at least one page.  Huge pages charges only succeed when
> > these batches add up over a series of huge faults, which is unlikely
> > under any significant load involving order-0 allocations in the group.
> >
> > Remove that loop on the memcg side in favor of passing the actual
> > reclaim goal to direct reclaim, which is already set up and optimized
> > to meet higher-order goals efficiently.
> >
> > This brings memcg's THP policy in line with the system policy: if the
> > allocator painstakingly assembles a hugepage, memcg will at least make
> > an honest effort to charge it.  As a result, transparent hugepage
> > allocation rates amid cache activity are drastically improved:
> >
> >                                       vanilla                 patched
> > pgalloc                 4717530.80 (  +0.00%)   4451376.40 (  -5.64%)
> > pgfault                  491370.60 (  +0.00%)    225477.40 ( -54.11%)
> > pgmajfault                    2.00 (  +0.00%)         1.80 (  -6.67%)
> > thp_fault_alloc               0.00 (  +0.00%)       531.60 (+100.00%)
> > thp_fault_fallback          749.00 (  +0.00%)       217.40 ( -70.88%)
> >
> > [ Note: this may in turn increase memory consumption from internal
> >   fragmentation, which is an inherent risk of transparent hugepages.
> >   Some setups may have to adjust the memcg limits accordingly to
> >   accomodate this - or, if the machine is already packed to capacity,
> >   disable the transparent huge page feature. ]
> 
> We're using an earlier version of this patch, so I approve of the
> general direction.  But I have some feedback.
> 
> The memsw aspect of this change seems somewhat separate.  Can it be
> split into a different patch?
> 
> The memsw aspect of this patch seems to change behavior.  Is this
> intended?  If so, a mention of it in the commit log would assuage the
> reader.  I'll explain...  Assume a machine with swap enabled and
> res.limit==memsw.limit, thus memsw_is_minimum is true.  My understanding
> is that memsw.usage represents sum(ram_usage, swap_usage).  So when
> memsw_is_minimum=true, then both swap_usage=0 and
> memsw.usage==res.usage.  In this condition, if res usage is at limit
> then there's no point in swapping because memsw.usage is already
> maximal.  Prior to this patch I think the kernel did the right thing,
> but not afterwards.
> 
> Before this patch:
>   if res.usage == res.limit, try_charge() indirectly calls
>   try_to_free_mem_cgroup_pages(noswap=true)
> 
> After this patch:
>   if res.usage == res.limit, try_charge() calls
>   try_to_free_mem_cgroup_pages(may_swap=true)
> 
> Notice the inverted swap-is-allowed value.

For some reason I had myself convinced that this is dead code due to a
change in callsites a long time ago, but you are right that currently
try_charge() relies on it, thanks for pointing it out.

However, memsw is always equal to or bigger than the memory limit - so
instead of keeping a separate state variable to track when memory
failure implies memsw failure, couldn't we just charge memsw first?

How about the following?  But yeah, I'd split this into a separate
patch now.

---
 mm/memcontrol.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e2def11f1ec1..7c9a8971d0f4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2497,16 +2497,17 @@ retry:
 		goto done;
 
 	size = batch * PAGE_SIZE;
-	if (!res_counter_charge(&memcg->res, size, &fail_res)) {
-		if (!do_swap_account)
+	if (!do_swap_account ||
+	    !res_counter_charge(&memcg->memsw, size, &fail_res)) {
+		if (!res_counter_charge(&memcg->res, size, &fail_res))
 			goto done_restock;
-		if (!res_counter_charge(&memcg->memsw, size, &fail_res))
-			goto done_restock;
-		res_counter_uncharge(&memcg->res, size);
+		if (do_swap_account)
+			res_counter_uncharge(&memcg->memsw, size);
+		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
+	} else {
 		mem_over_limit = mem_cgroup_from_res_counter(fail_res, memsw);
 		may_swap = false;
-	} else
-		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
+	}
 
 	if (batch > nr_pages) {
 		batch = nr_pages;
-- 
2.1.0

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