[PATCH v5 0/4] memcg: introduce per-memcg proactive reclaim

[PATCH v5 0/4] memcg: introduce per-memcg proactive reclaim

[Yosry Ahmed]

This patch series adds a memory.reclaim proactive reclaim interface.
The rationale behind the interface and how it works are in the first
patch.

---

Changes in V5:
- Fixed comment formating and added Co-developed-by in patch 1.
- Modified selftest to work if swap is enabled or not, and retry
  multiple times to wait for background allocation before failing
  with a clear message.

Changes in V4:
mm/memcontrol.c:
- Return -EINTR on signal_pending().
- On the final retry, drain percpu lru caches hoping that it might
  introduce some evictable pages for reclaim.
- Simplified the retry loop as suggested by Dan Schatzberg.

selftests:
- Always return -errno on failure from cg_write() (whether open() or
  write() fail), also update cg_read() and read_text() to return -errno
  as well for consistency. Also make sure to correctly check that the
  whole buffer was written in cg_write().
- Added a maximum number of retries for the reclaim selftest.

Changes in V3:
- Fix cg_write() (in patch 2) to properly return -1 if open() fails
  and not fail if len == errno.
- Remove debug printf() in patch 3.

Changes in V2:
- Add the interface to root as well.
- Added a selftest.
- Documented the interface as a nested-keyed interface, which makes
  adding optional arguments in the future easier (see doc updates in the
  first patch).
- Modified the commit message to reflect changes and added a timeout
  argument as a suggested possible extension
- Return -EAGAIN if the kernel fails to reclaim the full requested
  amount.

---

Shakeel Butt (1):
  memcg: introduce per-memcg reclaim interface

Yosry Ahmed (3):
  selftests: cgroup: return -errno from cg_read()/cg_write() on failure
  selftests: cgroup: fix alloc_anon_noexit() instantly freeing memory
  selftests: cgroup: add a selftest for memory.reclaim

 Documentation/admin-guide/cgroup-v2.rst       |  21 ++++
 mm/memcontrol.c                               |  45 +++++++
 tools/testing/selftests/cgroup/cgroup_util.c  |  44 +++----
 .../selftests/cgroup/test_memcontrol.c        | 114 +++++++++++++++++-
 4 files changed, 197 insertions(+), 27 deletions(-)

-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

[PATCH v5 0/4] memcg: introduce per-memcg proactive reclaim

[Yosry Ahmed]

This patch series adds a memory.reclaim proactive reclaim interface.
The rationale behind the interface and how it works are in the first
patch.

---

Changes in V5:
- Fixed comment formating and added Co-developed-by in patch 1.
- Modified selftest to work if swap is enabled or not, and retry
  multiple times to wait for background allocation before failing
  with a clear message.

Changes in V4:
mm/memcontrol.c:
- Return -EINTR on signal_pending().
- On the final retry, drain percpu lru caches hoping that it might
  introduce some evictable pages for reclaim.
- Simplified the retry loop as suggested by Dan Schatzberg.

selftests:
- Always return -errno on failure from cg_write() (whether open() or
  write() fail), also update cg_read() and read_text() to return -errno
  as well for consistency. Also make sure to correctly check that the
  whole buffer was written in cg_write().
- Added a maximum number of retries for the reclaim selftest.

Changes in V3:
- Fix cg_write() (in patch 2) to properly return -1 if open() fails
  and not fail if len == errno.
- Remove debug printf() in patch 3.

Changes in V2:
- Add the interface to root as well.
- Added a selftest.
- Documented the interface as a nested-keyed interface, which makes
  adding optional arguments in the future easier (see doc updates in the
  first patch).
- Modified the commit message to reflect changes and added a timeout
  argument as a suggested possible extension
- Return -EAGAIN if the kernel fails to reclaim the full requested
  amount.

---

Shakeel Butt (1):
  memcg: introduce per-memcg reclaim interface

Yosry Ahmed (3):
  selftests: cgroup: return -errno from cg_read()/cg_write() on failure
  selftests: cgroup: fix alloc_anon_noexit() instantly freeing memory
  selftests: cgroup: add a selftest for memory.reclaim

 Documentation/admin-guide/cgroup-v2.rst       |  21 ++++
 mm/memcontrol.c                               |  45 +++++++
 tools/testing/selftests/cgroup/cgroup_util.c  |  44 +++----
 .../selftests/cgroup/test_memcontrol.c        | 114 +++++++++++++++++-
 4 files changed, 197 insertions(+), 27 deletions(-)

-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

[PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Yosry Ahmed]

From: Shakeel Butt <shakeelb@google.com>

Introduce a memcg interface to trigger memory reclaim on a memory cgroup.

Use case: Proactive Reclaim
---------------------------

A userspace proactive reclaimer can continuously probe the memcg to
reclaim a small amount of memory. This gives more accurate and
up-to-date workingset estimation as the LRUs are continuously
sorted and can potentially provide more deterministic memory
overcommit behavior. The memory overcommit controller can provide
more proactive response to the changing behavior of the running
applications instead of being reactive.

A userspace reclaimer's purpose in this case is not a complete replacement
for kswapd or direct reclaim, it is to proactively identify memory savings
opportunities and reclaim some amount of cold pages set by the policy
to free up the memory for more demanding jobs or scheduling new jobs.

A user space proactive reclaimer is used in Google data centers.
Additionally, Meta's TMO paper recently referenced a very similar
interface used for user space proactive reclaim:
https://dl.acm.org/doi/pdf/10.1145/3503222.3507731

Benefits of a user space reclaimer:
-----------------------------------

1) More flexible on who should be charged for the cpu of the memory
reclaim. For proactive reclaim, it makes more sense to be centralized.

2) More flexible on dedicating the resources (like cpu). The memory
overcommit controller can balance the cost between the cpu usage and
the memory reclaimed.

3) Provides a way to the applications to keep their LRUs sorted, so,
under memory pressure better reclaim candidates are selected. This also
gives more accurate and uptodate notion of working set for an
application.

Why memory.high is not enough?
------------------------------

- memory.high can be used to trigger reclaim in a memcg and can
  potentially be used for proactive reclaim.
  However there is a big downside in using memory.high. It can potentially
  introduce high reclaim stalls in the target application as the
  allocations from the processes or the threads of the application can hit
  the temporary memory.high limit.

- Userspace proactive reclaimers usually use feedback loops to decide
  how much memory to proactively reclaim from a workload. The metrics
  used for this are usually either refaults or PSI, and these metrics
  will become messy if the application gets throttled by hitting the
  high limit.

- memory.high is a stateful interface, if the userspace proactive
  reclaimer crashes for any reason while triggering reclaim it can leave
  the application in a bad state.

- If a workload is rapidly expanding, setting memory.high to proactively
  reclaim memory can result in actually reclaiming more memory than
  intended.

The benefits of such interface and shortcomings of existing interface
were further discussed in this RFC thread:
https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/

Interface:
----------

Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
trigger reclaim in the target memory cgroup.

The interface is introduced as a nested-keyed file to allow for future
optional arguments to be easily added to configure the behavior of
reclaim.

Possible Extensions:
--------------------

- This interface can be extended with an additional parameter or flags
  to allow specifying one or more types of memory to reclaim from (e.g.
  file, anon, ..).

- The interface can also be extended with a node mask to reclaim from
  specific nodes. This has use cases for reclaim-based demotion in memory
  tiering systens.

- A similar per-node interface can also be added to support proactive
  reclaim and reclaim-based demotion in systems without memcg.

- Add a timeout parameter to make it easier for user space to call the
  interface without worrying about being blocked for an undefined amount
  of time.

For now, let's keep things simple by adding the basic functionality.

[yosryahmed@google.com: worked on versions v2 onwards, refreshed to
current master, updated commit message based on recent
discussions and use cases]

Signed-off-by: Shakeel Butt <shakeelb@google.com>
Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Wei Xu <weixugc@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
---
 Documentation/admin-guide/cgroup-v2.rst | 21 ++++++++++++
 mm/memcontrol.c                         | 45 +++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 69d7a6983f78..19bcd73cad03 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1208,6 +1208,27 @@ PAGE_SIZE multiple when read back.
 	high limit is used and monitored properly, this limit's
 	utility is limited to providing the final safety net.
 
+  memory.reclaim
+	A write-only nested-keyed file which exists for all cgroups.
+
+	This is a simple interface to trigger memory reclaim in the
+	target cgroup.
+
+	This file accepts a single key, the number of bytes to reclaim.
+	No nested keys are currently supported.
+
+	Example::
+
+	  echo "1G" > memory.reclaim
+
+	The interface can be later extended with nested keys to
+	configure the reclaim behavior. For example, specify the
+	type of memory to reclaim from (anon, file, ..).
+
+	Please note that the kernel can over or under reclaim from
+	the target cgroup. If less bytes are reclaimed than the
+	specified amount, -EAGAIN is returned.
+
   memory.oom.group
 	A read-write single value file which exists on non-root
 	cgroups.  The default value is "0".
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 725f76723220..879f1716d6e9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6355,6 +6355,46 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
+			      size_t nbytes, loff_t off)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+	unsigned int nr_retries = MAX_RECLAIM_RETRIES;
+	unsigned long nr_to_reclaim, nr_reclaimed = 0;
+	int err;
+
+	buf = strstrip(buf);
+	err = page_counter_memparse(buf, "", &nr_to_reclaim);
+	if (err)
+		return err;
+
+	while (nr_reclaimed < nr_to_reclaim) {
+		unsigned long reclaimed;
+
+		if (signal_pending(current))
+			return -EINTR;
+
+		/*
+		 * This is the final attempt, drain percpu lru caches in the
+		 * hope of introducing more evictable pages for
+		 * try_to_free_mem_cgroup_pages().
+		 */
+		if (!nr_retries)
+			lru_add_drain_all();
+
+		reclaimed = try_to_free_mem_cgroup_pages(memcg,
+						nr_to_reclaim - nr_reclaimed,
+						GFP_KERNEL, true);
+
+		if (!reclaimed && !nr_retries--)
+			return -EAGAIN;
+
+		nr_reclaimed += reclaimed;
+	}
+
+	return nbytes;
+}
+
 static struct cftype memory_files[] = {
 	{
 		.name = "current",
@@ -6413,6 +6453,11 @@ static struct cftype memory_files[] = {
 		.seq_show = memory_oom_group_show,
 		.write = memory_oom_group_write,
 	},
+	{
+		.name = "reclaim",
+		.flags = CFTYPE_NS_DELEGATABLE,
+		.write = memory_reclaim,
+	},
 	{ }	/* terminate */
 };
 
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

[PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Yosry Ahmed]

From: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

Introduce a memcg interface to trigger memory reclaim on a memory cgroup.

Use case: Proactive Reclaim
---------------------------

A userspace proactive reclaimer can continuously probe the memcg to
reclaim a small amount of memory. This gives more accurate and
up-to-date workingset estimation as the LRUs are continuously
sorted and can potentially provide more deterministic memory
overcommit behavior. The memory overcommit controller can provide
more proactive response to the changing behavior of the running
applications instead of being reactive.

A userspace reclaimer's purpose in this case is not a complete replacement
for kswapd or direct reclaim, it is to proactively identify memory savings
opportunities and reclaim some amount of cold pages set by the policy
to free up the memory for more demanding jobs or scheduling new jobs.

A user space proactive reclaimer is used in Google data centers.
Additionally, Meta's TMO paper recently referenced a very similar
interface used for user space proactive reclaim:
https://dl.acm.org/doi/pdf/10.1145/3503222.3507731

Benefits of a user space reclaimer:
-----------------------------------

1) More flexible on who should be charged for the cpu of the memory
reclaim. For proactive reclaim, it makes more sense to be centralized.

2) More flexible on dedicating the resources (like cpu). The memory
overcommit controller can balance the cost between the cpu usage and
the memory reclaimed.

3) Provides a way to the applications to keep their LRUs sorted, so,
under memory pressure better reclaim candidates are selected. This also
gives more accurate and uptodate notion of working set for an
application.

Why memory.high is not enough?
------------------------------

- memory.high can be used to trigger reclaim in a memcg and can
  potentially be used for proactive reclaim.
  However there is a big downside in using memory.high. It can potentially
  introduce high reclaim stalls in the target application as the
  allocations from the processes or the threads of the application can hit
  the temporary memory.high limit.

- Userspace proactive reclaimers usually use feedback loops to decide
  how much memory to proactively reclaim from a workload. The metrics
  used for this are usually either refaults or PSI, and these metrics
  will become messy if the application gets throttled by hitting the
  high limit.

- memory.high is a stateful interface, if the userspace proactive
  reclaimer crashes for any reason while triggering reclaim it can leave
  the application in a bad state.

- If a workload is rapidly expanding, setting memory.high to proactively
  reclaim memory can result in actually reclaiming more memory than
  intended.

The benefits of such interface and shortcomings of existing interface
were further discussed in this RFC thread:
https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org/

Interface:
----------

Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
trigger reclaim in the target memory cgroup.

The interface is introduced as a nested-keyed file to allow for future
optional arguments to be easily added to configure the behavior of
reclaim.

Possible Extensions:
--------------------

- This interface can be extended with an additional parameter or flags
  to allow specifying one or more types of memory to reclaim from (e.g.
  file, anon, ..).

- The interface can also be extended with a node mask to reclaim from
  specific nodes. This has use cases for reclaim-based demotion in memory
  tiering systens.

- A similar per-node interface can also be added to support proactive
  reclaim and reclaim-based demotion in systems without memcg.

- Add a timeout parameter to make it easier for user space to call the
  interface without worrying about being blocked for an undefined amount
  of time.

For now, let's keep things simple by adding the basic functionality.

[yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org: worked on versions v2 onwards, refreshed to
current master, updated commit message based on recent
discussions and use cases]

Signed-off-by: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Co-developed-by: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Signed-off-by: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
Acked-by: Wei Xu <weixugc-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Acked-by: Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>
---
 Documentation/admin-guide/cgroup-v2.rst | 21 ++++++++++++
 mm/memcontrol.c                         | 45 +++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 69d7a6983f78..19bcd73cad03 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1208,6 +1208,27 @@ PAGE_SIZE multiple when read back.
 	high limit is used and monitored properly, this limit's
 	utility is limited to providing the final safety net.
 
+  memory.reclaim
+	A write-only nested-keyed file which exists for all cgroups.
+
+	This is a simple interface to trigger memory reclaim in the
+	target cgroup.
+
+	This file accepts a single key, the number of bytes to reclaim.
+	No nested keys are currently supported.
+
+	Example::
+
+	  echo "1G" > memory.reclaim
+
+	The interface can be later extended with nested keys to
+	configure the reclaim behavior. For example, specify the
+	type of memory to reclaim from (anon, file, ..).
+
+	Please note that the kernel can over or under reclaim from
+	the target cgroup. If less bytes are reclaimed than the
+	specified amount, -EAGAIN is returned.
+
   memory.oom.group
 	A read-write single value file which exists on non-root
 	cgroups.  The default value is "0".
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 725f76723220..879f1716d6e9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6355,6 +6355,46 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
 	return nbytes;
 }
 
+static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
+			      size_t nbytes, loff_t off)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+	unsigned int nr_retries = MAX_RECLAIM_RETRIES;
+	unsigned long nr_to_reclaim, nr_reclaimed = 0;
+	int err;
+
+	buf = strstrip(buf);
+	err = page_counter_memparse(buf, "", &nr_to_reclaim);
+	if (err)
+		return err;
+
+	while (nr_reclaimed < nr_to_reclaim) {
+		unsigned long reclaimed;
+
+		if (signal_pending(current))
+			return -EINTR;
+
+		/*
+		 * This is the final attempt, drain percpu lru caches in the
+		 * hope of introducing more evictable pages for
+		 * try_to_free_mem_cgroup_pages().
+		 */
+		if (!nr_retries)
+			lru_add_drain_all();
+
+		reclaimed = try_to_free_mem_cgroup_pages(memcg,
+						nr_to_reclaim - nr_reclaimed,
+						GFP_KERNEL, true);
+
+		if (!reclaimed && !nr_retries--)
+			return -EAGAIN;
+
+		nr_reclaimed += reclaimed;
+	}
+
+	return nbytes;
+}
+
 static struct cftype memory_files[] = {
 	{
 		.name = "current",
@@ -6413,6 +6453,11 @@ static struct cftype memory_files[] = {
 		.seq_show = memory_oom_group_show,
 		.write = memory_oom_group_write,
 	},
+	{
+		.name = "reclaim",
+		.flags = CFTYPE_NS_DELEGATABLE,
+		.write = memory_reclaim,
+	},
 	{ }	/* terminate */
 };
 
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[David Rientjes]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

> From: Shakeel Butt <shakeelb@google.com>
> 
> Introduce a memcg interface to trigger memory reclaim on a memory cgroup.
> 
> Use case: Proactive Reclaim
> ---------------------------
> 
> A userspace proactive reclaimer can continuously probe the memcg to
> reclaim a small amount of memory. This gives more accurate and
> up-to-date workingset estimation as the LRUs are continuously
> sorted and can potentially provide more deterministic memory
> overcommit behavior. The memory overcommit controller can provide
> more proactive response to the changing behavior of the running
> applications instead of being reactive.
> 
> A userspace reclaimer's purpose in this case is not a complete replacement
> for kswapd or direct reclaim, it is to proactively identify memory savings
> opportunities and reclaim some amount of cold pages set by the policy
> to free up the memory for more demanding jobs or scheduling new jobs.
> 
> A user space proactive reclaimer is used in Google data centers.
> Additionally, Meta's TMO paper recently referenced a very similar
> interface used for user space proactive reclaim:
> https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
> 
> Benefits of a user space reclaimer:
> -----------------------------------
> 
> 1) More flexible on who should be charged for the cpu of the memory
> reclaim. For proactive reclaim, it makes more sense to be centralized.
> 
> 2) More flexible on dedicating the resources (like cpu). The memory
> overcommit controller can balance the cost between the cpu usage and
> the memory reclaimed.
> 
> 3) Provides a way to the applications to keep their LRUs sorted, so,
> under memory pressure better reclaim candidates are selected. This also
> gives more accurate and uptodate notion of working set for an
> application.
> 
> Why memory.high is not enough?
> ------------------------------
> 
> - memory.high can be used to trigger reclaim in a memcg and can
>   potentially be used for proactive reclaim.
>   However there is a big downside in using memory.high. It can potentially
>   introduce high reclaim stalls in the target application as the
>   allocations from the processes or the threads of the application can hit
>   the temporary memory.high limit.
> 
> - Userspace proactive reclaimers usually use feedback loops to decide
>   how much memory to proactively reclaim from a workload. The metrics
>   used for this are usually either refaults or PSI, and these metrics
>   will become messy if the application gets throttled by hitting the
>   high limit.
> 
> - memory.high is a stateful interface, if the userspace proactive
>   reclaimer crashes for any reason while triggering reclaim it can leave
>   the application in a bad state.
> 
> - If a workload is rapidly expanding, setting memory.high to proactively
>   reclaim memory can result in actually reclaiming more memory than
>   intended.
> 
> The benefits of such interface and shortcomings of existing interface
> were further discussed in this RFC thread:
> https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/
> 
> Interface:
> ----------
> 
> Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
> trigger reclaim in the target memory cgroup.
> 
> The interface is introduced as a nested-keyed file to allow for future
> optional arguments to be easily added to configure the behavior of
> reclaim.
> 
> Possible Extensions:
> --------------------
> 
> - This interface can be extended with an additional parameter or flags
>   to allow specifying one or more types of memory to reclaim from (e.g.
>   file, anon, ..).
> 
> - The interface can also be extended with a node mask to reclaim from
>   specific nodes. This has use cases for reclaim-based demotion in memory
>   tiering systens.
> 
> - A similar per-node interface can also be added to support proactive
>   reclaim and reclaim-based demotion in systems without memcg.
> 
> - Add a timeout parameter to make it easier for user space to call the
>   interface without worrying about being blocked for an undefined amount
>   of time.
> 
> For now, let's keep things simple by adding the basic functionality.
> 
> [yosryahmed@google.com: worked on versions v2 onwards, refreshed to
> current master, updated commit message based on recent
> discussions and use cases]
> 
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Acked-by: Wei Xu <weixugc@google.com>
> Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Acked-by: David Rientjes <rientjes@google.com>

"can over or under reclaim from the target cgroup" begs the question of 
how much more memory the kernel can decide to reclaim :)  I think it's 
assumed that it's minimal and that matches the current implementation that 
rounds up to SWAP_CLUSTER_MAX, though, so looks good.

Thanks Yosry!

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[David Rientjes]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

> From: Shakeel Butt <shakeelb@google.com>
> 
> Introduce a memcg interface to trigger memory reclaim on a memory cgroup.
> 
> Use case: Proactive Reclaim
> ---------------------------
> 
> A userspace proactive reclaimer can continuously probe the memcg to
> reclaim a small amount of memory. This gives more accurate and
> up-to-date workingset estimation as the LRUs are continuously
> sorted and can potentially provide more deterministic memory
> overcommit behavior. The memory overcommit controller can provide
> more proactive response to the changing behavior of the running
> applications instead of being reactive.
> 
> A userspace reclaimer's purpose in this case is not a complete replacement
> for kswapd or direct reclaim, it is to proactively identify memory savings
> opportunities and reclaim some amount of cold pages set by the policy
> to free up the memory for more demanding jobs or scheduling new jobs.
> 
> A user space proactive reclaimer is used in Google data centers.
> Additionally, Meta's TMO paper recently referenced a very similar
> interface used for user space proactive reclaim:
> https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
> 
> Benefits of a user space reclaimer:
> -----------------------------------
> 
> 1) More flexible on who should be charged for the cpu of the memory
> reclaim. For proactive reclaim, it makes more sense to be centralized.
> 
> 2) More flexible on dedicating the resources (like cpu). The memory
> overcommit controller can balance the cost between the cpu usage and
> the memory reclaimed.
> 
> 3) Provides a way to the applications to keep their LRUs sorted, so,
> under memory pressure better reclaim candidates are selected. This also
> gives more accurate and uptodate notion of working set for an
> application.
> 
> Why memory.high is not enough?
> ------------------------------
> 
> - memory.high can be used to trigger reclaim in a memcg and can
>   potentially be used for proactive reclaim.
>   However there is a big downside in using memory.high. It can potentially
>   introduce high reclaim stalls in the target application as the
>   allocations from the processes or the threads of the application can hit
>   the temporary memory.high limit.
> 
> - Userspace proactive reclaimers usually use feedback loops to decide
>   how much memory to proactively reclaim from a workload. The metrics
>   used for this are usually either refaults or PSI, and these metrics
>   will become messy if the application gets throttled by hitting the
>   high limit.
> 
> - memory.high is a stateful interface, if the userspace proactive
>   reclaimer crashes for any reason while triggering reclaim it can leave
>   the application in a bad state.
> 
> - If a workload is rapidly expanding, setting memory.high to proactively
>   reclaim memory can result in actually reclaiming more memory than
>   intended.
> 
> The benefits of such interface and shortcomings of existing interface
> were further discussed in this RFC thread:
> https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/
> 
> Interface:
> ----------
> 
> Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
> trigger reclaim in the target memory cgroup.
> 
> The interface is introduced as a nested-keyed file to allow for future
> optional arguments to be easily added to configure the behavior of
> reclaim.
> 
> Possible Extensions:
> --------------------
> 
> - This interface can be extended with an additional parameter or flags
>   to allow specifying one or more types of memory to reclaim from (e.g.
>   file, anon, ..).
> 
> - The interface can also be extended with a node mask to reclaim from
>   specific nodes. This has use cases for reclaim-based demotion in memory
>   tiering systens.
> 
> - A similar per-node interface can also be added to support proactive
>   reclaim and reclaim-based demotion in systems without memcg.
> 
> - Add a timeout parameter to make it easier for user space to call the
>   interface without worrying about being blocked for an undefined amount
>   of time.
> 
> For now, let's keep things simple by adding the basic functionality.
> 
> [yosryahmed@google.com: worked on versions v2 onwards, refreshed to
> current master, updated commit message based on recent
> discussions and use cases]
> 
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Acked-by: Wei Xu <weixugc@google.com>
> Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Acked-by: David Rientjes <rientjes@google.com>

"can over or under reclaim from the target cgroup" begs the question of 
how much more memory the kernel can decide to reclaim :)  I think it's 
assumed that it's minimal and that matches the current implementation that 
rounds up to SWAP_CLUSTER_MAX, though, so looks good.

Thanks Yosry!

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Yosry Ahmed]

On Mon, Apr 25, 2022 at 12:15 PM David Rientjes <rientjes@google.com> wrote:
>
> On Mon, 25 Apr 2022, Yosry Ahmed wrote:
>
> > From: Shakeel Butt <shakeelb@google.com>
> >
> > Introduce a memcg interface to trigger memory reclaim on a memory cgroup.
> >
> > Use case: Proactive Reclaim
> > ---------------------------
> >
> > A userspace proactive reclaimer can continuously probe the memcg to
> > reclaim a small amount of memory. This gives more accurate and
> > up-to-date workingset estimation as the LRUs are continuously
> > sorted and can potentially provide more deterministic memory
> > overcommit behavior. The memory overcommit controller can provide
> > more proactive response to the changing behavior of the running
> > applications instead of being reactive.
> >
> > A userspace reclaimer's purpose in this case is not a complete replacement
> > for kswapd or direct reclaim, it is to proactively identify memory savings
> > opportunities and reclaim some amount of cold pages set by the policy
> > to free up the memory for more demanding jobs or scheduling new jobs.
> >
> > A user space proactive reclaimer is used in Google data centers.
> > Additionally, Meta's TMO paper recently referenced a very similar
> > interface used for user space proactive reclaim:
> > https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
> >
> > Benefits of a user space reclaimer:
> > -----------------------------------
> >
> > 1) More flexible on who should be charged for the cpu of the memory
> > reclaim. For proactive reclaim, it makes more sense to be centralized.
> >
> > 2) More flexible on dedicating the resources (like cpu). The memory
> > overcommit controller can balance the cost between the cpu usage and
> > the memory reclaimed.
> >
> > 3) Provides a way to the applications to keep their LRUs sorted, so,
> > under memory pressure better reclaim candidates are selected. This also
> > gives more accurate and uptodate notion of working set for an
> > application.
> >
> > Why memory.high is not enough?
> > ------------------------------
> >
> > - memory.high can be used to trigger reclaim in a memcg and can
> >   potentially be used for proactive reclaim.
> >   However there is a big downside in using memory.high. It can potentially
> >   introduce high reclaim stalls in the target application as the
> >   allocations from the processes or the threads of the application can hit
> >   the temporary memory.high limit.
> >
> > - Userspace proactive reclaimers usually use feedback loops to decide
> >   how much memory to proactively reclaim from a workload. The metrics
> >   used for this are usually either refaults or PSI, and these metrics
> >   will become messy if the application gets throttled by hitting the
> >   high limit.
> >
> > - memory.high is a stateful interface, if the userspace proactive
> >   reclaimer crashes for any reason while triggering reclaim it can leave
> >   the application in a bad state.
> >
> > - If a workload is rapidly expanding, setting memory.high to proactively
> >   reclaim memory can result in actually reclaiming more memory than
> >   intended.
> >
> > The benefits of such interface and shortcomings of existing interface
> > were further discussed in this RFC thread:
> > https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/
> >
> > Interface:
> > ----------
> >
> > Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
> > trigger reclaim in the target memory cgroup.
> >
> > The interface is introduced as a nested-keyed file to allow for future
> > optional arguments to be easily added to configure the behavior of
> > reclaim.
> >
> > Possible Extensions:
> > --------------------
> >
> > - This interface can be extended with an additional parameter or flags
> >   to allow specifying one or more types of memory to reclaim from (e.g.
> >   file, anon, ..).
> >
> > - The interface can also be extended with a node mask to reclaim from
> >   specific nodes. This has use cases for reclaim-based demotion in memory
> >   tiering systens.
> >
> > - A similar per-node interface can also be added to support proactive
> >   reclaim and reclaim-based demotion in systems without memcg.
> >
> > - Add a timeout parameter to make it easier for user space to call the
> >   interface without worrying about being blocked for an undefined amount
> >   of time.
> >
> > For now, let's keep things simple by adding the basic functionality.
> >
> > [yosryahmed@google.com: worked on versions v2 onwards, refreshed to
> > current master, updated commit message based on recent
> > discussions and use cases]
> >
> > Signed-off-by: Shakeel Butt <shakeelb@google.com>
> > Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > Acked-by: Michal Hocko <mhocko@suse.com>
> > Acked-by: Wei Xu <weixugc@google.com>
> > Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
>
> Acked-by: David Rientjes <rientjes@google.com>
>
> "can over or under reclaim from the target cgroup" begs the question of
> how much more memory the kernel can decide to reclaim :)  I think it's
> assumed that it's minimal and that matches the current implementation that
> rounds up to SWAP_CLUSTER_MAX, though, so looks good.
>
> Thanks Yosry!

I think it could be more complex than this. Some functions that get
called during reclaim only use the nr_to_reclaim parameter to check if
they need one more iteration, but not to limit the actual reclaimed
pages per say. For example, nr_to_reclaim is not even passed to
shrink_slab() or mem_cgroup_soft_limit_reclaim(), so they have no way
to know that they should stop if nr_to_reclaim was already satisfied.
I think the general assumption is that each of these calls normally
does not reclaim a huge number of pages, so like you said, the kernel
should not over-reclaim too much. However, I don't think there are
guarantees about this.

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Yosry Ahmed]

On Mon, Apr 25, 2022 at 12:15 PM David Rientjes <rientjes@google.com> wrote:
>
> On Mon, 25 Apr 2022, Yosry Ahmed wrote:
>
> > From: Shakeel Butt <shakeelb@google.com>
> >
> > Introduce a memcg interface to trigger memory reclaim on a memory cgroup.
> >
> > Use case: Proactive Reclaim
> > ---------------------------
> >
> > A userspace proactive reclaimer can continuously probe the memcg to
> > reclaim a small amount of memory. This gives more accurate and
> > up-to-date workingset estimation as the LRUs are continuously
> > sorted and can potentially provide more deterministic memory
> > overcommit behavior. The memory overcommit controller can provide
> > more proactive response to the changing behavior of the running
> > applications instead of being reactive.
> >
> > A userspace reclaimer's purpose in this case is not a complete replacement
> > for kswapd or direct reclaim, it is to proactively identify memory savings
> > opportunities and reclaim some amount of cold pages set by the policy
> > to free up the memory for more demanding jobs or scheduling new jobs.
> >
> > A user space proactive reclaimer is used in Google data centers.
> > Additionally, Meta's TMO paper recently referenced a very similar
> > interface used for user space proactive reclaim:
> > https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
> >
> > Benefits of a user space reclaimer:
> > -----------------------------------
> >
> > 1) More flexible on who should be charged for the cpu of the memory
> > reclaim. For proactive reclaim, it makes more sense to be centralized.
> >
> > 2) More flexible on dedicating the resources (like cpu). The memory
> > overcommit controller can balance the cost between the cpu usage and
> > the memory reclaimed.
> >
> > 3) Provides a way to the applications to keep their LRUs sorted, so,
> > under memory pressure better reclaim candidates are selected. This also
> > gives more accurate and uptodate notion of working set for an
> > application.
> >
> > Why memory.high is not enough?
> > ------------------------------
> >
> > - memory.high can be used to trigger reclaim in a memcg and can
> >   potentially be used for proactive reclaim.
> >   However there is a big downside in using memory.high. It can potentially
> >   introduce high reclaim stalls in the target application as the
> >   allocations from the processes or the threads of the application can hit
> >   the temporary memory.high limit.
> >
> > - Userspace proactive reclaimers usually use feedback loops to decide
> >   how much memory to proactively reclaim from a workload. The metrics
> >   used for this are usually either refaults or PSI, and these metrics
> >   will become messy if the application gets throttled by hitting the
> >   high limit.
> >
> > - memory.high is a stateful interface, if the userspace proactive
> >   reclaimer crashes for any reason while triggering reclaim it can leave
> >   the application in a bad state.
> >
> > - If a workload is rapidly expanding, setting memory.high to proactively
> >   reclaim memory can result in actually reclaiming more memory than
> >   intended.
> >
> > The benefits of such interface and shortcomings of existing interface
> > were further discussed in this RFC thread:
> > https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/
> >
> > Interface:
> > ----------
> >
> > Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
> > trigger reclaim in the target memory cgroup.
> >
> > The interface is introduced as a nested-keyed file to allow for future
> > optional arguments to be easily added to configure the behavior of
> > reclaim.
> >
> > Possible Extensions:
> > --------------------
> >
> > - This interface can be extended with an additional parameter or flags
> >   to allow specifying one or more types of memory to reclaim from (e.g.
> >   file, anon, ..).
> >
> > - The interface can also be extended with a node mask to reclaim from
> >   specific nodes. This has use cases for reclaim-based demotion in memory
> >   tiering systens.
> >
> > - A similar per-node interface can also be added to support proactive
> >   reclaim and reclaim-based demotion in systems without memcg.
> >
> > - Add a timeout parameter to make it easier for user space to call the
> >   interface without worrying about being blocked for an undefined amount
> >   of time.
> >
> > For now, let's keep things simple by adding the basic functionality.
> >
> > [yosryahmed@google.com: worked on versions v2 onwards, refreshed to
> > current master, updated commit message based on recent
> > discussions and use cases]
> >
> > Signed-off-by: Shakeel Butt <shakeelb@google.com>
> > Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > Acked-by: Michal Hocko <mhocko@suse.com>
> > Acked-by: Wei Xu <weixugc@google.com>
> > Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
>
> Acked-by: David Rientjes <rientjes@google.com>
>
> "can over or under reclaim from the target cgroup" begs the question of
> how much more memory the kernel can decide to reclaim :)  I think it's
> assumed that it's minimal and that matches the current implementation that
> rounds up to SWAP_CLUSTER_MAX, though, so looks good.
>
> Thanks Yosry!

I think it could be more complex than this. Some functions that get
called during reclaim only use the nr_to_reclaim parameter to check if
they need one more iteration, but not to limit the actual reclaimed
pages per say. For example, nr_to_reclaim is not even passed to
shrink_slab() or mem_cgroup_soft_limit_reclaim(), so they have no way
to know that they should stop if nr_to_reclaim was already satisfied.
I think the general assumption is that each of these calls normally
does not reclaim a huge number of pages, so like you said, the kernel
should not over-reclaim too much. However, I don't think there are
guarantees about this.

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Michal Hocko]

On Mon 25-04-22 12:31:51, Yosry Ahmed wrote:
> On Mon, Apr 25, 2022 at 12:15 PM David Rientjes <rientjes@google.com> wrote:
[...]
> > "can over or under reclaim from the target cgroup" begs the question of
> > how much more memory the kernel can decide to reclaim :)  I think it's
> > assumed that it's minimal and that matches the current implementation that
> > rounds up to SWAP_CLUSTER_MAX, though, so looks good.
> >
> > Thanks Yosry!
> 
> I think it could be more complex than this. Some functions that get
> called during reclaim only use the nr_to_reclaim parameter to check if
> they need one more iteration, but not to limit the actual reclaimed
> pages per say. For example, nr_to_reclaim is not even passed to
> shrink_slab() or mem_cgroup_soft_limit_reclaim(), so they have no way
> to know that they should stop if nr_to_reclaim was already satisfied.
> I think the general assumption is that each of these calls normally
> does not reclaim a huge number of pages, so like you said, the kernel
> should not over-reclaim too much. However, I don't think there are
> guarantees about this.

There are no guarantees indeed and it si definitely good to be explicit
about that so that userspace tools expect that and consider that in
the imeplementation. Sure we do not want to go overboard and huge excess
should be considered a bug. I am not sure we do agree on the notion of
"huge" so let's see.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Michal Hocko]

On Mon 25-04-22 12:31:51, Yosry Ahmed wrote:
> On Mon, Apr 25, 2022 at 12:15 PM David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
[...]
> > "can over or under reclaim from the target cgroup" begs the question of
> > how much more memory the kernel can decide to reclaim :)  I think it's
> > assumed that it's minimal and that matches the current implementation that
> > rounds up to SWAP_CLUSTER_MAX, though, so looks good.
> >
> > Thanks Yosry!
> 
> I think it could be more complex than this. Some functions that get
> called during reclaim only use the nr_to_reclaim parameter to check if
> they need one more iteration, but not to limit the actual reclaimed
> pages per say. For example, nr_to_reclaim is not even passed to
> shrink_slab() or mem_cgroup_soft_limit_reclaim(), so they have no way
> to know that they should stop if nr_to_reclaim was already satisfied.
> I think the general assumption is that each of these calls normally
> does not reclaim a huge number of pages, so like you said, the kernel
> should not over-reclaim too much. However, I don't think there are
> guarantees about this.

There are no guarantees indeed and it si definitely good to be explicit
about that so that userspace tools expect that and consider that in
the imeplementation. Sure we do not want to go overboard and huge excess
should be considered a bug. I am not sure we do agree on the notion of
"huge" so let's see.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Davidlohr Bueso]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

>From: Shakeel Butt <shakeelb@google.com>
>
>Introduce a memcg interface to trigger memory reclaim on a memory cgroup.
>
>Use case: Proactive Reclaim
>---------------------------
>
>A userspace proactive reclaimer can continuously probe the memcg to
>reclaim a small amount of memory. This gives more accurate and
>up-to-date workingset estimation as the LRUs are continuously
>sorted and can potentially provide more deterministic memory
>overcommit behavior. The memory overcommit controller can provide
>more proactive response to the changing behavior of the running
>applications instead of being reactive.
>
>A userspace reclaimer's purpose in this case is not a complete replacement
>for kswapd or direct reclaim, it is to proactively identify memory savings
>opportunities and reclaim some amount of cold pages set by the policy
>to free up the memory for more demanding jobs or scheduling new jobs.
>
>A user space proactive reclaimer is used in Google data centers.
>Additionally, Meta's TMO paper recently referenced a very similar
>interface used for user space proactive reclaim:
>https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
>
>Benefits of a user space reclaimer:
>-----------------------------------
>
>1) More flexible on who should be charged for the cpu of the memory
>reclaim. For proactive reclaim, it makes more sense to be centralized.
>
>2) More flexible on dedicating the resources (like cpu). The memory
>overcommit controller can balance the cost between the cpu usage and
>the memory reclaimed.
>
>3) Provides a way to the applications to keep their LRUs sorted, so,
>under memory pressure better reclaim candidates are selected. This also
>gives more accurate and uptodate notion of working set for an
>application.
>
>Why memory.high is not enough?
>------------------------------
>
>- memory.high can be used to trigger reclaim in a memcg and can
>  potentially be used for proactive reclaim.
>  However there is a big downside in using memory.high. It can potentially
>  introduce high reclaim stalls in the target application as the
>  allocations from the processes or the threads of the application can hit
>  the temporary memory.high limit.
>
>- Userspace proactive reclaimers usually use feedback loops to decide
>  how much memory to proactively reclaim from a workload. The metrics
>  used for this are usually either refaults or PSI, and these metrics
>  will become messy if the application gets throttled by hitting the
>  high limit.
>
>- memory.high is a stateful interface, if the userspace proactive
>  reclaimer crashes for any reason while triggering reclaim it can leave
>  the application in a bad state.
>
>- If a workload is rapidly expanding, setting memory.high to proactively
>  reclaim memory can result in actually reclaiming more memory than
>  intended.
>
>The benefits of such interface and shortcomings of existing interface
>were further discussed in this RFC thread:
>https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/
>
>Interface:
>----------
>
>Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
>trigger reclaim in the target memory cgroup.
>
>The interface is introduced as a nested-keyed file to allow for future
>optional arguments to be easily added to configure the behavior of
>reclaim.
>
>Possible Extensions:
>--------------------
>
>- This interface can be extended with an additional parameter or flags
>  to allow specifying one or more types of memory to reclaim from (e.g.
>  file, anon, ..).
>
>- The interface can also be extended with a node mask to reclaim from
>  specific nodes. This has use cases for reclaim-based demotion in memory
>  tiering systens.
>
>- A similar per-node interface can also be added to support proactive
>  reclaim and reclaim-based demotion in systems without memcg.
>
>- Add a timeout parameter to make it easier for user space to call the
>  interface without worrying about being blocked for an undefined amount
>  of time.
>
>For now, let's keep things simple by adding the basic functionality.
>
>[yosryahmed@google.com: worked on versions v2 onwards, refreshed to
>current master, updated commit message based on recent
>discussions and use cases]
>
>Signed-off-by: Shakeel Butt <shakeelb@google.com>
>Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
>Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>Acked-by: Michal Hocko <mhocko@suse.com>
>Acked-by: Wei Xu <weixugc@google.com>
>Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>

Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

[Davidlohr Bueso]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

>From: Shakeel Butt <shakeelb@google.com>
>
>Introduce a memcg interface to trigger memory reclaim on a memory cgroup.
>
>Use case: Proactive Reclaim
>---------------------------
>
>A userspace proactive reclaimer can continuously probe the memcg to
>reclaim a small amount of memory. This gives more accurate and
>up-to-date workingset estimation as the LRUs are continuously
>sorted and can potentially provide more deterministic memory
>overcommit behavior. The memory overcommit controller can provide
>more proactive response to the changing behavior of the running
>applications instead of being reactive.
>
>A userspace reclaimer's purpose in this case is not a complete replacement
>for kswapd or direct reclaim, it is to proactively identify memory savings
>opportunities and reclaim some amount of cold pages set by the policy
>to free up the memory for more demanding jobs or scheduling new jobs.
>
>A user space proactive reclaimer is used in Google data centers.
>Additionally, Meta's TMO paper recently referenced a very similar
>interface used for user space proactive reclaim:
>https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
>
>Benefits of a user space reclaimer:
>-----------------------------------
>
>1) More flexible on who should be charged for the cpu of the memory
>reclaim. For proactive reclaim, it makes more sense to be centralized.
>
>2) More flexible on dedicating the resources (like cpu). The memory
>overcommit controller can balance the cost between the cpu usage and
>the memory reclaimed.
>
>3) Provides a way to the applications to keep their LRUs sorted, so,
>under memory pressure better reclaim candidates are selected. This also
>gives more accurate and uptodate notion of working set for an
>application.
>
>Why memory.high is not enough?
>------------------------------
>
>- memory.high can be used to trigger reclaim in a memcg and can
>  potentially be used for proactive reclaim.
>  However there is a big downside in using memory.high. It can potentially
>  introduce high reclaim stalls in the target application as the
>  allocations from the processes or the threads of the application can hit
>  the temporary memory.high limit.
>
>- Userspace proactive reclaimers usually use feedback loops to decide
>  how much memory to proactively reclaim from a workload. The metrics
>  used for this are usually either refaults or PSI, and these metrics
>  will become messy if the application gets throttled by hitting the
>  high limit.
>
>- memory.high is a stateful interface, if the userspace proactive
>  reclaimer crashes for any reason while triggering reclaim it can leave
>  the application in a bad state.
>
>- If a workload is rapidly expanding, setting memory.high to proactively
>  reclaim memory can result in actually reclaiming more memory than
>  intended.
>
>The benefits of such interface and shortcomings of existing interface
>were further discussed in this RFC thread:
>https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/
>
>Interface:
>----------
>
>Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
>trigger reclaim in the target memory cgroup.
>
>The interface is introduced as a nested-keyed file to allow for future
>optional arguments to be easily added to configure the behavior of
>reclaim.
>
>Possible Extensions:
>--------------------
>
>- This interface can be extended with an additional parameter or flags
>  to allow specifying one or more types of memory to reclaim from (e.g.
>  file, anon, ..).
>
>- The interface can also be extended with a node mask to reclaim from
>  specific nodes. This has use cases for reclaim-based demotion in memory
>  tiering systens.
>
>- A similar per-node interface can also be added to support proactive
>  reclaim and reclaim-based demotion in systems without memcg.
>
>- Add a timeout parameter to make it easier for user space to call the
>  interface without worrying about being blocked for an undefined amount
>  of time.
>
>For now, let's keep things simple by adding the basic functionality.
>
>[yosryahmed@google.com: worked on versions v2 onwards, refreshed to
>current master, updated commit message based on recent
>discussions and use cases]
>
>Signed-off-by: Shakeel Butt <shakeelb@google.com>
>Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
>Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>Acked-by: Michal Hocko <mhocko@suse.com>
>Acked-by: Wei Xu <weixugc@google.com>
>Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>

[PATCH v5 2/4] selftests: cgroup: return -errno from cg_read()/cg_write() on failure

[Yosry Ahmed]

Currently, cg_read()/cg_write() returns 0 on success and -1 on failure.
Modify them to return the -errno on failure.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
---
 tools/testing/selftests/cgroup/cgroup_util.c | 44 +++++++++-----------
 1 file changed, 19 insertions(+), 25 deletions(-)

diff --git a/tools/testing/selftests/cgroup/cgroup_util.c b/tools/testing/selftests/cgroup/cgroup_util.c
index dbaa7aabbb4a..e6f3679cdcc0 100644
--- a/tools/testing/selftests/cgroup/cgroup_util.c
+++ b/tools/testing/selftests/cgroup/cgroup_util.c
@@ -19,6 +19,7 @@
 #include "cgroup_util.h"
 #include "../clone3/clone3_selftests.h"
 
+/* Returns read len on success, or -errno on failure. */
 static ssize_t read_text(const char *path, char *buf, size_t max_len)
 {
 	ssize_t len;
@@ -26,35 +27,29 @@ static ssize_t read_text(const char *path, char *buf, size_t max_len)
 
 	fd = open(path, O_RDONLY);
 	if (fd < 0)
-		return fd;
+		return -errno;
 
 	len = read(fd, buf, max_len - 1);
-	if (len < 0)
-		goto out;
 
-	buf[len] = 0;
-out:
+	if (len >= 0)
+		buf[len] = 0;
+
 	close(fd);
-	return len;
+	return len < 0 ? -errno : len;
 }
 
+/* Returns written len on success, or -errno on failure. */
 static ssize_t write_text(const char *path, char *buf, ssize_t len)
 {
 	int fd;
 
 	fd = open(path, O_WRONLY | O_APPEND);
 	if (fd < 0)
-		return fd;
+		return -errno;
 
 	len = write(fd, buf, len);
-	if (len < 0) {
-		close(fd);
-		return len;
-	}
-
 	close(fd);
-
-	return len;
+	return len < 0 ? -errno : len;
 }
 
 char *cg_name(const char *root, const char *name)
@@ -87,16 +82,16 @@ char *cg_control(const char *cgroup, const char *control)
 	return ret;
 }
 
+/* Returns 0 on success, or -errno on failure. */
 int cg_read(const char *cgroup, const char *control, char *buf, size_t len)
 {
 	char path[PATH_MAX];
+	ssize_t ret;
 
 	snprintf(path, sizeof(path), "%s/%s", cgroup, control);
 
-	if (read_text(path, buf, len) >= 0)
-		return 0;
-
-	return -1;
+	ret = read_text(path, buf, len);
+	return ret >= 0 ? 0 : ret;
 }
 
 int cg_read_strcmp(const char *cgroup, const char *control,
@@ -177,17 +172,15 @@ long cg_read_lc(const char *cgroup, const char *control)
 	return cnt;
 }
 
+/* Returns 0 on success, or -errno on failure. */
 int cg_write(const char *cgroup, const char *control, char *buf)
 {
 	char path[PATH_MAX];
-	ssize_t len = strlen(buf);
+	ssize_t len = strlen(buf), ret;
 
 	snprintf(path, sizeof(path), "%s/%s", cgroup, control);
-
-	if (write_text(path, buf, len) == len)
-		return 0;
-
-	return -1;
+	ret = write_text(path, buf, len);
+	return ret == len ? 0 : ret;
 }
 
 int cg_find_unified_root(char *root, size_t len)
@@ -545,7 +538,8 @@ ssize_t proc_read_text(int pid, bool thread, const char *item, char *buf, size_t
 	else
 		snprintf(path, sizeof(path), "/proc/%d/%s", pid, item);
 
-	return read_text(path, buf, size);
+	size = read_text(path, buf, size);
+	return size < 0 ? -1 : size;
 }
 
 int proc_read_strstr(int pid, bool thread, const char *item, const char *needle)
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

Re: [PATCH v5 2/4] selftests: cgroup: return -errno from cg_read()/cg_write() on failure

[David Rientjes]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

> Currently, cg_read()/cg_write() returns 0 on success and -1 on failure.
> Modify them to return the -errno on failure.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Shakeel Butt <shakeelb@google.com>

Acked-by: David Rientjes <rientjes@google.com>

Re: [PATCH v5 2/4] selftests: cgroup: return -errno from cg_read()/cg_write() on failure

[David Rientjes]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

> Currently, cg_read()/cg_write() returns 0 on success and -1 on failure.
> Modify them to return the -errno on failure.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Acked-by: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

Acked-by: David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

Re: [PATCH v5 2/4] selftests: cgroup: return -errno from cg_read()/cg_write() on failure

[Roman Gushchin]

On Mon, Apr 25, 2022 at 07:00:38PM +0000, Yosry Ahmed wrote:
> Currently, cg_read()/cg_write() returns 0 on success and -1 on failure.
> Modify them to return the -errno on failure.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Shakeel Butt <shakeelb@google.com>

Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!

Re: [PATCH v5 2/4] selftests: cgroup: return -errno from cg_read()/cg_write() on failure

[Roman Gushchin]

On Mon, Apr 25, 2022 at 07:00:38PM +0000, Yosry Ahmed wrote:
> Currently, cg_read()/cg_write() returns 0 on success and -1 on failure.
> Modify them to return the -errno on failure.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> Acked-by: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

Acked-by: Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>

Thanks!

[PATCH v5 3/4] selftests: cgroup: fix alloc_anon_noexit() instantly freeing memory

[Yosry Ahmed]

Currently, alloc_anon_noexit() calls alloc_anon() which instantly frees
the allocated memory. alloc_anon_noexit() is usually used with
cg_run_nowait() to run a process in the background that allocates
memory. It makes sense for the background process to keep the memory
allocated and not instantly free it (otherwise there is no point of
running it in the background).

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Shakeel Butt <shakeelb@google.com>
---
 tools/testing/selftests/cgroup/test_memcontrol.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index 36ccf2322e21..f2ffb3a30194 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -211,13 +211,17 @@ static int alloc_pagecache_50M_noexit(const char *cgroup, void *arg)
 static int alloc_anon_noexit(const char *cgroup, void *arg)
 {
 	int ppid = getppid();
+	size_t size = (unsigned long)arg;
+	char *buf, *ptr;
 
-	if (alloc_anon(cgroup, arg))
-		return -1;
+	buf = malloc(size);
+	for (ptr = buf; ptr < buf + size; ptr += PAGE_SIZE)
+		*ptr = 0;
 
 	while (getppid() == ppid)
 		sleep(1);
 
+	free(buf);
 	return 0;
 }
 
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

Re: [PATCH v5 3/4] selftests: cgroup: fix alloc_anon_noexit() instantly freeing memory

[David Rientjes]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

> Currently, alloc_anon_noexit() calls alloc_anon() which instantly frees
> the allocated memory. alloc_anon_noexit() is usually used with
> cg_run_nowait() to run a process in the background that allocates
> memory. It makes sense for the background process to keep the memory
> allocated and not instantly free it (otherwise there is no point of
> running it in the background).
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
> Acked-by: Shakeel Butt <shakeelb@google.com>

Acked-by: David Rientjes <rientjes@google.com>

[PATCH v5 4/4] selftests: cgroup: add a selftest for memory.reclaim

[Yosry Ahmed]

Add a new test for memory.reclaim that verifies that the interface
correctly reclaims memory as intended, from both anon and file pages.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
---
 .../selftests/cgroup/test_memcontrol.c        | 106 ++++++++++++++++++
 1 file changed, 106 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index f2ffb3a30194..a9c691fb94c0 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -760,6 +760,111 @@ static int test_memcg_max(const char *root)
 	return ret;
 }
 
+/*
+ * This test checks that memory.reclaim reclaims the given
+ * amount of memory (from both anon and file, if possible).
+ */
+static int test_memcg_reclaim(const char *root)
+{
+	int ret = KSFT_FAIL, fd, retries;
+	char *memcg;
+	long current, expected_usage, to_reclaim;
+	char buf[64];
+
+	memcg = cg_name(root, "memcg_test");
+	if (!memcg)
+		goto cleanup;
+
+	if (cg_create(memcg))
+		goto cleanup;
+
+	current = cg_read_long(memcg, "memory.current");
+	if (current != 0)
+		goto cleanup;
+
+	fd = get_temp_fd();
+	if (fd < 0)
+		goto cleanup;
+
+	cg_run_nowait(memcg, alloc_pagecache_50M_noexit, (void *)(long)fd);
+
+	/*
+	 * If swap is enabled, try to reclaim from both anon and file, else try
+	 * to reclaim from file only.
+	 */
+	if (is_swap_enabled()) {
+		cg_run_nowait(memcg, alloc_anon_noexit, (void *) MB(50));
+		expected_usage = MB(100);
+	} else
+		expected_usage = MB(50);
+
+	/*
+	 * Wait until current usage reaches the expected usage (or we run out of
+	 * retries).
+	 */
+	retries = 5;
+	while (!values_close(cg_read_long(memcg, "memory.current"),
+			    expected_usage, 10)) {
+		if (retries--) {
+			sleep(1);
+			continue;
+		} else {
+			fprintf(stderr,
+				"failed to allocate %ld for memcg reclaim test\n",
+				expected_usage);
+			goto cleanup;
+		}
+	}
+
+	/*
+	 * Reclaim until current reaches 30M, this makes sure we hit both anon
+	 * and file if swap is enabled.
+	 */
+	retries = 5;
+	while (true) {
+		int err;
+
+		current = cg_read_long(memcg, "memory.current");
+		to_reclaim = current - MB(30);
+
+		/*
+		 * We only keep looping if we get EAGAIN, which means we could
+		 * not reclaim the full amount.
+		 */
+		if (to_reclaim <= 0)
+			goto cleanup;
+
+
+		snprintf(buf, sizeof(buf), "%ld", to_reclaim);
+		err = cg_write(memcg, "memory.reclaim", buf);
+		if (!err) {
+			/*
+			 * If writing succeeds, then the written amount should have been
+			 * fully reclaimed (and maybe more).
+			 */
+			current = cg_read_long(memcg, "memory.current");
+			if (!values_close(current, MB(30), 3) && current > MB(30))
+				goto cleanup;
+			break;
+		}
+
+		/* The kernel could not reclaim the full amount, try again. */
+		if (err == -EAGAIN && retries--)
+			continue;
+
+		/* We got an unexpected error or ran out of retries. */
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+cleanup:
+	cg_destroy(memcg);
+	free(memcg);
+	close(fd);
+
+	return ret;
+}
+
 static int alloc_anon_50M_check_swap(const char *cgroup, void *arg)
 {
 	long mem_max = (long)arg;
@@ -1263,6 +1368,7 @@ struct memcg_test {
 	T(test_memcg_high),
 	T(test_memcg_high_sync),
 	T(test_memcg_max),
+	T(test_memcg_reclaim),
 	T(test_memcg_oom_events),
 	T(test_memcg_swap_max),
 	T(test_memcg_sock),
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

[PATCH v5 4/4] selftests: cgroup: add a selftest for memory.reclaim

[Yosry Ahmed]

Add a new test for memory.reclaim that verifies that the interface
correctly reclaims memory as intended, from both anon and file pages.

Signed-off-by: Yosry Ahmed <yosryahmed-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
Acked-by: Roman Gushchin <roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org>
---
 .../selftests/cgroup/test_memcontrol.c        | 106 ++++++++++++++++++
 1 file changed, 106 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
index f2ffb3a30194..a9c691fb94c0 100644
--- a/tools/testing/selftests/cgroup/test_memcontrol.c
+++ b/tools/testing/selftests/cgroup/test_memcontrol.c
@@ -760,6 +760,111 @@ static int test_memcg_max(const char *root)
 	return ret;
 }
 
+/*
+ * This test checks that memory.reclaim reclaims the given
+ * amount of memory (from both anon and file, if possible).
+ */
+static int test_memcg_reclaim(const char *root)
+{
+	int ret = KSFT_FAIL, fd, retries;
+	char *memcg;
+	long current, expected_usage, to_reclaim;
+	char buf[64];
+
+	memcg = cg_name(root, "memcg_test");
+	if (!memcg)
+		goto cleanup;
+
+	if (cg_create(memcg))
+		goto cleanup;
+
+	current = cg_read_long(memcg, "memory.current");
+	if (current != 0)
+		goto cleanup;
+
+	fd = get_temp_fd();
+	if (fd < 0)
+		goto cleanup;
+
+	cg_run_nowait(memcg, alloc_pagecache_50M_noexit, (void *)(long)fd);
+
+	/*
+	 * If swap is enabled, try to reclaim from both anon and file, else try
+	 * to reclaim from file only.
+	 */
+	if (is_swap_enabled()) {
+		cg_run_nowait(memcg, alloc_anon_noexit, (void *) MB(50));
+		expected_usage = MB(100);
+	} else
+		expected_usage = MB(50);
+
+	/*
+	 * Wait until current usage reaches the expected usage (or we run out of
+	 * retries).
+	 */
+	retries = 5;
+	while (!values_close(cg_read_long(memcg, "memory.current"),
+			    expected_usage, 10)) {
+		if (retries--) {
+			sleep(1);
+			continue;
+		} else {
+			fprintf(stderr,
+				"failed to allocate %ld for memcg reclaim test\n",
+				expected_usage);
+			goto cleanup;
+		}
+	}
+
+	/*
+	 * Reclaim until current reaches 30M, this makes sure we hit both anon
+	 * and file if swap is enabled.
+	 */
+	retries = 5;
+	while (true) {
+		int err;
+
+		current = cg_read_long(memcg, "memory.current");
+		to_reclaim = current - MB(30);
+
+		/*
+		 * We only keep looping if we get EAGAIN, which means we could
+		 * not reclaim the full amount.
+		 */
+		if (to_reclaim <= 0)
+			goto cleanup;
+
+
+		snprintf(buf, sizeof(buf), "%ld", to_reclaim);
+		err = cg_write(memcg, "memory.reclaim", buf);
+		if (!err) {
+			/*
+			 * If writing succeeds, then the written amount should have been
+			 * fully reclaimed (and maybe more).
+			 */
+			current = cg_read_long(memcg, "memory.current");
+			if (!values_close(current, MB(30), 3) && current > MB(30))
+				goto cleanup;
+			break;
+		}
+
+		/* The kernel could not reclaim the full amount, try again. */
+		if (err == -EAGAIN && retries--)
+			continue;
+
+		/* We got an unexpected error or ran out of retries. */
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+cleanup:
+	cg_destroy(memcg);
+	free(memcg);
+	close(fd);
+
+	return ret;
+}
+
 static int alloc_anon_50M_check_swap(const char *cgroup, void *arg)
 {
 	long mem_max = (long)arg;
@@ -1263,6 +1368,7 @@ struct memcg_test {
 	T(test_memcg_high),
 	T(test_memcg_high_sync),
 	T(test_memcg_max),
+	T(test_memcg_reclaim),
 	T(test_memcg_oom_events),
 	T(test_memcg_swap_max),
 	T(test_memcg_sock),
-- 
2.36.0.rc2.479.g8af0fa9b8e-goog

Re: [PATCH v5 4/4] selftests: cgroup: add a selftest for memory.reclaim

[David Rientjes]

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

> Add a new test for memory.reclaim that verifies that the interface
> correctly reclaims memory as intended, from both anon and file pages.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Acked-by: David Rientjes <rientjes@google.com>