[PATCH v4 0/2] xenbus/backend: Add a memory pressure handler callback

[PATCH v4 0/2] xenbus/backend: Add a memory pressure handler callback

[SeongJae Park]

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lacks a
flexibility.

To mitigate such problems, this patchset adds a memory reclaim callback
to 'xenbus_driver' (patch 1) and use it to mitigate the problem in
'xen-blkback' (patch 2).

Base Version
------------

This patch is based on v5.4.  A complete tree is also available at my
public git repo:
https://github.com/sjp38/linux/tree/blkback_squeezing_v4


Patch History
-------------

Changes from v3 (https://lore.kernel.org/xen-devel/20191209085839.21215-1-sjpark@amazon.com/)
 - Add general callback in xen_driver and use it (suggested by Juergen
   Gross)

Changes from v2 (https://lore.kernel.org/linux-block/af195033-23d5-38ed-b73b-f6e2e3b34541@amazon.com)
 - Rename the module parameter and variables for brevity (aggressive
   shrinking -> squeezing)

Changes from v1 (https://lore.kernel.org/xen-devel/20191204113419.2298-1-sjpark@amazon.com/)
 - Adjust the description to not use the term, `arbitrarily` (suggested
   by Paul Durrant)
 - Specify time unit of the duration in the parameter description,
   (suggested by Maximilian Heyne)
 - Change default aggressive shrinking duration from 1ms to 10ms
 - Merge two patches into one single patch

SeongJae Park (1):
  xen/blkback: Squeeze page pools if a memory pressure is detected

 drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

SeongJae Park (2):
  xenbus/backend: Add memory pressure handler callback
  xen/blkback: Squeeze page pools if a memory pressure is detected

 drivers/block/xen-blkback/blkback.c       | 23 +++++++++++++++--
 drivers/block/xen-blkback/common.h        |  1 +
 drivers/block/xen-blkback/xenbus.c        |  3 ++-
 drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 5 files changed, 56 insertions(+), 3 deletions(-)

-- 
2.17.1

[Xen-devel] [PATCH v4 0/2] xenbus/backend: Add a memory pressure handler callback

[SeongJae Park]

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lacks a
flexibility.

To mitigate such problems, this patchset adds a memory reclaim callback
to 'xenbus_driver' (patch 1) and use it to mitigate the problem in
'xen-blkback' (patch 2).

Base Version
------------

This patch is based on v5.4.  A complete tree is also available at my
public git repo:
https://github.com/sjp38/linux/tree/blkback_squeezing_v4


Patch History
-------------

Changes from v3 (https://lore.kernel.org/xen-devel/20191209085839.21215-1-sjpark@amazon.com/)
 - Add general callback in xen_driver and use it (suggested by Juergen
   Gross)

Changes from v2 (https://lore.kernel.org/linux-block/af195033-23d5-38ed-b73b-f6e2e3b34541@amazon.com)
 - Rename the module parameter and variables for brevity (aggressive
   shrinking -> squeezing)

Changes from v1 (https://lore.kernel.org/xen-devel/20191204113419.2298-1-sjpark@amazon.com/)
 - Adjust the description to not use the term, `arbitrarily` (suggested
   by Paul Durrant)
 - Specify time unit of the duration in the parameter description,
   (suggested by Maximilian Heyne)
 - Change default aggressive shrinking duration from 1ms to 10ms
 - Merge two patches into one single patch

SeongJae Park (1):
  xen/blkback: Squeeze page pools if a memory pressure is detected

 drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

SeongJae Park (2):
  xenbus/backend: Add memory pressure handler callback
  xen/blkback: Squeeze page pools if a memory pressure is detected

 drivers/block/xen-blkback/blkback.c       | 23 +++++++++++++++--
 drivers/block/xen-blkback/common.h        |  1 +
 drivers/block/xen-blkback/xenbus.c        |  3 ++-
 drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 5 files changed, 56 insertions(+), 3 deletions(-)

-- 
2.17.1


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

[PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lacks a
flexibility.

To mitigate such problems, this commit adds a memory reclaim callback to
'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
a memory pressure and request specific domains of specific backend
drivers which causing the given pressure to voluntarily release its
memory.

That said, this commit simply requests every callback registered driver
to release its memory for every domain, rather than issueing the
requests to the drivers and domain in charge.  Such things would be a
future work.  Also, this commit focuses on memory only.  However, it
would be ablt to be extended for general resources.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 2 files changed, 32 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index b0bed4faf44c..cd5fd1cd8de3 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
 	return NOTIFY_DONE;
 }
 
+static int xenbus_backend_reclaim(struct device *dev, void *data)
+{
+	struct xenbus_driver *drv;
+	if (!dev->driver)
+		return -ENOENT;
+	drv = to_xenbus_driver(dev->driver);
+	if (drv && drv->reclaim)
+		drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
+	return 0;
+}
+
+/*
+ * Returns 0 always because we are using shrinker to only detect memory
+ * pressure.
+ */
+static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
+				struct shrink_control *sc)
+{
+	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
+			xenbus_backend_reclaim);
+	return 0;
+}
+
+static struct shrinker xenbus_backend_shrinker = {
+	.count_objects = xenbus_backend_shrink_count,
+	.seeks = DEFAULT_SEEKS,
+};
+
 static int __init xenbus_probe_backend_init(void)
 {
 	static struct notifier_block xenstore_notifier = {
@@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
 
 	register_xenstore_notifier(&xenstore_notifier);
 
+	if (register_shrinker(&xenbus_backend_shrinker))
+		pr_warn("shrinker registration failed\n");
+
 	return 0;
 }
 subsys_initcall(xenbus_probe_backend_init);
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index 869c816d5f8c..52aaf4f78400 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -104,6 +104,7 @@ struct xenbus_driver {
 	struct device_driver driver;
 	int (*read_otherend_details)(struct xenbus_device *dev);
 	int (*is_ready)(struct xenbus_device *dev);
+	unsigned (*reclaim)(struct xenbus_device *dev, domid_t domid);
 };
 
 static inline struct xenbus_driver *to_xenbus_driver(struct device_driver *drv)
-- 
2.17.1

[Xen-devel] [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

Granting pages consumes backend system memory.  In systems configured
with insufficient spare memory for those pages, it can cause a memory
pressure situation.  However, finding the optimal amount of the spare
memory is challenging for large systems having dynamic resource
utilization patterns.  Also, such a static configuration might lacks a
flexibility.

To mitigate such problems, this commit adds a memory reclaim callback to
'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
a memory pressure and request specific domains of specific backend
drivers which causing the given pressure to voluntarily release its
memory.

That said, this commit simply requests every callback registered driver
to release its memory for every domain, rather than issueing the
requests to the drivers and domain in charge.  Such things would be a
future work.  Also, this commit focuses on memory only.  However, it
would be ablt to be extended for general resources.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 2 files changed, 32 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index b0bed4faf44c..cd5fd1cd8de3 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
 	return NOTIFY_DONE;
 }
 
+static int xenbus_backend_reclaim(struct device *dev, void *data)
+{
+	struct xenbus_driver *drv;
+	if (!dev->driver)
+		return -ENOENT;
+	drv = to_xenbus_driver(dev->driver);
+	if (drv && drv->reclaim)
+		drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
+	return 0;
+}
+
+/*
+ * Returns 0 always because we are using shrinker to only detect memory
+ * pressure.
+ */
+static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
+				struct shrink_control *sc)
+{
+	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
+			xenbus_backend_reclaim);
+	return 0;
+}
+
+static struct shrinker xenbus_backend_shrinker = {
+	.count_objects = xenbus_backend_shrink_count,
+	.seeks = DEFAULT_SEEKS,
+};
+
 static int __init xenbus_probe_backend_init(void)
 {
 	static struct notifier_block xenstore_notifier = {
@@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
 
 	register_xenstore_notifier(&xenstore_notifier);
 
+	if (register_shrinker(&xenbus_backend_shrinker))
+		pr_warn("shrinker registration failed\n");
+
 	return 0;
 }
 subsys_initcall(xenbus_probe_backend_init);
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index 869c816d5f8c..52aaf4f78400 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -104,6 +104,7 @@ struct xenbus_driver {
 	struct device_driver driver;
 	int (*read_otherend_details)(struct xenbus_device *dev);
 	int (*is_ready)(struct xenbus_device *dev);
+	unsigned (*reclaim)(struct xenbus_device *dev, domid_t domid);
 };
 
 static inline struct xenbus_driver *to_xenbus_driver(struct device_driver *drv)
-- 
2.17.1


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

[PATCH v4 2/2] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

Each `blkif` has a free pages pool for the grant mapping.  The size of
the pool starts from zero and be increased on demand while processing
the I/O requests.  If current I/O requests handling is finished or 100
milliseconds has passed since last I/O requests handling, it checks and
shrinks the pool to not exceed the size limit, `max_buffer_pages`.

Therefore, `blkfront` running guests can cause a memory pressure in the
`blkback` running guest by attaching a large number of block devices and
inducing I/O.  System administrators can avoid such problematic
situations by limiting the maximum number of devices each guest can
attach.  However, finding the optimal limit is not so easy.  Improper
set of the limit can results in the memory pressure or a resource
underutilization.  This commit avoids such problematic situations by
squeezing the pools (returns every free page in the pool to the system)
for a while (users can set this duration via a module parameter) if a
memory pressure is detected.

Discussions
===========

The `blkback`'s original shrinking mechanism returns only pages in the
pool, which are not currently be used by `blkback`, to the system.  In
other words, the pages are not mapped with foreign pages.  Because this
commit is changing only the shrink limit but uses the mechanism as is,
this commit does not introduce improper mappings related security
issues.

Once a memory pressure is detected, this commit keeps the squeezing
limit for a user-specified time duration.  The duration should be
neither too long nor too short.  If it is too long, the squeezing
incurring overhead can reduce the I/O performance.  If it is too short,
`blkback` will not free enough pages to reduce the memory pressure.
This commit sets the value as `10 milliseconds` by default because it is
a short time in terms of I/O while it is a long time in terms of memory
operations.  Also, as the original shrinking mechanism works for at
least every 100 milliseconds, this could be a somewhat reasonable
choice.  I also tested other durations (refer to the below section for
more details) and confirmed that 10 milliseconds is the one that works
best with the test.  That said, the proper duration depends on actual
configurations and workloads.  That's why this commit is allowing users
to set it as their optimal value via the module parameter.

Memory Pressure Test
====================

To show how this commit fixes the memory pressure situation well, I
configured a test environment on a xen-running virtualization system.
On the `blkfront` running guest instances, I attach a large number of
network-backed volume devices and induce I/O to those.  Meanwhile, I
measure the number of pages that swapped in and out on the `blkback`
running guest.  The test ran twice, once for the `blkback` before this
commit and once for that after this commit.  As shown below, this commit
has dramatically reduced the memory pressure:

                pswpin  pswpout
    before      76,672  185,799
    after          212    3,325

Optimal Aggressive Shrinking Duration
-------------------------------------

To find a best squeezing duration, I repeated the test with three
different durations (1ms, 10ms, and 100ms).  The results are as below:

    duration    pswpin  pswpout
    1           852     6,424
    10          212     3,325
    100         203     3,340

As expected, the memory pressure has decreased as the duration is
increased, but the reduction stopped from the `10ms`.  Based on this
results, I chose the default duration as 10ms.

Performance Overhead Test
=========================

This commit could incur I/O performance degradation under severe memory
pressure because the squeezing will require more page allocations per
I/O.  To show the overhead, I artificially made a worst-case squeezing
situation and measured the I/O performance of a `blkfront` running
guest.

For the artificial squeezing, I set the `blkback.max_buffer_pages` using
the `/sys/module/xen_blkback/parameters/max_buffer_pages` file.  We set
the value to `1024` and `0`.  The `1024` is the default value.  Setting
the value as `0` is same to a situation doing the squeezing always
(worst-case).

For the I/O performance measurement, I use a simple `dd` command.

Default Performance
-------------------

    [dom0]# echo 1024 > /sys/module/xen_blkback/parameters/max_buffer_pages
    [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8827 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8781 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8737 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8702 s, 38.7 MB/s

Worst-case Performance
----------------------

    [dom0]# echo 0 > /sys/module/xen_blkback/parameters/max_buffer_pages
    [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.878 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8746 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8786 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8749 s, 38.7 MB/s

In short, even worst case squeezing makes no visible performance
degradation.  I think this is due to the slow speed of the I/O.  In
other words, the additional page allocation overhead is hidden under the
much slower I/O latency.

Nevertheless, pleaset note that this is just a very simple and minimal
test.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/block/xen-blkback/blkback.c | 23 +++++++++++++++++++++--
 drivers/block/xen-blkback/common.h  |  1 +
 drivers/block/xen-blkback/xenbus.c  |  3 ++-
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index fd1e19f1a49f..7e3994c20843 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -142,6 +142,22 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
 		HZ * xen_blkif_pgrant_timeout);
 }
 
+/* Once a memory pressure is detected, squeeze free page pools for a while. */
+static int xen_blkif_buffer_squeeze_duration_ms = 10;
+module_param_named(buffer_squeeze_duration_ms,
+		xen_blkif_buffer_squeeze_duration_ms, int, 0644);
+MODULE_PARM_DESC(buffer_squeeze_duration_ms,
+"Duration in ms to squeeze pages buffer when a memory pressure is detected");
+
+static unsigned long xen_blk_buffer_squeeze_end;
+
+unsigned xen_blkbk_reclaim(struct xenbus_device *dev, domid_t domid)
+{
+	xen_blk_buffer_squeeze_end = jiffies +
+		msecs_to_jiffies(xen_blkif_buffer_squeeze_duration_ms);
+	return 0;
+}
+
 static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
 {
 	unsigned long flags;
@@ -656,8 +672,11 @@ int xen_blkif_schedule(void *arg)
 			ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL);
 		}
 
-		/* Shrink if we have more than xen_blkif_max_buffer_pages */
-		shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
+		/* Shrink the free pages pool if it is too large. */
+		if (time_before(jiffies, xen_blk_buffer_squeeze_end))
+			shrink_free_pagepool(ring, 0);
+		else
+			shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
 
 		if (log_stats && time_after(jiffies, ring->st_print))
 			print_stats(ring);
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 1d3002d773f7..de5c6a301f44 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -383,6 +383,7 @@ irqreturn_t xen_blkif_be_int(int irq, void *dev_id);
 int xen_blkif_schedule(void *arg);
 int xen_blkif_purge_persistent(void *arg);
 void xen_blkbk_free_caches(struct xen_blkif_ring *ring);
+unsigned xen_blkbk_reclaim(struct xenbus_device *dev, domid_t domid);
 
 int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
 			      struct backend_info *be, int state);
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index b90dbcd99c03..de49a09e6933 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -1115,7 +1115,8 @@ static struct xenbus_driver xen_blkbk_driver = {
 	.ids  = xen_blkbk_ids,
 	.probe = xen_blkbk_probe,
 	.remove = xen_blkbk_remove,
-	.otherend_changed = frontend_changed
+	.otherend_changed = frontend_changed,
+	.reclaim = xen_blkbk_reclaim
 };
 
 int xen_blkif_xenbus_init(void)
-- 
2.17.1

[Xen-devel] [PATCH v4 2/2] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

From: SeongJae Park <sjpark@amazon.de>

Each `blkif` has a free pages pool for the grant mapping.  The size of
the pool starts from zero and be increased on demand while processing
the I/O requests.  If current I/O requests handling is finished or 100
milliseconds has passed since last I/O requests handling, it checks and
shrinks the pool to not exceed the size limit, `max_buffer_pages`.

Therefore, `blkfront` running guests can cause a memory pressure in the
`blkback` running guest by attaching a large number of block devices and
inducing I/O.  System administrators can avoid such problematic
situations by limiting the maximum number of devices each guest can
attach.  However, finding the optimal limit is not so easy.  Improper
set of the limit can results in the memory pressure or a resource
underutilization.  This commit avoids such problematic situations by
squeezing the pools (returns every free page in the pool to the system)
for a while (users can set this duration via a module parameter) if a
memory pressure is detected.

Discussions
===========

The `blkback`'s original shrinking mechanism returns only pages in the
pool, which are not currently be used by `blkback`, to the system.  In
other words, the pages are not mapped with foreign pages.  Because this
commit is changing only the shrink limit but uses the mechanism as is,
this commit does not introduce improper mappings related security
issues.

Once a memory pressure is detected, this commit keeps the squeezing
limit for a user-specified time duration.  The duration should be
neither too long nor too short.  If it is too long, the squeezing
incurring overhead can reduce the I/O performance.  If it is too short,
`blkback` will not free enough pages to reduce the memory pressure.
This commit sets the value as `10 milliseconds` by default because it is
a short time in terms of I/O while it is a long time in terms of memory
operations.  Also, as the original shrinking mechanism works for at
least every 100 milliseconds, this could be a somewhat reasonable
choice.  I also tested other durations (refer to the below section for
more details) and confirmed that 10 milliseconds is the one that works
best with the test.  That said, the proper duration depends on actual
configurations and workloads.  That's why this commit is allowing users
to set it as their optimal value via the module parameter.

Memory Pressure Test
====================

To show how this commit fixes the memory pressure situation well, I
configured a test environment on a xen-running virtualization system.
On the `blkfront` running guest instances, I attach a large number of
network-backed volume devices and induce I/O to those.  Meanwhile, I
measure the number of pages that swapped in and out on the `blkback`
running guest.  The test ran twice, once for the `blkback` before this
commit and once for that after this commit.  As shown below, this commit
has dramatically reduced the memory pressure:

                pswpin  pswpout
    before      76,672  185,799
    after          212    3,325

Optimal Aggressive Shrinking Duration
-------------------------------------

To find a best squeezing duration, I repeated the test with three
different durations (1ms, 10ms, and 100ms).  The results are as below:

    duration    pswpin  pswpout
    1           852     6,424
    10          212     3,325
    100         203     3,340

As expected, the memory pressure has decreased as the duration is
increased, but the reduction stopped from the `10ms`.  Based on this
results, I chose the default duration as 10ms.

Performance Overhead Test
=========================

This commit could incur I/O performance degradation under severe memory
pressure because the squeezing will require more page allocations per
I/O.  To show the overhead, I artificially made a worst-case squeezing
situation and measured the I/O performance of a `blkfront` running
guest.

For the artificial squeezing, I set the `blkback.max_buffer_pages` using
the `/sys/module/xen_blkback/parameters/max_buffer_pages` file.  We set
the value to `1024` and `0`.  The `1024` is the default value.  Setting
the value as `0` is same to a situation doing the squeezing always
(worst-case).

For the I/O performance measurement, I use a simple `dd` command.

Default Performance
-------------------

    [dom0]# echo 1024 > /sys/module/xen_blkback/parameters/max_buffer_pages
    [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8827 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8781 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8737 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8702 s, 38.7 MB/s

Worst-case Performance
----------------------

    [dom0]# echo 0 > /sys/module/xen_blkback/parameters/max_buffer_pages
    [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.878 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8746 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8786 s, 38.7 MB/s
    131072+0 records in
    131072+0 records out
    536870912 bytes (537 MB) copied, 13.8749 s, 38.7 MB/s

In short, even worst case squeezing makes no visible performance
degradation.  I think this is due to the slow speed of the I/O.  In
other words, the additional page allocation overhead is hidden under the
much slower I/O latency.

Nevertheless, pleaset note that this is just a very simple and minimal
test.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/block/xen-blkback/blkback.c | 23 +++++++++++++++++++++--
 drivers/block/xen-blkback/common.h  |  1 +
 drivers/block/xen-blkback/xenbus.c  |  3 ++-
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index fd1e19f1a49f..7e3994c20843 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -142,6 +142,22 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
 		HZ * xen_blkif_pgrant_timeout);
 }
 
+/* Once a memory pressure is detected, squeeze free page pools for a while. */
+static int xen_blkif_buffer_squeeze_duration_ms = 10;
+module_param_named(buffer_squeeze_duration_ms,
+		xen_blkif_buffer_squeeze_duration_ms, int, 0644);
+MODULE_PARM_DESC(buffer_squeeze_duration_ms,
+"Duration in ms to squeeze pages buffer when a memory pressure is detected");
+
+static unsigned long xen_blk_buffer_squeeze_end;
+
+unsigned xen_blkbk_reclaim(struct xenbus_device *dev, domid_t domid)
+{
+	xen_blk_buffer_squeeze_end = jiffies +
+		msecs_to_jiffies(xen_blkif_buffer_squeeze_duration_ms);
+	return 0;
+}
+
 static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
 {
 	unsigned long flags;
@@ -656,8 +672,11 @@ int xen_blkif_schedule(void *arg)
 			ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL);
 		}
 
-		/* Shrink if we have more than xen_blkif_max_buffer_pages */
-		shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
+		/* Shrink the free pages pool if it is too large. */
+		if (time_before(jiffies, xen_blk_buffer_squeeze_end))
+			shrink_free_pagepool(ring, 0);
+		else
+			shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
 
 		if (log_stats && time_after(jiffies, ring->st_print))
 			print_stats(ring);
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 1d3002d773f7..de5c6a301f44 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -383,6 +383,7 @@ irqreturn_t xen_blkif_be_int(int irq, void *dev_id);
 int xen_blkif_schedule(void *arg);
 int xen_blkif_purge_persistent(void *arg);
 void xen_blkbk_free_caches(struct xen_blkif_ring *ring);
+unsigned xen_blkbk_reclaim(struct xenbus_device *dev, domid_t domid);
 
 int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
 			      struct backend_info *be, int state);
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index b90dbcd99c03..de49a09e6933 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -1115,7 +1115,8 @@ static struct xenbus_driver xen_blkbk_driver = {
 	.ids  = xen_blkbk_ids,
 	.probe = xen_blkbk_probe,
 	.remove = xen_blkbk_remove,
-	.otherend_changed = frontend_changed
+	.otherend_changed = frontend_changed,
+	.reclaim = xen_blkbk_reclaim
 };
 
 int xen_blkif_xenbus_init(void)
-- 
2.17.1


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

Re: [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[Jürgen Groß]

On 09.12.19 20:43, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> Granting pages consumes backend system memory.  In systems configured
> with insufficient spare memory for those pages, it can cause a memory
> pressure situation.  However, finding the optimal amount of the spare
> memory is challenging for large systems having dynamic resource
> utilization patterns.  Also, such a static configuration might lacks a
> flexibility.
> 
> To mitigate such problems, this commit adds a memory reclaim callback to
> 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> a memory pressure and request specific domains of specific backend
> drivers which causing the given pressure to voluntarily release its
> memory.
> 
> That said, this commit simply requests every callback registered driver
> to release its memory for every domain, rather than issueing the
> requests to the drivers and domain in charge.  Such things would be a
> future work.  Also, this commit focuses on memory only.  However, it
> would be ablt to be extended for general resources.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
>   include/xen/xenbus.h                      |  1 +
>   2 files changed, 32 insertions(+)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index b0bed4faf44c..cd5fd1cd8de3 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>   	return NOTIFY_DONE;
>   }
>   
> +static int xenbus_backend_reclaim(struct device *dev, void *data)
> +{
> +	struct xenbus_driver *drv;
> +	if (!dev->driver)
> +		return -ENOENT;
> +	drv = to_xenbus_driver(dev->driver);
> +	if (drv && drv->reclaim)
> +		drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
> +	return 0;
> +}
> +
> +/*
> + * Returns 0 always because we are using shrinker to only detect memory
> + * pressure.
> + */
> +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> +				struct shrink_control *sc)
> +{
> +	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> +			xenbus_backend_reclaim);
> +	return 0;
> +}
> +
> +static struct shrinker xenbus_backend_shrinker = {
> +	.count_objects = xenbus_backend_shrink_count,
> +	.seeks = DEFAULT_SEEKS,
> +};
> +
>   static int __init xenbus_probe_backend_init(void)
>   {
>   	static struct notifier_block xenstore_notifier = {
> @@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
>   
>   	register_xenstore_notifier(&xenstore_notifier);
>   
> +	if (register_shrinker(&xenbus_backend_shrinker))
> +		pr_warn("shrinker registration failed\n");
> +
>   	return 0;
>   }
>   subsys_initcall(xenbus_probe_backend_init);
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index 869c816d5f8c..52aaf4f78400 100644
> --- a/include/xen/xenbus.h
> +++ b/include/xen/xenbus.h
> @@ -104,6 +104,7 @@ struct xenbus_driver {
>   	struct device_driver driver;
>   	int (*read_otherend_details)(struct xenbus_device *dev);
>   	int (*is_ready)(struct xenbus_device *dev);
> +	unsigned (*reclaim)(struct xenbus_device *dev, domid_t domid);

Can you please add a comment here regarding semantics of specifying
DOMID_INVALID as domid?

Block maintainers, would you be fine with me carrying this series
through the Xen tree?


Juergen

Re: [Xen-devel] [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[Jürgen Groß]

On 09.12.19 20:43, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> Granting pages consumes backend system memory.  In systems configured
> with insufficient spare memory for those pages, it can cause a memory
> pressure situation.  However, finding the optimal amount of the spare
> memory is challenging for large systems having dynamic resource
> utilization patterns.  Also, such a static configuration might lacks a
> flexibility.
> 
> To mitigate such problems, this commit adds a memory reclaim callback to
> 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> a memory pressure and request specific domains of specific backend
> drivers which causing the given pressure to voluntarily release its
> memory.
> 
> That said, this commit simply requests every callback registered driver
> to release its memory for every domain, rather than issueing the
> requests to the drivers and domain in charge.  Such things would be a
> future work.  Also, this commit focuses on memory only.  However, it
> would be ablt to be extended for general resources.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
>   include/xen/xenbus.h                      |  1 +
>   2 files changed, 32 insertions(+)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index b0bed4faf44c..cd5fd1cd8de3 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>   	return NOTIFY_DONE;
>   }
>   
> +static int xenbus_backend_reclaim(struct device *dev, void *data)
> +{
> +	struct xenbus_driver *drv;
> +	if (!dev->driver)
> +		return -ENOENT;
> +	drv = to_xenbus_driver(dev->driver);
> +	if (drv && drv->reclaim)
> +		drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
> +	return 0;
> +}
> +
> +/*
> + * Returns 0 always because we are using shrinker to only detect memory
> + * pressure.
> + */
> +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> +				struct shrink_control *sc)
> +{
> +	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> +			xenbus_backend_reclaim);
> +	return 0;
> +}
> +
> +static struct shrinker xenbus_backend_shrinker = {
> +	.count_objects = xenbus_backend_shrink_count,
> +	.seeks = DEFAULT_SEEKS,
> +};
> +
>   static int __init xenbus_probe_backend_init(void)
>   {
>   	static struct notifier_block xenstore_notifier = {
> @@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
>   
>   	register_xenstore_notifier(&xenstore_notifier);
>   
> +	if (register_shrinker(&xenbus_backend_shrinker))
> +		pr_warn("shrinker registration failed\n");
> +
>   	return 0;
>   }
>   subsys_initcall(xenbus_probe_backend_init);
> diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> index 869c816d5f8c..52aaf4f78400 100644
> --- a/include/xen/xenbus.h
> +++ b/include/xen/xenbus.h
> @@ -104,6 +104,7 @@ struct xenbus_driver {
>   	struct device_driver driver;
>   	int (*read_otherend_details)(struct xenbus_device *dev);
>   	int (*is_ready)(struct xenbus_device *dev);
> +	unsigned (*reclaim)(struct xenbus_device *dev, domid_t domid);

Can you please add a comment here regarding semantics of specifying
DOMID_INVALID as domid?

Block maintainers, would you be fine with me carrying this series
through the Xen tree?


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

Re: [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

On Tue, Dec 10, 2019 at 7:11 AM Jürgen Groß <jgross@suse.com> wrote:
>
> On 09.12.19 20:43, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > Granting pages consumes backend system memory.  In systems configured
> > with insufficient spare memory for those pages, it can cause a memory
> > pressure situation.  However, finding the optimal amount of the spare
> > memory is challenging for large systems having dynamic resource
> > utilization patterns.  Also, such a static configuration might lacks a
> > flexibility.
> >
> > To mitigate such problems, this commit adds a memory reclaim callback to
> > 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> > a memory pressure and request specific domains of specific backend
> > drivers which causing the given pressure to voluntarily release its
> > memory.
> >
> > That said, this commit simply requests every callback registered driver
> > to release its memory for every domain, rather than issueing the
> > requests to the drivers and domain in charge.  Such things would be a
> > future work.  Also, this commit focuses on memory only.  However, it
> > would be ablt to be extended for general resources.
> >
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
> >   include/xen/xenbus.h                      |  1 +
> >   2 files changed, 32 insertions(+)
> >
> > diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> > index b0bed4faf44c..cd5fd1cd8de3 100644
> > --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> > +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> > @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
> >       return NOTIFY_DONE;
> >   }
> >
> > +static int xenbus_backend_reclaim(struct device *dev, void *data)
> > +{
> > +     struct xenbus_driver *drv;
> > +     if (!dev->driver)
> > +             return -ENOENT;
> > +     drv = to_xenbus_driver(dev->driver);
> > +     if (drv && drv->reclaim)
> > +             drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
> > +     return 0;
> > +}
> > +
> > +/*
> > + * Returns 0 always because we are using shrinker to only detect memory
> > + * pressure.
> > + */
> > +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> > +                             struct shrink_control *sc)
> > +{
> > +     bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> > +                     xenbus_backend_reclaim);
> > +     return 0;
> > +}
> > +
> > +static struct shrinker xenbus_backend_shrinker = {
> > +     .count_objects = xenbus_backend_shrink_count,
> > +     .seeks = DEFAULT_SEEKS,
> > +};
> > +
> >   static int __init xenbus_probe_backend_init(void)
> >   {
> >       static struct notifier_block xenstore_notifier = {
> > @@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
> >
> >       register_xenstore_notifier(&xenstore_notifier);
> >
> > +     if (register_shrinker(&xenbus_backend_shrinker))
> > +             pr_warn("shrinker registration failed\n");
> > +
> >       return 0;
> >   }
> >   subsys_initcall(xenbus_probe_backend_init);
> > diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> > index 869c816d5f8c..52aaf4f78400 100644
> > --- a/include/xen/xenbus.h
> > +++ b/include/xen/xenbus.h
> > @@ -104,6 +104,7 @@ struct xenbus_driver {
> >       struct device_driver driver;
> >       int (*read_otherend_details)(struct xenbus_device *dev);
> >       int (*is_ready)(struct xenbus_device *dev);
> > +     unsigned (*reclaim)(struct xenbus_device *dev, domid_t domid);
>
> Can you please add a comment here regarding semantics of specifying
> DOMID_INVALID as domid?

Yes, of course.  Will do with the next version.


Thanks,
SeongJae Park

>
> Block maintainers, would you be fine with me carrying this series
> through the Xen tree?
>
>
> Juergen

Re: [Xen-devel] [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

On Tue, Dec 10, 2019 at 7:11 AM Jürgen Groß <jgross@suse.com> wrote:
>
> On 09.12.19 20:43, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > Granting pages consumes backend system memory.  In systems configured
> > with insufficient spare memory for those pages, it can cause a memory
> > pressure situation.  However, finding the optimal amount of the spare
> > memory is challenging for large systems having dynamic resource
> > utilization patterns.  Also, such a static configuration might lacks a
> > flexibility.
> >
> > To mitigate such problems, this commit adds a memory reclaim callback to
> > 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> > a memory pressure and request specific domains of specific backend
> > drivers which causing the given pressure to voluntarily release its
> > memory.
> >
> > That said, this commit simply requests every callback registered driver
> > to release its memory for every domain, rather than issueing the
> > requests to the drivers and domain in charge.  Such things would be a
> > future work.  Also, this commit focuses on memory only.  However, it
> > would be ablt to be extended for general resources.
> >
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
> >   include/xen/xenbus.h                      |  1 +
> >   2 files changed, 32 insertions(+)
> >
> > diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> > index b0bed4faf44c..cd5fd1cd8de3 100644
> > --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> > +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> > @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
> >       return NOTIFY_DONE;
> >   }
> >
> > +static int xenbus_backend_reclaim(struct device *dev, void *data)
> > +{
> > +     struct xenbus_driver *drv;
> > +     if (!dev->driver)
> > +             return -ENOENT;
> > +     drv = to_xenbus_driver(dev->driver);
> > +     if (drv && drv->reclaim)
> > +             drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
> > +     return 0;
> > +}
> > +
> > +/*
> > + * Returns 0 always because we are using shrinker to only detect memory
> > + * pressure.
> > + */
> > +static unsigned long xenbus_backend_shrink_count(struct shrinker *shrinker,
> > +                             struct shrink_control *sc)
> > +{
> > +     bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
> > +                     xenbus_backend_reclaim);
> > +     return 0;
> > +}
> > +
> > +static struct shrinker xenbus_backend_shrinker = {
> > +     .count_objects = xenbus_backend_shrink_count,
> > +     .seeks = DEFAULT_SEEKS,
> > +};
> > +
> >   static int __init xenbus_probe_backend_init(void)
> >   {
> >       static struct notifier_block xenstore_notifier = {
> > @@ -264,6 +292,9 @@ static int __init xenbus_probe_backend_init(void)
> >
> >       register_xenstore_notifier(&xenstore_notifier);
> >
> > +     if (register_shrinker(&xenbus_backend_shrinker))
> > +             pr_warn("shrinker registration failed\n");
> > +
> >       return 0;
> >   }
> >   subsys_initcall(xenbus_probe_backend_init);
> > diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
> > index 869c816d5f8c..52aaf4f78400 100644
> > --- a/include/xen/xenbus.h
> > +++ b/include/xen/xenbus.h
> > @@ -104,6 +104,7 @@ struct xenbus_driver {
> >       struct device_driver driver;
> >       int (*read_otherend_details)(struct xenbus_device *dev);
> >       int (*is_ready)(struct xenbus_device *dev);
> > +     unsigned (*reclaim)(struct xenbus_device *dev, domid_t domid);
>
> Can you please add a comment here regarding semantics of specifying
> DOMID_INVALID as domid?

Yes, of course.  Will do with the next version.


Thanks,
SeongJae Park

>
> Block maintainers, would you be fine with me carrying this series
> through the Xen tree?
>
>
> Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

Re: [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[Jürgen Groß]

On 09.12.19 20:43, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> Granting pages consumes backend system memory.  In systems configured
> with insufficient spare memory for those pages, it can cause a memory
> pressure situation.  However, finding the optimal amount of the spare
> memory is challenging for large systems having dynamic resource
> utilization patterns.  Also, such a static configuration might lacks a
> flexibility.
> 
> To mitigate such problems, this commit adds a memory reclaim callback to
> 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> a memory pressure and request specific domains of specific backend
> drivers which causing the given pressure to voluntarily release its
> memory.
> 
> That said, this commit simply requests every callback registered driver
> to release its memory for every domain, rather than issueing the
> requests to the drivers and domain in charge.  Such things would be a
> future work.  Also, this commit focuses on memory only.  However, it
> would be ablt to be extended for general resources.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
>   include/xen/xenbus.h                      |  1 +
>   2 files changed, 32 insertions(+)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index b0bed4faf44c..cd5fd1cd8de3 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>   	return NOTIFY_DONE;
>   }
>   
> +static int xenbus_backend_reclaim(struct device *dev, void *data)
> +{
> +	struct xenbus_driver *drv;
> +	if (!dev->driver)
> +		return -ENOENT;
> +	drv = to_xenbus_driver(dev->driver);
> +	if (drv && drv->reclaim)
> +		drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);

Oh, sorry for first requesting you to add the domid as a parameter,
but now I realize this could be handled in the xenbus driver, as
struct xenbus_device already contains the otherend_id.

Would you mind dropping the parameter again, please?


Juergen

Re: [Xen-devel] [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[Jürgen Groß]

On 09.12.19 20:43, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> Granting pages consumes backend system memory.  In systems configured
> with insufficient spare memory for those pages, it can cause a memory
> pressure situation.  However, finding the optimal amount of the spare
> memory is challenging for large systems having dynamic resource
> utilization patterns.  Also, such a static configuration might lacks a
> flexibility.
> 
> To mitigate such problems, this commit adds a memory reclaim callback to
> 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> a memory pressure and request specific domains of specific backend
> drivers which causing the given pressure to voluntarily release its
> memory.
> 
> That said, this commit simply requests every callback registered driver
> to release its memory for every domain, rather than issueing the
> requests to the drivers and domain in charge.  Such things would be a
> future work.  Also, this commit focuses on memory only.  However, it
> would be ablt to be extended for general resources.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
>   include/xen/xenbus.h                      |  1 +
>   2 files changed, 32 insertions(+)
> 
> diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> index b0bed4faf44c..cd5fd1cd8de3 100644
> --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
>   	return NOTIFY_DONE;
>   }
>   
> +static int xenbus_backend_reclaim(struct device *dev, void *data)
> +{
> +	struct xenbus_driver *drv;
> +	if (!dev->driver)
> +		return -ENOENT;
> +	drv = to_xenbus_driver(dev->driver);
> +	if (drv && drv->reclaim)
> +		drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);

Oh, sorry for first requesting you to add the domid as a parameter,
but now I realize this could be handled in the xenbus driver, as
struct xenbus_device already contains the otherend_id.

Would you mind dropping the parameter again, please?


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

Re: [PATCH v4 2/2] xen/blkback: Squeeze page pools if a memory pressure is detected

[Jürgen Groß]

On 09.12.19 20:43, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> Each `blkif` has a free pages pool for the grant mapping.  The size of
> the pool starts from zero and be increased on demand while processing
> the I/O requests.  If current I/O requests handling is finished or 100
> milliseconds has passed since last I/O requests handling, it checks and
> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
> 
> Therefore, `blkfront` running guests can cause a memory pressure in the
> `blkback` running guest by attaching a large number of block devices and
> inducing I/O.  System administrators can avoid such problematic
> situations by limiting the maximum number of devices each guest can
> attach.  However, finding the optimal limit is not so easy.  Improper
> set of the limit can results in the memory pressure or a resource
> underutilization.  This commit avoids such problematic situations by
> squeezing the pools (returns every free page in the pool to the system)
> for a while (users can set this duration via a module parameter) if a
> memory pressure is detected.
> 
> Discussions
> ===========
> 
> The `blkback`'s original shrinking mechanism returns only pages in the
> pool, which are not currently be used by `blkback`, to the system.  In
> other words, the pages are not mapped with foreign pages.  Because this
> commit is changing only the shrink limit but uses the mechanism as is,
> this commit does not introduce improper mappings related security
> issues.
> 
> Once a memory pressure is detected, this commit keeps the squeezing
> limit for a user-specified time duration.  The duration should be
> neither too long nor too short.  If it is too long, the squeezing
> incurring overhead can reduce the I/O performance.  If it is too short,
> `blkback` will not free enough pages to reduce the memory pressure.
> This commit sets the value as `10 milliseconds` by default because it is
> a short time in terms of I/O while it is a long time in terms of memory
> operations.  Also, as the original shrinking mechanism works for at
> least every 100 milliseconds, this could be a somewhat reasonable
> choice.  I also tested other durations (refer to the below section for
> more details) and confirmed that 10 milliseconds is the one that works
> best with the test.  That said, the proper duration depends on actual
> configurations and workloads.  That's why this commit is allowing users
> to set it as their optimal value via the module parameter.
> 
> Memory Pressure Test
> ====================
> 
> To show how this commit fixes the memory pressure situation well, I
> configured a test environment on a xen-running virtualization system.
> On the `blkfront` running guest instances, I attach a large number of
> network-backed volume devices and induce I/O to those.  Meanwhile, I
> measure the number of pages that swapped in and out on the `blkback`
> running guest.  The test ran twice, once for the `blkback` before this
> commit and once for that after this commit.  As shown below, this commit
> has dramatically reduced the memory pressure:
> 
>                  pswpin  pswpout
>      before      76,672  185,799
>      after          212    3,325
> 
> Optimal Aggressive Shrinking Duration
> -------------------------------------
> 
> To find a best squeezing duration, I repeated the test with three
> different durations (1ms, 10ms, and 100ms).  The results are as below:
> 
>      duration    pswpin  pswpout
>      1           852     6,424
>      10          212     3,325
>      100         203     3,340
> 
> As expected, the memory pressure has decreased as the duration is
> increased, but the reduction stopped from the `10ms`.  Based on this
> results, I chose the default duration as 10ms.
> 
> Performance Overhead Test
> =========================
> 
> This commit could incur I/O performance degradation under severe memory
> pressure because the squeezing will require more page allocations per
> I/O.  To show the overhead, I artificially made a worst-case squeezing
> situation and measured the I/O performance of a `blkfront` running
> guest.
> 
> For the artificial squeezing, I set the `blkback.max_buffer_pages` using
> the `/sys/module/xen_blkback/parameters/max_buffer_pages` file.  We set
> the value to `1024` and `0`.  The `1024` is the default value.  Setting
> the value as `0` is same to a situation doing the squeezing always
> (worst-case).
> 
> For the I/O performance measurement, I use a simple `dd` command.
> 
> Default Performance
> -------------------
> 
>      [dom0]# echo 1024 > /sys/module/xen_blkback/parameters/max_buffer_pages
>      [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8827 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8781 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8737 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8702 s, 38.7 MB/s
> 
> Worst-case Performance
> ----------------------
> 
>      [dom0]# echo 0 > /sys/module/xen_blkback/parameters/max_buffer_pages
>      [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.878 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8746 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8786 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8749 s, 38.7 MB/s
> 
> In short, even worst case squeezing makes no visible performance
> degradation.  I think this is due to the slow speed of the I/O.  In
> other words, the additional page allocation overhead is hidden under the
> much slower I/O latency.
> 
> Nevertheless, pleaset note that this is just a very simple and minimal
> test.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>

Reviewed-by: Juergen Gross <jgross@suse.com>

When dropping the domid parameter you can keep the Reviewed-by, of
course.


Juergen

Re: [Xen-devel] [PATCH v4 2/2] xen/blkback: Squeeze page pools if a memory pressure is detected

[Jürgen Groß]

On 09.12.19 20:43, SeongJae Park wrote:
> From: SeongJae Park <sjpark@amazon.de>
> 
> Each `blkif` has a free pages pool for the grant mapping.  The size of
> the pool starts from zero and be increased on demand while processing
> the I/O requests.  If current I/O requests handling is finished or 100
> milliseconds has passed since last I/O requests handling, it checks and
> shrinks the pool to not exceed the size limit, `max_buffer_pages`.
> 
> Therefore, `blkfront` running guests can cause a memory pressure in the
> `blkback` running guest by attaching a large number of block devices and
> inducing I/O.  System administrators can avoid such problematic
> situations by limiting the maximum number of devices each guest can
> attach.  However, finding the optimal limit is not so easy.  Improper
> set of the limit can results in the memory pressure or a resource
> underutilization.  This commit avoids such problematic situations by
> squeezing the pools (returns every free page in the pool to the system)
> for a while (users can set this duration via a module parameter) if a
> memory pressure is detected.
> 
> Discussions
> ===========
> 
> The `blkback`'s original shrinking mechanism returns only pages in the
> pool, which are not currently be used by `blkback`, to the system.  In
> other words, the pages are not mapped with foreign pages.  Because this
> commit is changing only the shrink limit but uses the mechanism as is,
> this commit does not introduce improper mappings related security
> issues.
> 
> Once a memory pressure is detected, this commit keeps the squeezing
> limit for a user-specified time duration.  The duration should be
> neither too long nor too short.  If it is too long, the squeezing
> incurring overhead can reduce the I/O performance.  If it is too short,
> `blkback` will not free enough pages to reduce the memory pressure.
> This commit sets the value as `10 milliseconds` by default because it is
> a short time in terms of I/O while it is a long time in terms of memory
> operations.  Also, as the original shrinking mechanism works for at
> least every 100 milliseconds, this could be a somewhat reasonable
> choice.  I also tested other durations (refer to the below section for
> more details) and confirmed that 10 milliseconds is the one that works
> best with the test.  That said, the proper duration depends on actual
> configurations and workloads.  That's why this commit is allowing users
> to set it as their optimal value via the module parameter.
> 
> Memory Pressure Test
> ====================
> 
> To show how this commit fixes the memory pressure situation well, I
> configured a test environment on a xen-running virtualization system.
> On the `blkfront` running guest instances, I attach a large number of
> network-backed volume devices and induce I/O to those.  Meanwhile, I
> measure the number of pages that swapped in and out on the `blkback`
> running guest.  The test ran twice, once for the `blkback` before this
> commit and once for that after this commit.  As shown below, this commit
> has dramatically reduced the memory pressure:
> 
>                  pswpin  pswpout
>      before      76,672  185,799
>      after          212    3,325
> 
> Optimal Aggressive Shrinking Duration
> -------------------------------------
> 
> To find a best squeezing duration, I repeated the test with three
> different durations (1ms, 10ms, and 100ms).  The results are as below:
> 
>      duration    pswpin  pswpout
>      1           852     6,424
>      10          212     3,325
>      100         203     3,340
> 
> As expected, the memory pressure has decreased as the duration is
> increased, but the reduction stopped from the `10ms`.  Based on this
> results, I chose the default duration as 10ms.
> 
> Performance Overhead Test
> =========================
> 
> This commit could incur I/O performance degradation under severe memory
> pressure because the squeezing will require more page allocations per
> I/O.  To show the overhead, I artificially made a worst-case squeezing
> situation and measured the I/O performance of a `blkfront` running
> guest.
> 
> For the artificial squeezing, I set the `blkback.max_buffer_pages` using
> the `/sys/module/xen_blkback/parameters/max_buffer_pages` file.  We set
> the value to `1024` and `0`.  The `1024` is the default value.  Setting
> the value as `0` is same to a situation doing the squeezing always
> (worst-case).
> 
> For the I/O performance measurement, I use a simple `dd` command.
> 
> Default Performance
> -------------------
> 
>      [dom0]# echo 1024 > /sys/module/xen_blkback/parameters/max_buffer_pages
>      [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8827 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8781 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8737 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8702 s, 38.7 MB/s
> 
> Worst-case Performance
> ----------------------
> 
>      [dom0]# echo 0 > /sys/module/xen_blkback/parameters/max_buffer_pages
>      [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.878 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8746 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8786 s, 38.7 MB/s
>      131072+0 records in
>      131072+0 records out
>      536870912 bytes (537 MB) copied, 13.8749 s, 38.7 MB/s
> 
> In short, even worst case squeezing makes no visible performance
> degradation.  I think this is due to the slow speed of the I/O.  In
> other words, the additional page allocation overhead is hidden under the
> much slower I/O latency.
> 
> Nevertheless, pleaset note that this is just a very simple and minimal
> test.
> 
> Signed-off-by: SeongJae Park <sjpark@amazon.de>

Reviewed-by: Juergen Gross <jgross@suse.com>

When dropping the domid parameter you can keep the Reviewed-by, of
course.


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

Re: [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

On Tue, Dec 10, 2019 at 7:23 AM Jürgen Groß <jgross@suse.com> wrote:
>
> On 09.12.19 20:43, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > Granting pages consumes backend system memory.  In systems configured
> > with insufficient spare memory for those pages, it can cause a memory
> > pressure situation.  However, finding the optimal amount of the spare
> > memory is challenging for large systems having dynamic resource
> > utilization patterns.  Also, such a static configuration might lacks a
> > flexibility.
> >
> > To mitigate such problems, this commit adds a memory reclaim callback to
> > 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> > a memory pressure and request specific domains of specific backend
> > drivers which causing the given pressure to voluntarily release its
> > memory.
> >
> > That said, this commit simply requests every callback registered driver
> > to release its memory for every domain, rather than issueing the
> > requests to the drivers and domain in charge.  Such things would be a
> > future work.  Also, this commit focuses on memory only.  However, it
> > would be ablt to be extended for general resources.
> >
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
> >   include/xen/xenbus.h                      |  1 +
> >   2 files changed, 32 insertions(+)
> >
> > diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> > index b0bed4faf44c..cd5fd1cd8de3 100644
> > --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> > +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> > @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
> >       return NOTIFY_DONE;
> >   }
> >
> > +static int xenbus_backend_reclaim(struct device *dev, void *data)
> > +{
> > +     struct xenbus_driver *drv;
> > +     if (!dev->driver)
> > +             return -ENOENT;
> > +     drv = to_xenbus_driver(dev->driver);
> > +     if (drv && drv->reclaim)
> > +             drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
>
> Oh, sorry for first requesting you to add the domid as a parameter,
> but now I realize this could be handled in the xenbus driver, as
> struct xenbus_device already contains the otherend_id.
>
> Would you mind dropping the parameter again, please?

Oh, I also missed it!  Will do!


Thanks,
SeongJae Park

>
>
> Juergen

Re: [Xen-devel] [PATCH v4 1/2] xenbus/backend: Add memory pressure handler callback

[SeongJae Park]

On Tue, Dec 10, 2019 at 7:23 AM Jürgen Groß <jgross@suse.com> wrote:
>
> On 09.12.19 20:43, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > Granting pages consumes backend system memory.  In systems configured
> > with insufficient spare memory for those pages, it can cause a memory
> > pressure situation.  However, finding the optimal amount of the spare
> > memory is challenging for large systems having dynamic resource
> > utilization patterns.  Also, such a static configuration might lacks a
> > flexibility.
> >
> > To mitigate such problems, this commit adds a memory reclaim callback to
> > 'xenbus_driver'.  Using this facility, 'xenbus' would be able to monitor
> > a memory pressure and request specific domains of specific backend
> > drivers which causing the given pressure to voluntarily release its
> > memory.
> >
> > That said, this commit simply requests every callback registered driver
> > to release its memory for every domain, rather than issueing the
> > requests to the drivers and domain in charge.  Such things would be a
> > future work.  Also, this commit focuses on memory only.  However, it
> > would be ablt to be extended for general resources.
> >
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >   drivers/xen/xenbus/xenbus_probe_backend.c | 31 +++++++++++++++++++++++
> >   include/xen/xenbus.h                      |  1 +
> >   2 files changed, 32 insertions(+)
> >
> > diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
> > index b0bed4faf44c..cd5fd1cd8de3 100644
> > --- a/drivers/xen/xenbus/xenbus_probe_backend.c
> > +++ b/drivers/xen/xenbus/xenbus_probe_backend.c
> > @@ -248,6 +248,34 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
> >       return NOTIFY_DONE;
> >   }
> >
> > +static int xenbus_backend_reclaim(struct device *dev, void *data)
> > +{
> > +     struct xenbus_driver *drv;
> > +     if (!dev->driver)
> > +             return -ENOENT;
> > +     drv = to_xenbus_driver(dev->driver);
> > +     if (drv && drv->reclaim)
> > +             drv->reclaim(to_xenbus_device(dev), DOMID_INVALID);
>
> Oh, sorry for first requesting you to add the domid as a parameter,
> but now I realize this could be handled in the xenbus driver, as
> struct xenbus_device already contains the otherend_id.
>
> Would you mind dropping the parameter again, please?

Oh, I also missed it!  Will do!


Thanks,
SeongJae Park

>
>
> Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel