[PATCH 0/3] mm: memcontrol: recursive memory.low protection

[PATCH 0/3] mm: memcontrol: recursive memory.low protection

[Johannes Weiner]

Changes since v2:
- Changelog & documentation updates (Michal Hocko, Michal Koutny)

Changes since v1:
- improved Changelogs based on the discussion with Roman. Thanks!
- fix div0 when recursive & fixed protection is combined
- fix an unused compiler warning

The current memory.low (and memory.min) semantics require protection
to be assigned to a cgroup in an untinterrupted chain from the
top-level cgroup all the way to the leaf.

In practice, we want to protect entire cgroup subtrees from each other
(system management software vs. workload), but we would like the VM to
balance memory optimally *within* each subtree, without having to make
explicit weight allocations among individual components. The current
semantics make that impossible.

They also introduce unmanageable complexity into more advanced
resource trees. For example:

          host root
          `- system.slice
             `- rpm upgrades
             `- logging
          `- workload.slice
             `- a container
                `- system.slice
                `- workload.slice
                   `- job A
                      `- component 1
                      `- component 2
                   `- job B

From a host-level perspective, we would like to protect the outer
workload.slice subtree as a whole from rpm upgrades, logging etc. But
for that to be effective, right now we'd have to propagate it down
through the container, the inner workload.slice, into the job cgroup
and ultimately the component cgroups where memory is actually,
physically allocated. This may cross several tree delegation points
and namespace boundaries, which make such a setup near impossible.

CPU and IO on the other hand are already distributed recursively. The
user would simply configure allowances at the host level, and they
would apply to the entire subtree without any downward propagation.

To enable the above-mentioned usecases and bring memory in line with
other resource controllers, this patch series extends memory.low/min
such that settings apply recursively to the entire subtree. Users can
still assign explicit shares in subgroups, but if they don't, any
ancestral protection will be distributed such that children compete
freely amongst each other - as if no memory control were enabled
inside the subtree - but enjoy protection from neighboring trees.

In the above example, the user would then be able to configure shares
of CPU, IO and memory at the host level to comprehensively protect and
isolate the workload.slice as a whole from system.slice activity.

Patch #1 fixes an existing bug that can give a cgroup tree more
protection than it should receive as per ancestor configuration.

Patch #2 simplifies and documents the existing code to make it easier
to reason about the changes in the next patch.

Patch #3 finally implements recursive memory protection semantics.

Because of a risk of regressing legacy setups, the new semantics are
hidden behind a cgroup2 mount option, 'memory_recursiveprot'.

More details in patch #3.

 Documentation/admin-guide/cgroup-v2.rst |  11 ++
 include/linux/cgroup-defs.h             |   5 +
 kernel/cgroup/cgroup.c                  |  17 ++-
 mm/memcontrol.c                         | 220 +++++++++++++++++-------------
 mm/page_counter.c                       |  12 +-
 5 files changed, 160 insertions(+), 105 deletions(-)

[PATCH 1/3] mm: memcontrol: fix memory.low proportional distribution

[Johannes Weiner]

When memory.low is overcommitted - i.e. the children claim more
protection than their shared ancestor grants them - the allowance is
distributed in proportion to how much each sibling uses their own
declared protection:

	low_usage = min(memory.low, memory.current)
	elow = parent_elow * (low_usage / siblings_low_usage)

However, siblings_low_usage is not the sum of all low_usages. It sums
up the usages of *only those cgroups that are within their memory.low*
That means that low_usage can be *bigger* than siblings_low_usage, and
consequently the total protection afforded to the children can be
bigger than what the ancestor grants the subtree.

Consider three groups where two are in excess of their protection:

  A/memory.low = 10G
  A/A1/memory.low = 10G, memory.current = 20G
  A/A2/memory.low = 10G, memory.current = 20G
  A/A3/memory.low = 10G, memory.current =  8G
  siblings_low_usage = 8G (only A3 contributes)

  A1/elow = parent_elow(10G) * low_usage(10G) / siblings_low_usage(8G) = 12.5G -> 10G
  A2/elow = parent_elow(10G) * low_usage(10G) / siblings_low_usage(8G) = 12.5G -> 10G
  A3/elow = parent_elow(10G) * low_usage(8G) / siblings_low_usage(8G) = 10.0G

  (the 12.5G are capped to the explicit memory.low setting of 10G)

With that, the sum of all awarded protection below A is 30G, when A
only grants 10G for the entire subtree.

What does this mean in practice? A1 and A2 would still be in excess of
their 10G allowance and would be reclaimed, whereas A3 would not. As
they eventually drop below their protection setting, they would be
counted in siblings_low_usage again and the error would right itself.

When reclaim was applied in a binary fashion (cgroup is reclaimed when
it's above its protection, otherwise it's skipped) this would actually
work out just fine. However, since 1bc63fb1272b ("mm, memcg: make scan
aggression always exclude protection"), reclaim pressure is scaled to
how much a cgroup is above its protection. As a result this
calculation error unduly skews pressure away from A1 and A2 toward the
rest of the system.

But why did we do it like this in the first place?

The reasoning behind exempting groups in excess from
siblings_low_usage was to go after them first during reclaim in an
overcommitted subtree:

  A/memory.low = 2G, memory.current = 4G
  A/A1/memory.low = 3G, memory.current = 2G
  A/A2/memory.low = 1G, memory.current = 2G

  siblings_low_usage = 2G (only A1 contributes)
  A1/elow = parent_elow(2G) * low_usage(2G) / siblings_low_usage(2G) = 2G
  A2/elow = parent_elow(2G) * low_usage(1G) / siblings_low_usage(2G) = 1G

While the children combined are overcomitting A and are technically
both at fault, A2 is actively declaring unprotected memory and we
would like to reclaim that first.

However, while this sounds like a noble goal on the face of it, it
doesn't make much difference in actual memory distribution: Because A
is overcommitted, reclaim will not stop once A2 gets pushed back to
within its allowance; we'll have to reclaim A1 either way. The end
result is still that protection is distributed proportionally, with A1
getting 3/4 (1.5G) and A2 getting 1/4 (0.5G) of A's allowance.

[ If A weren't overcommitted, it wouldn't make a difference since each
  cgroup would just get the protection it declares:

  A/memory.low = 2G, memory.current = 3G
  A/A1/memory.low = 1G, memory.current = 1G
  A/A2/memory.low = 1G, memory.current = 2G

  With the current calculation:

  siblings_low_usage = 1G (only A1 contributes)
  A1/elow = parent_elow(2G) * low_usage(1G) / siblings_low_usage(1G) = 2G -> 1G
  A2/elow = parent_elow(2G) * low_usage(1G) / siblings_low_usage(1G) = 2G -> 1G

  Including excess groups in siblings_low_usage:

  siblings_low_usage = 2G
  A1/elow = parent_elow(2G) * low_usage(1G) / siblings_low_usage(2G) = 1G -> 1G
  A2/elow = parent_elow(2G) * low_usage(1G) / siblings_low_usage(2G) = 1G -> 1G ]

Simplify the calculation and fix the proportional reclaim bug by
including excess cgroups in siblings_low_usage.

After this patch, the effective memory.low distribution from the
example above would be as follows:

  A/memory.low = 10G
  A/A1/memory.low = 10G, memory.current = 20G
  A/A2/memory.low = 10G, memory.current = 20G
  A/A3/memory.low = 10G, memory.current =  8G
  siblings_low_usage = 28G

  A1/elow = parent_elow(10G) * low_usage(10G) / siblings_low_usage(28G) = 3.5G
  A2/elow = parent_elow(10G) * low_usage(10G) / siblings_low_usage(28G) = 3.5G
  A3/elow = parent_elow(10G) * low_usage(8G) / siblings_low_usage(28G) = 2.8G

Fixes: 1bc63fb1272b ("mm, memcg: make scan aggression always exclude protection")
Fixes: 230671533d64 ("mm: memory.low hierarchical behavior")
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Chris Down <chris@chrisdown.name>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memcontrol.c   |  4 +---
 mm/page_counter.c | 12 ++----------
 2 files changed, 3 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5b5f74cfd4d..874a0b00f89b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6236,9 +6236,7 @@ struct cgroup_subsys memory_cgrp_subsys = {
  * elow = min( memory.low, parent->elow * ------------------ ),
  *                                        siblings_low_usage
  *
- *             | memory.current, if memory.current < memory.low
- * low_usage = |
- *	       | 0, otherwise.
+ * low_usage = min(memory.low, memory.current)
  *
  *
  * Such definition of the effective memory.low provides the expected
diff --git a/mm/page_counter.c b/mm/page_counter.c
index de31470655f6..75d53f15f040 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -23,11 +23,7 @@ static void propagate_protected_usage(struct page_counter *c,
 		return;
 
 	if (c->min || atomic_long_read(&c->min_usage)) {
-		if (usage <= c->min)
-			protected = usage;
-		else
-			protected = 0;
-
+		protected = min(usage, c->min);
 		old_protected = atomic_long_xchg(&c->min_usage, protected);
 		delta = protected - old_protected;
 		if (delta)
@@ -35,11 +31,7 @@ static void propagate_protected_usage(struct page_counter *c,
 	}
 
 	if (c->low || atomic_long_read(&c->low_usage)) {
-		if (usage <= c->low)
-			protected = usage;
-		else
-			protected = 0;
-
+		protected = min(usage, c->low);
 		old_protected = atomic_long_xchg(&c->low_usage, protected);
 		delta = protected - old_protected;
 		if (delta)
-- 
2.24.1

[PATCH 2/3] mm: memcontrol: clean up and document effective low/min calculations

[Johannes Weiner]

The effective protection of any given cgroup is a somewhat complicated
construct that depends on the ancestor's configuration, siblings'
configurations, as well as current memory utilization in all these
groups. It's done this way to satisfy hierarchical delegation
requirements while also making the configuration semantics flexible
and expressive in complex real life scenarios.

Unfortunately, all the rules and requirements are sparsely documented,
and the code is a little too clever in merging different scenarios
into a single min() expression. This makes it hard to reason about the
implementation and avoid breaking semantics when making changes to it.

This patch documents each semantic rule individually and splits out
the handling of the overcommit case from the regular case.

Michal Koutný also points out that the points of equilibrium as
described in the existing example scenarios aren't actually
accurate. Delete these examples for now to avoid confusion.

Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Chris Down <chris@chrisdown.name>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/memcontrol.c | 175 +++++++++++++++++++++++-------------------------
 1 file changed, 83 insertions(+), 92 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 874a0b00f89b..836c521bd61f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6204,6 +6204,76 @@ struct cgroup_subsys memory_cgrp_subsys = {
 	.early_init = 0,
 };
 
+/*
+ * This function calculates an individual cgroup's effective
+ * protection which is derived from its own memory.min/low, its
+ * parent's and siblings' settings, as well as the actual memory
+ * distribution in the tree.
+ *
+ * The following rules apply to the effective protection values:
+ *
+ * 1. At the first level of reclaim, effective protection is equal to
+ *    the declared protection in memory.min and memory.low.
+ *
+ * 2. To enable safe delegation of the protection configuration, at
+ *    subsequent levels the effective protection is capped to the
+ *    parent's effective protection.
+ *
+ * 3. To make complex and dynamic subtrees easier to configure, the
+ *    user is allowed to overcommit the declared protection at a given
+ *    level. If that is the case, the parent's effective protection is
+ *    distributed to the children in proportion to how much protection
+ *    they have declared and how much of it they are utilizing.
+ *
+ *    This makes distribution proportional, but also work-conserving:
+ *    if one cgroup claims much more protection than it uses memory,
+ *    the unused remainder is available to its siblings.
+ *
+ * 4. Conversely, when the declared protection is undercommitted at a
+ *    given level, the distribution of the larger parental protection
+ *    budget is NOT proportional. A cgroup's protection from a sibling
+ *    is capped to its own memory.min/low setting.
+ *
+ */
+static unsigned long effective_protection(unsigned long usage,
+					  unsigned long setting,
+					  unsigned long parent_effective,
+					  unsigned long siblings_protected)
+{
+	unsigned long protected;
+
+	protected = min(usage, setting);
+	/*
+	 * If all cgroups at this level combined claim and use more
+	 * protection then what the parent affords them, distribute
+	 * shares in proportion to utilization.
+	 *
+	 * We are using actual utilization rather than the statically
+	 * claimed protection in order to be work-conserving: claimed
+	 * but unused protection is available to siblings that would
+	 * otherwise get a smaller chunk than what they claimed.
+	 */
+	if (siblings_protected > parent_effective)
+		return protected * parent_effective / siblings_protected;
+
+	/*
+	 * Ok, utilized protection of all children is within what the
+	 * parent affords them, so we know whatever this child claims
+	 * and utilizes is effectively protected.
+	 *
+	 * If there is unprotected usage beyond this value, reclaim
+	 * will apply pressure in proportion to that amount.
+	 *
+	 * If there is unutilized protection, the cgroup will be fully
+	 * shielded from reclaim, but we do return a smaller value for
+	 * protection than what the group could enjoy in theory. This
+	 * is okay. With the overcommit distribution above, effective
+	 * protection is always dependent on how memory is actually
+	 * consumed among the siblings anyway.
+	 */
+	return protected;
+}
+
 /**
  * mem_cgroup_protected - check if memory consumption is in the normal range
  * @root: the top ancestor of the sub-tree being checked
@@ -6217,67 +6287,11 @@ struct cgroup_subsys memory_cgrp_subsys = {
  *   MEMCG_PROT_LOW: cgroup memory is protected as long there is
  *     an unprotected supply of reclaimable memory from other cgroups.
  *   MEMCG_PROT_MIN: cgroup memory is protected
- *
- * @root is exclusive; it is never protected when looked at directly
- *
- * To provide a proper hierarchical behavior, effective memory.min/low values
- * are used. Below is the description of how effective memory.low is calculated.
- * Effective memory.min values is calculated in the same way.
- *
- * Effective memory.low is always equal or less than the original memory.low.
- * If there is no memory.low overcommittment (which is always true for
- * top-level memory cgroups), these two values are equal.
- * Otherwise, it's a part of parent's effective memory.low,
- * calculated as a cgroup's memory.low usage divided by sum of sibling's
- * memory.low usages, where memory.low usage is the size of actually
- * protected memory.
- *
- *                                             low_usage
- * elow = min( memory.low, parent->elow * ------------------ ),
- *                                        siblings_low_usage
- *
- * low_usage = min(memory.low, memory.current)
- *
- *
- * Such definition of the effective memory.low provides the expected
- * hierarchical behavior: parent's memory.low value is limiting
- * children, unprotected memory is reclaimed first and cgroups,
- * which are not using their guarantee do not affect actual memory
- * distribution.
- *
- * For example, if there are memcgs A, A/B, A/C, A/D and A/E:
- *
- *     A      A/memory.low = 2G, A/memory.current = 6G
- *    //\\
- *   BC  DE   B/memory.low = 3G  B/memory.current = 2G
- *            C/memory.low = 1G  C/memory.current = 2G
- *            D/memory.low = 0   D/memory.current = 2G
- *            E/memory.low = 10G E/memory.current = 0
- *
- * and the memory pressure is applied, the following memory distribution
- * is expected (approximately):
- *
- *     A/memory.current = 2G
- *
- *     B/memory.current = 1.3G
- *     C/memory.current = 0.6G
- *     D/memory.current = 0
- *     E/memory.current = 0
- *
- * These calculations require constant tracking of the actual low usages
- * (see propagate_protected_usage()), as well as recursive calculation of
- * effective memory.low values. But as we do call mem_cgroup_protected()
- * path for each memory cgroup top-down from the reclaim,
- * it's possible to optimize this part, and save calculated elow
- * for next usage. This part is intentionally racy, but it's ok,
- * as memory.low is a best-effort mechanism.
  */
 enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 						struct mem_cgroup *memcg)
 {
 	struct mem_cgroup *parent;
-	unsigned long emin, parent_emin;
-	unsigned long elow, parent_elow;
 	unsigned long usage;
 
 	if (mem_cgroup_disabled())
@@ -6292,52 +6306,29 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 	if (!usage)
 		return MEMCG_PROT_NONE;
 
-	emin = memcg->memory.min;
-	elow = memcg->memory.low;
-
 	parent = parent_mem_cgroup(memcg);
 	/* No parent means a non-hierarchical mode on v1 memcg */
 	if (!parent)
 		return MEMCG_PROT_NONE;
 
-	if (parent == root)
-		goto exit;
-
-	parent_emin = READ_ONCE(parent->memory.emin);
-	emin = min(emin, parent_emin);
-	if (emin && parent_emin) {
-		unsigned long min_usage, siblings_min_usage;
-
-		min_usage = min(usage, memcg->memory.min);
-		siblings_min_usage = atomic_long_read(
-			&parent->memory.children_min_usage);
-
-		if (min_usage && siblings_min_usage)
-			emin = min(emin, parent_emin * min_usage /
-				   siblings_min_usage);
+	if (parent == root) {
+		memcg->memory.emin = memcg->memory.min;
+		memcg->memory.elow = memcg->memory.low;
+		goto out;
 	}
 
-	parent_elow = READ_ONCE(parent->memory.elow);
-	elow = min(elow, parent_elow);
-	if (elow && parent_elow) {
-		unsigned long low_usage, siblings_low_usage;
-
-		low_usage = min(usage, memcg->memory.low);
-		siblings_low_usage = atomic_long_read(
-			&parent->memory.children_low_usage);
+	memcg->memory.emin = effective_protection(usage,
+			memcg->memory.min, READ_ONCE(parent->memory.emin),
+			atomic_long_read(&parent->memory.children_min_usage));
 
-		if (low_usage && siblings_low_usage)
-			elow = min(elow, parent_elow * low_usage /
-				   siblings_low_usage);
-	}
+	memcg->memory.elow = effective_protection(usage,
+			memcg->memory.low, READ_ONCE(parent->memory.elow),
+			atomic_long_read(&parent->memory.children_low_usage));
 
-exit:
-	memcg->memory.emin = emin;
-	memcg->memory.elow = elow;
-
-	if (usage <= emin)
+out:
+	if (usage <= memcg->memory.emin)
 		return MEMCG_PROT_MIN;
-	else if (usage <= elow)
+	else if (usage <= memcg->memory.elow)
 		return MEMCG_PROT_LOW;
 	else
 		return MEMCG_PROT_NONE;
-- 
2.24.1

[PATCH 3/3] mm: memcontrol: recursive memory.low protection

[Johannes Weiner]

Right now, the effective protection of any given cgroup is capped by
its own explicit memory.low setting, regardless of what the parent
says. The reasons for this are mostly historical and ease of
implementation: to make delegation of memory.low safe, effective
protection is the min() of all memory.low up the tree.

Unfortunately, this limitation makes it impossible to protect an
entire subtree from another without forcing the user to make explicit
protection allocations all the way to the leaf cgroups - something
that is highly undesirable in real life scenarios.

Consider memory in a data center host. At the cgroup top level, we
have a distinction between system management software and the actual
workload the system is executing. Both branches are further subdivided
into individual services, job components etc.

We want to protect the workload as a whole from the system management
software, but that doesn't mean we want to protect and prioritize
individual workload wrt each other. Their memory demand can vary over
time, and we'd want the VM to simply cache the hottest data within the
workload subtree. Yet, the current memory.low limitations force us to
allocate a fixed amount of protection to each workload component in
order to get protection from system management software in
general. This results in very inefficient resource distribution.

Another concern with mandating downward allocation is that, as the
complexity of the cgroup tree grows, it gets harder for the lower
levels to be informed about decisions made at the host-level. Consider
a container inside a namespace that in turn creates its own nested
tree of cgroups to run multiple workloads. It'd be extremely difficult
to configure memory.low parameters in those leaf cgroups that on one
hand balance pressure among siblings as the container desires, while
also reflecting the host-level protection from e.g. rpm upgrades, that
lie beyond one or more delegation and namespacing points in the tree.

It's highly unusual from a cgroup interface POV that nested levels
have to be aware of and reflect decisions made at higher levels for
them to be effective.

To enable such use cases and scale configurability for complex trees,
this patch implements a resource inheritance model for memory that is
similar to how the CPU and the IO controller implement work-conserving
resource allocations: a share of a resource allocated to a subree
always applies to the entire subtree recursively, while allowing, but
not mandating, children to further specify distribution rules.

That means that if protection is explicitly allocated among siblings,
those configured shares are being followed during page reclaim just
like they are now. However, if the memory.low set at a higher level is
not fully claimed by the children in that subtree, the "floating"
remainder is applied to each cgroup in the tree in proportion to its
size. Since reclaim pressure is applied in proportion to size as well,
each child in that tree gets the same boost, and the effect is neutral
among siblings - with respect to each other, they behave as if no
memory control was enabled at all, and the VM simply balances the
memory demands optimally within the subtree. But collectively those
cgroups enjoy a boost over the cgroups in neighboring trees.

E.g. a leaf cgroup with a memory.low setting of 0 no longer means that
it's not getting a share of the hierarchically assigned resource, just
that it doesn't claim a fixed amount of it to protect from its siblings.

This allows us to recursively protect one subtree (workload) from
another (system management), while letting subgroups compete freely
among each other - without having to assign fixed shares to each leaf,
and without nested groups having to echo higher-level settings.

The floating protection composes naturally with fixed
protection. Consider the following example tree:

		A            A: low = 2G
               / \          A1: low = 1G
              A1 A2         A2: low = 0G

As outside pressure is applied to this tree, A1 will enjoy a fixed
protection from A2 of 1G, but the remaining, unclaimed 1G from A is
split evenly among A1 and A2, coming out to 1.5G and 0.5G.

There is a slight risk of regressing theoretical setups where the
top-level cgroups don't know about the true budgeting and set bogusly
high "bypass" values that are meaningfully allocated down the
tree. Such setups would rely on unclaimed protection to be discarded,
and distributing it would change the intended behavior. Be safe and
hide the new behavior behind a mount option, 'memory_recursiveprot'.

Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Acked-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 Documentation/admin-guide/cgroup-v2.rst | 11 ++++++
 include/linux/cgroup-defs.h             |  5 +++
 kernel/cgroup/cgroup.c                  | 17 ++++++++-
 mm/memcontrol.c                         | 51 +++++++++++++++++++++++--
 4 files changed, 79 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 0636bcb60b5a..e569d83621da 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -186,6 +186,17 @@ cgroup v2 currently supports the following mount options.
         modified through remount from the init namespace. The mount
         option is ignored on non-init namespace mounts.
 
+  memory_recursiveprot
+
+        Recursively apply memory.min and memory.low protection to
+        entire subtrees, without requiring explicit downward
+        propagation into leaf cgroups.  This allows protecting entire
+        subtrees from one another, while retaining free competition
+        within those subtrees.  This should have been the default
+        behavior but is a mount-option to avoid regressing setups
+        relying on the original semantics (e.g. specifying bogusly
+        high 'bypass' protection values at higher tree levels).
+
 
 Organizing Processes and Threads
 --------------------------------
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 63097cb243cb..e1fafed22db1 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -94,6 +94,11 @@ enum {
 	 * Enable legacy local memory.events.
 	 */
 	CGRP_ROOT_MEMORY_LOCAL_EVENTS = (1 << 5),
+
+	/*
+	 * Enable recursive subtree protection
+	 */
+	CGRP_ROOT_MEMORY_RECURSIVE_PROT = (1 << 6),
 };
 
 /* cftype->flags */
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 735af8f15f95..a2f8d2ab8dec 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1813,12 +1813,14 @@ int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
 enum cgroup2_param {
 	Opt_nsdelegate,
 	Opt_memory_localevents,
+	Opt_memory_recursiveprot,
 	nr__cgroup2_params
 };
 
 static const struct fs_parameter_spec cgroup2_param_specs[] = {
 	fsparam_flag("nsdelegate",		Opt_nsdelegate),
 	fsparam_flag("memory_localevents",	Opt_memory_localevents),
+	fsparam_flag("memory_recursiveprot",	Opt_memory_recursiveprot),
 	{}
 };
 
@@ -1844,6 +1846,9 @@ static int cgroup2_parse_param(struct fs_context *fc, struct fs_parameter *param
 	case Opt_memory_localevents:
 		ctx->flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
 		return 0;
+	case Opt_memory_recursiveprot:
+		ctx->flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
+		return 0;
 	}
 	return -EINVAL;
 }
@@ -1860,6 +1865,11 @@ static void apply_cgroup_root_flags(unsigned int root_flags)
 			cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
 		else
 			cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_LOCAL_EVENTS;
+
+		if (root_flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
+			cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
+		else
+			cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_RECURSIVE_PROT;
 	}
 }
 
@@ -1869,6 +1879,8 @@ static int cgroup_show_options(struct seq_file *seq, struct kernfs_root *kf_root
 		seq_puts(seq, ",nsdelegate");
 	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
 		seq_puts(seq, ",memory_localevents");
+	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
+		seq_puts(seq, ",memory_recursiveprot");
 	return 0;
 }
 
@@ -6364,7 +6376,10 @@ static struct kobj_attribute cgroup_delegate_attr = __ATTR_RO(delegate);
 static ssize_t features_show(struct kobject *kobj, struct kobj_attribute *attr,
 			     char *buf)
 {
-	return snprintf(buf, PAGE_SIZE, "nsdelegate\nmemory_localevents\n");
+	return snprintf(buf, PAGE_SIZE,
+			"nsdelegate\n"
+			"memory_localevents\n"
+			"memory_recursiveprot\n");
 }
 static struct kobj_attribute cgroup_features_attr = __ATTR_RO(features);
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 836c521bd61f..0dd5d5f70593 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6234,13 +6234,27 @@ struct cgroup_subsys memory_cgrp_subsys = {
  *    budget is NOT proportional. A cgroup's protection from a sibling
  *    is capped to its own memory.min/low setting.
  *
+ * 5. However, to allow protecting recursive subtrees from each other
+ *    without having to declare each individual cgroup's fixed share
+ *    of the ancestor's claim to protection, any unutilized -
+ *    "floating" - protection from up the tree is distributed in
+ *    proportion to each cgroup's *usage*. This makes the protection
+ *    neutral wrt sibling cgroups and lets them compete freely over
+ *    the shared parental protection budget, but it protects the
+ *    subtree as a whole from neighboring subtrees.
+ *
+ * Note that 4. and 5. are not in conflict: 4. is about protecting
+ * against immediate siblings whereas 5. is about protecting against
+ * neighboring subtrees.
  */
 static unsigned long effective_protection(unsigned long usage,
+					  unsigned long parent_usage,
 					  unsigned long setting,
 					  unsigned long parent_effective,
 					  unsigned long siblings_protected)
 {
 	unsigned long protected;
+	unsigned long ep;
 
 	protected = min(usage, setting);
 	/*
@@ -6271,7 +6285,34 @@ static unsigned long effective_protection(unsigned long usage,
 	 * protection is always dependent on how memory is actually
 	 * consumed among the siblings anyway.
 	 */
-	return protected;
+	ep = protected;
+
+	/*
+	 * If the children aren't claiming (all of) the protection
+	 * afforded to them by the parent, distribute the remainder in
+	 * proportion to the (unprotected) memory of each cgroup. That
+	 * way, cgroups that aren't explicitly prioritized wrt each
+	 * other compete freely over the allowance, but they are
+	 * collectively protected from neighboring trees.
+	 *
+	 * We're using unprotected memory for the weight so that if
+	 * some cgroups DO claim explicit protection, we don't protect
+	 * the same bytes twice.
+	 */
+	if (!(cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT))
+		return ep;
+
+	if (parent_effective > siblings_protected && usage > protected) {
+		unsigned long unclaimed;
+
+		unclaimed = parent_effective - siblings_protected;
+		unclaimed *= usage - protected;
+		unclaimed /= parent_usage - siblings_protected;
+
+		ep += unclaimed;
+	}
+
+	return ep;
 }
 
 /**
@@ -6291,8 +6332,8 @@ static unsigned long effective_protection(unsigned long usage,
 enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 						struct mem_cgroup *memcg)
 {
+	unsigned long usage, parent_usage;
 	struct mem_cgroup *parent;
-	unsigned long usage;
 
 	if (mem_cgroup_disabled())
 		return MEMCG_PROT_NONE;
@@ -6317,11 +6358,13 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 		goto out;
 	}
 
-	memcg->memory.emin = effective_protection(usage,
+	parent_usage = page_counter_read(&parent->memory);
+
+	memcg->memory.emin = effective_protection(usage, parent_usage,
 			memcg->memory.min, READ_ONCE(parent->memory.emin),
 			atomic_long_read(&parent->memory.children_min_usage));
 
-	memcg->memory.elow = effective_protection(usage,
+	memcg->memory.elow = effective_protection(usage, parent_usage,
 			memcg->memory.low, READ_ONCE(parent->memory.elow),
 			atomic_long_read(&parent->memory.children_low_usage));
 
-- 
2.24.1

Re: [PATCH 3/3] mm: memcontrol: recursive memory.low protection

[Johannes Weiner]

On Fri, Dec 13, 2019 at 02:21:58PM -0500, Johannes Weiner wrote:
> +	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT) {
> +		unsigned long unclaimed;
> +		/*
> +		 * If the children aren't claiming (all of) the
> +		 * protection afforded to them by the parent,
> +		 * distribute the remainder in proportion to the
> +		 * (unprotected) size of each cgroup. That way,
> +		 * cgroups that aren't explicitly prioritized wrt each
> +		 * other compete freely over the allowance, but they
> +		 * are collectively protected from neighboring trees.
> +		 *
> +		 * We're using unprotected size for the weight so that
> +		 * if some cgroups DO claim explicit protection, we
> +		 * don't protect the same bytes twice.
> +		 */
> +		unclaimed = parent_effective - siblings_protected;
> +		unclaimed *= usage - protected;
> +		unclaimed /= parent_usage - siblings_protected;

Brainfart I noticed just after sending it out - naturally. If there is
unclaimed protection in the parent, but the children use exactly how
much they claim, this will div0. We have to check for usage that isn't
explicitly protected in the child to which to apply the float. Fixlet
below. Doesn't change the overall logic, though.

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2e352cd6c38d..8d7e9490740b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6328,21 +6328,24 @@ static unsigned long effective_protection(unsigned long usage,
 	 */
 	ep = protected;
 
-	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT) {
+	/*
+	 * If the children aren't claiming (all of) the protection
+	 * afforded to them by the parent, distribute the remainder in
+	 * proportion to the (unprotected) size of each cgroup. That
+	 * way, cgroups that aren't explicitly prioritized wrt each
+	 * other compete freely over the allowance, but they are
+	 * collectively protected from neighboring trees.
+	 *
+	 * We're using unprotected size for the weight so that if some
+	 * cgroups DO claim explicit protection, we don't protect the
+	 * same bytes twice.
+	 */
+	if (!(cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT))
+		return ep;
+
+	if (usage > protected && parent_effective > siblings_protected) {
 		unsigned long unclaimed;
-		/*
-		 * If the children aren't claiming (all of) the
-		 * protection afforded to them by the parent,
-		 * distribute the remainder in proportion to the
-		 * (unprotected) size of each cgroup. That way,
-		 * cgroups that aren't explicitly prioritized wrt each
-		 * other compete freely over the allowance, but they
-		 * are collectively protected from neighboring trees.
-		 *
-		 * We're using unprotected size for the weight so that
-		 * if some cgroups DO claim explicit protection, we
-		 * don't protect the same bytes twice.
-		 */
+
 		unclaimed = parent_effective - siblings_protected;
 		unclaimed *= usage - protected;
 		unclaimed /= parent_usage - siblings_protected;

[PATCH 3/3] mm: memcontrol: recursive memory.low protection

[Johannes Weiner]

Right now, the effective protection of any given cgroup is capped by
its own explicit memory.low setting, regardless of what the parent
says. The reasons for this are mostly historical and ease of
implementation: to make delegation of memory.low safe, effective
protection is the min() of all memory.low up the tree.

Unfortunately, this limitation makes it impossible to protect an
entire subtree from another without forcing the user to make explicit
protection allocations all the way to the leaf cgroups - something
that is highly undesirable in real life scenarios.

Consider memory in a data center node. At the cgroup top level, we
have a distinction between system management software and the actual
workload the system is executing. Both branches are further subdivided
into individual services, job components etc.

We want to protect the workload as a whole from the system management
software, but we don't want to protect individual workload components
from each other! Their memory demand can vary over time, and we want
the VM to simply cache the hottest data within the workload subtree.
Yet, the current memory.low limitations force us to hard-allocate
protection to each workload cgroup in order to get any protection from
system management software. This is basically useless in practice.

This patch adds the concept of recursive protection to the memory.low
configurable, while retaining the abilty to assign fixed protection in
leaf groups as well.

That means that if protection is explicitly allocated among siblings,
those configured weights are being followed during page reclaim just
like they are now.

However, if the memory.low set at a higher level is not fully claimed
by the children in that subtree, that "floating" protection is applied
to each cgroup in the tree in proportion to its size. Since reclaim
pressure is applied in proportion to size as well, each child in that
tree gets the same boost, and the effect is neutral among siblings -
with respect to each other, they behave as if no memory control was
enabled at all, and the VM simply balances the memory demands
optimally within the subtree. But collectively those cgroups enjoy a
boost over the cgroups in neighboring trees.

This allows us to recursively protect one subtree (workload) from
another (system management), but let subgroups compete freely among
each other without having to assign fixed weights to each leaf.

This floating protection composes with fixed protection. Consider the
following example tree:

		A            A: low = 2G
               / \          A1: low = 1G
              A1 A2         A2: low = 0G

As outside pressure is applied to this tree, A1 will enjoy a fixed
protection from A2 of 1G, but the remaining, unclaimed 1G from A is
split evenly among A1 and A2. Assuming equal memory demand in both,
memory usage will converge on A1 using 1.5G and A2 using 0.5G.

There is a slight risk of regressing theoretical setups where the
top-level cgroups don't know about the true budgeting and set bogusly
high "bypass" values that are meaningfully allocated down the
tree. Such setups would rely on unclaimed protection to be discarded,
and distributing it would change the intended behavior. Be safe and
hide the new behavior behind a mount option, 'memory_recursiveprot'.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 Documentation/admin-guide/cgroup-v2.rst | 11 +++++
 include/linux/cgroup-defs.h             |  5 ++
 kernel/cgroup/cgroup.c                  | 17 ++++++-
 mm/memcontrol.c                         | 62 +++++++++++++++++++++++--
 4 files changed, 90 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 0636bcb60b5a..e569d83621da 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -186,6 +186,17 @@ cgroup v2 currently supports the following mount options.
         modified through remount from the init namespace. The mount
         option is ignored on non-init namespace mounts.
 
+  memory_recursiveprot
+
+        Recursively apply memory.min and memory.low protection to
+        entire subtrees, without requiring explicit downward
+        propagation into leaf cgroups.  This allows protecting entire
+        subtrees from one another, while retaining free competition
+        within those subtrees.  This should have been the default
+        behavior but is a mount-option to avoid regressing setups
+        relying on the original semantics (e.g. specifying bogusly
+        high 'bypass' protection values at higher tree levels).
+
 
 Organizing Processes and Threads
 --------------------------------
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 63097cb243cb..e1fafed22db1 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -94,6 +94,11 @@ enum {
 	 * Enable legacy local memory.events.
 	 */
 	CGRP_ROOT_MEMORY_LOCAL_EVENTS = (1 << 5),
+
+	/*
+	 * Enable recursive subtree protection
+	 */
+	CGRP_ROOT_MEMORY_RECURSIVE_PROT = (1 << 6),
 };
 
 /* cftype->flags */
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 735af8f15f95..a2f8d2ab8dec 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1813,12 +1813,14 @@ int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
 enum cgroup2_param {
 	Opt_nsdelegate,
 	Opt_memory_localevents,
+	Opt_memory_recursiveprot,
 	nr__cgroup2_params
 };
 
 static const struct fs_parameter_spec cgroup2_param_specs[] = {
 	fsparam_flag("nsdelegate",		Opt_nsdelegate),
 	fsparam_flag("memory_localevents",	Opt_memory_localevents),
+	fsparam_flag("memory_recursiveprot",	Opt_memory_recursiveprot),
 	{}
 };
 
@@ -1844,6 +1846,9 @@ static int cgroup2_parse_param(struct fs_context *fc, struct fs_parameter *param
 	case Opt_memory_localevents:
 		ctx->flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
 		return 0;
+	case Opt_memory_recursiveprot:
+		ctx->flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
+		return 0;
 	}
 	return -EINVAL;
 }
@@ -1860,6 +1865,11 @@ static void apply_cgroup_root_flags(unsigned int root_flags)
 			cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
 		else
 			cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_LOCAL_EVENTS;
+
+		if (root_flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
+			cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_RECURSIVE_PROT;
+		else
+			cgrp_dfl_root.flags &= ~CGRP_ROOT_MEMORY_RECURSIVE_PROT;
 	}
 }
 
@@ -1869,6 +1879,8 @@ static int cgroup_show_options(struct seq_file *seq, struct kernfs_root *kf_root
 		seq_puts(seq, ",nsdelegate");
 	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
 		seq_puts(seq, ",memory_localevents");
+	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)
+		seq_puts(seq, ",memory_recursiveprot");
 	return 0;
 }
 
@@ -6364,7 +6376,10 @@ static struct kobj_attribute cgroup_delegate_attr = __ATTR_RO(delegate);
 static ssize_t features_show(struct kobject *kobj, struct kobj_attribute *attr,
 			     char *buf)
 {
-	return snprintf(buf, PAGE_SIZE, "nsdelegate\nmemory_localevents\n");
+	return snprintf(buf, PAGE_SIZE,
+			"nsdelegate\n"
+			"memory_localevents\n"
+			"memory_recursiveprot\n");
 }
 static struct kobj_attribute cgroup_features_attr = __ATTR_RO(features);
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ac9a3a170bec..2e352cd6c38d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6254,6 +6254,32 @@ struct cgroup_subsys memory_cgrp_subsys = {
  *    budget is NOT proportional. A cgroup's protection from a sibling
  *    is capped to its own memory.min/low setting.
  *
+ * 5. However, to allow protecting recursive subtrees from each other
+ *    without having to declare each individual cgroup's fixed share
+ *    of the ancestor's claim to protection, any unutilized -
+ *    "floating" - protection from up the tree is distributed in
+ *    proportion to each cgroup's *usage*. This makes the protection
+ *    neutral wrt sibling cgroups and lets them compete freely over
+ *    the shared parental protection budget, but it protects the
+ *    subtree as a whole from neighboring subtrees.
+ *
+ *    Consider the following example tree:
+ *
+ *        A            A: low = 2G
+ *       / \           B: low = 1G
+ *      B   C          C: low = 0G
+ *
+ *    As memory pressure is applied, the following memory distribution
+ *    is expected (approximately):
+ *
+ *      A/memory.current = 2G
+ *      B/memory.current = 1.5G
+ *      C/memory.current = 0.5G
+ *
+ * Note that 4. and 5. are not in conflict: 4. is about protecting
+ * against immediate siblings whereas 5. is about protecting against
+ * neighboring subtrees.
+ *
  * These calculations require constant tracking of the actual low usages
  * (see propagate_protected_usage()), as well as recursive calculation of
  * effective memory.low values. But as we do call mem_cgroup_protected()
@@ -6263,11 +6289,13 @@ struct cgroup_subsys memory_cgrp_subsys = {
  * as memory.low is a best-effort mechanism.
  */
 static unsigned long effective_protection(unsigned long usage,
+					  unsigned long parent_usage,
 					  unsigned long setting,
 					  unsigned long parent_effective,
 					  unsigned long siblings_protected)
 {
 	unsigned long protected;
+	unsigned long ep;
 
 	protected = min(usage, setting);
 	/*
@@ -6298,7 +6326,31 @@ static unsigned long effective_protection(unsigned long usage,
 	 * protection is always dependent on how memory is actually
 	 * consumed among the siblings anyway.
 	 */
-	return protected;
+	ep = protected;
+
+	if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT) {
+		unsigned long unclaimed;
+		/*
+		 * If the children aren't claiming (all of) the
+		 * protection afforded to them by the parent,
+		 * distribute the remainder in proportion to the
+		 * (unprotected) size of each cgroup. That way,
+		 * cgroups that aren't explicitly prioritized wrt each
+		 * other compete freely over the allowance, but they
+		 * are collectively protected from neighboring trees.
+		 *
+		 * We're using unprotected size for the weight so that
+		 * if some cgroups DO claim explicit protection, we
+		 * don't protect the same bytes twice.
+		 */
+		unclaimed = parent_effective - siblings_protected;
+		unclaimed *= usage - protected;
+		unclaimed /= parent_usage - siblings_protected;
+
+		ep += unclaimed;
+	}
+
+	return ep;
 }
 
 /**
@@ -6318,9 +6370,9 @@ static unsigned long effective_protection(unsigned long usage,
 enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 						struct mem_cgroup *memcg)
 {
+	unsigned long usage, parent_usage;
 	struct mem_cgroup *parent;
 	unsigned long emin, elow;
-	unsigned long usage;
 
 	if (mem_cgroup_disabled())
 		return MEMCG_PROT_NONE;
@@ -6345,11 +6397,13 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
 		goto out;
 	}
 
-	memcg->memory.emin = effective_protection(usage,
+	parent_usage = page_counter_read(&parent->memory);
+
+	memcg->memory.emin = effective_protection(usage, parent_usage,
 			memcg->memory.min, READ_ONCE(parent->memory.emin),
 			atomic_long_read(&parent->memory.children_min_usage));
 
-	memcg->memory.elow = effective_protection(usage,
+	memcg->memory.elow = effective_protection(usage, parent_usage,
 			memcg->memory.low, READ_ONCE(parent->memory.elow),
 			atomic_long_read(&parent->memory.children_low_usage));
 
-- 
2.24.0