[RFC PATCH 1/6] mm/page_counter: Move calculating protection values to page_counter

Maarten Lankhorst
2024-06-27T15:57:52Z

It's a lot of math, and there is nothing memcontrol specific about it.
This makes it easier to use inside of the drm cgroup controller.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
---
include/linux/page_counter.h | 4 +
mm/memcontrol.c | 154 +------------------------------
mm/page_counter.c | 173 +++++++++++++++++++++++++++++++++++
3 files changed, 180 insertions(+), 151 deletions(-)

diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
index 8cd858d912c4..904c52f97284 100644
--- a/include/linux/page_counter.h
+++ b/include/linux/page_counter.h
@@ -81,4 +81,8 @@ static inline void page_counter_reset_watermark(struct page_counter *counter)
counter->watermark = page_counter_read(counter);
}

+void page_counter_calculate_protection(struct page_counter *root,
+ struct page_counter *counter,
+ bool recursive_protection);
+
#endif /* _LINUX_PAGE_COUNTER_H */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 71fe2a95b8bd..9454e1a3120e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7316,122 +7316,6 @@ 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.
- *
- * 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);
- /*
- * If all cgroups at this level combined claim and use more
- * protection than 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.
- */
- 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.
- *
- * Check both usage and parent_usage against the respective
- * protected values. One should imply the other, but they
- * aren't read atomically - make sure the division is sane.
- */
- if (!(cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT))
- return ep;
- if (parent_effective > siblings_protected &&
- parent_usage > siblings_protected &&
- usage > protected) {
- unsigned long unclaimed;
-
- unclaimed = parent_effective - siblings_protected;
- unclaimed *= usage - protected;
- unclaimed /= parent_usage - siblings_protected;
-
- ep += unclaimed;
- }
-
- return ep;
-}
-
/**
* mem_cgroup_calculate_protection - check if memory consumption is in the normal range
* @root: the top ancestor of the sub-tree being checked
@@ -7443,8 +7327,8 @@ static unsigned long effective_protection(unsigned long usage,
void mem_cgroup_calculate_protection(struct mem_cgroup *root,
struct mem_cgroup *memcg)
{
- unsigned long usage, parent_usage;
- struct mem_cgroup *parent;
+ bool recursive_protection =
+ cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT;

if (mem_cgroup_disabled())
return;
@@ -7452,39 +7336,7 @@ void mem_cgroup_calculate_protection(struct mem_cgroup *root,
if (!root)
root = root_mem_cgroup;

- /*
- * Effective values of the reclaim targets are ignored so they
- * can be stale. Have a look at mem_cgroup_protection for more
- * details.
- * TODO: calculation should be more robust so that we do not need
- * that special casing.
- */
- if (memcg == root)
- return;
-
- usage = page_counter_read(&memcg->memory);
- if (!usage)
- return;
-
- parent = parent_mem_cgroup(memcg);
-
- if (parent == root) {
- memcg->memory.emin = READ_ONCE(memcg->memory.min);
- memcg->memory.elow = READ_ONCE(memcg->memory.low);
- return;
- }
-
- parent_usage = page_counter_read(&parent->memory);
-
- WRITE_ONCE(memcg->memory.emin, effective_protection(usage, parent_usage,
- READ_ONCE(memcg->memory.min),
- READ_ONCE(parent->memory.emin),
- atomic_long_read(&parent->memory.children_min_usage)));
-
- WRITE_ONCE(memcg->memory.elow, effective_protection(usage, parent_usage,
- READ_ONCE(memcg->memory.low),
- READ_ONCE(parent->memory.elow),
- atomic_long_read(&parent->memory.children_low_usage)));
+ page_counter_calculate_protection(&root->memory, &memcg->memory, recursive_protection);
}

static int charge_memcg(struct folio *folio, struct mem_cgroup *memcg,
diff --git a/mm/page_counter.c b/mm/page_counter.c
index db20d6452b71..8ee49cbf71be 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -262,3 +262,176 @@ int page_counter_memparse(const char *buf, const char *max,

return 0;
}
+
+
+/*
+ * This function calculates an individual page counter'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 counter 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 counter'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 counter's fixed share
+ * of the ancestor's claim to protection, any unutilized -
+ * "floating" - protection from up the tree is distributed in
+ * proportion to each counter'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,
+ bool recursive_protection)
+{
+ unsigned long protected;
+ unsigned long ep;
+
+ protected = min(usage, setting);
+ /*
+ * If all cgroups at this level combined claim and use more
+ * protection than 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.
+ */
+ 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.
+ *
+ * Check both usage and parent_usage against the respective
+ * protected values. One should imply the other, but they
+ * aren't read atomically - make sure the division is sane.
+ */
+ if (!recursive_protection)
+ return ep;
+
+ if (parent_effective > siblings_protected &&
+ parent_usage > siblings_protected &&
+ usage > protected) {
+ unsigned long unclaimed;
+
+ unclaimed = parent_effective - siblings_protected;
+ unclaimed *= usage - protected;
+ unclaimed /= parent_usage - siblings_protected;
+
+ ep += unclaimed;
+ }
+
+ return ep;
+}
+
+
+/**
+ * page_counter_calculate_protection - check if memory consumption is in the normal range
+ * @root: the top ancestor of the sub-tree being checked
+ * @memcg: the memory cgroup to check
+ * @recursive_protection: Whether to use memory_recursiveprot behavior.
+ *
+ * Calculates elow/emin thresholds for given page_counter.
+ *
+ * WARNING: This function is not stateless! It can only be used as part
+ * of a top-down tree iteration, not for isolated queries.
+ */
+void page_counter_calculate_protection(struct page_counter *root,
+ struct page_counter *counter,
+ bool recursive_protection)
+{
+ unsigned long usage, parent_usage;
+ struct page_counter *parent = counter->parent;
+
+ /*
+ * Effective values of the reclaim targets are ignored so they
+ * can be stale. Have a look at mem_cgroup_protection for more
+ * details.
+ * TODO: calculation should be more robust so that we do not need
+ * that special casing.
+ */
+ if (root == counter)
+ return;
+
+ usage = page_counter_read(counter);
+ if (!usage)
+ return;
+
+ if (parent == root) {
+ counter->emin = READ_ONCE(counter->min);
+ counter->elow = READ_ONCE(counter->low);
+ return;
+ }
+
+ parent_usage = page_counter_read(parent);
+
+ WRITE_ONCE(counter->emin, effective_protection(usage, parent_usage,
+ READ_ONCE(counter->min),
+ READ_ONCE(parent->emin),
+ atomic_long_read(&parent->children_min_usage),
+ recursive_protection));
+
+ WRITE_ONCE(counter->elow, effective_protection(usage, parent_usage,
+ READ_ONCE(counter->low),
+ READ_ONCE(parent->elow),
+ atomic_long_read(&parent->children_low_usage),
+ recursive_protection));
+}
--
2.45.2