[Xen-devel] [PATCH v7 0/2] xenbus/backend: Add a memory pressure handler callback

[Xen-devel] [PATCH v7 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 lack
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).  The third patch is a trivial cleanup of
variable names.

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_v7


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

Changes from v6
(https://lore.kernel.org/linux-block/20191211042428.5961-1-sjpark@amazon.de/)
 - Remove more unnecessary prefixes (suggested by Roger Pau Monné)
 - Constify a variable (suggested by Roger Pau Monné)
 - Rename 'reclaim' into 'reclaim_memory' (suggested by Roger Pau Monné)
 - More wordsmith of the commit message (suggested by Roger Pau Monné)

Changes from v5
(https://lore.kernel.org/linux-block/20191210080628.5264-1-sjpark@amazon.de/)
 - Wordsmith the commit messages (suggested by Roger Pau Monné)
 - Change the reclaim callback return type (suggested by Roger Pau Monné)
 - Change the type of the blkback squeeze duration variable
   (suggested by Roger Pau Monné)
 - Add a patch for removal of unnecessary static variable name prefixes
   (suggested by Roger Pau Monné)
 - Fix checkpatch.pl warnings

Changes from v4
(https://lore.kernel.org/xen-devel/20191209194305.20828-1-sjpark@amazon.com/)
 - Remove domain id parameter from the callback (suggested by Juergen Gross)
 - Rename xen-blkback module parameter (suggested by Stefan Nuernburger)

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 (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


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[Xen-devel] [PATCH v7 1/3] xenbus/backend: Add 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 lack
flexibility.

To mitigate such problems, this commit adds a memory reclaim callback to
'xenbus_driver'.  If a memory pressure is detected, 'xenbus' requests
every backend driver to volunarily release its memory.

Note that it would be able to improve the callback facility for more
sophisticated handlings of general pressures.  For example, it would be
possible to monitor the memory consumption of each device and issue the
release requests to only devices which causing the pressure.  Also, the
callback could be extended to handle not only memory, but general
resources.  Nevertheless, this version of the implementation defers such
sophisticated goals as a future work.

Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/xen/xenbus/xenbus_probe_backend.c | 32 +++++++++++++++++++++++
 include/xen/xenbus.h                      |  1 +
 2 files changed, 33 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index b0bed4faf44c..7e78ebef7c54 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -248,6 +248,35 @@ static int backend_probe_and_watch(struct notifier_block *notifier,
 	return NOTIFY_DONE;
 }
 
+static int backend_reclaim_memory(struct device *dev, void *data)
+{
+	const struct xenbus_driver *drv;
+
+	if (!dev->driver)
+		return 0;
+	drv = to_xenbus_driver(dev->driver);
+	if (drv && drv->reclaim_memory)
+		drv->reclaim_memory(to_xenbus_device(dev));
+	return 0;
+}
+
+/*
+ * Returns 0 always because we are using shrinker to only detect memory
+ * pressure.
+ */
+static unsigned long backend_shrink_memory_count(struct shrinker *shrinker,
+				struct shrink_control *sc)
+{
+	bus_for_each_dev(&xenbus_backend.bus, NULL, NULL,
+			backend_reclaim_memory);
+	return 0;
+}
+
+static struct shrinker backend_memory_shrinker = {
+	.count_objects = backend_shrink_memory_count,
+	.seeks = DEFAULT_SEEKS,
+};
+
 static int __init xenbus_probe_backend_init(void)
 {
 	static struct notifier_block xenstore_notifier = {
@@ -264,6 +293,9 @@ static int __init xenbus_probe_backend_init(void)
 
 	register_xenstore_notifier(&xenstore_notifier);
 
+	if (register_shrinker(&backend_memory_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..c861cfb6f720 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);
+	void (*reclaim_memory)(struct xenbus_device *dev);
 };
 
 static inline struct xenbus_driver *to_xenbus_driver(struct device_driver *drv)
-- 
2.17.1


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Re: [Xen-devel] [PATCH v7 1/3] xenbus/backend: Add memory pressure handler callback

[Roger Pau Monné]

On Wed, Dec 11, 2019 at 06:10:14PM +0000, SeongJae Park wrote:
> 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 lack
> flexibility.
> 
> To mitigate such problems, this commit adds a memory reclaim callback to
> 'xenbus_driver'.  If a memory pressure is detected, 'xenbus' requests
> every backend driver to volunarily release its memory.
> 
> Note that it would be able to improve the callback facility for more
> sophisticated handlings of general pressures.  For example, it would be
> possible to monitor the memory consumption of each device and issue the
> release requests to only devices which causing the pressure.  Also, the
> callback could be extended to handle not only memory, but general
> resources.  Nevertheless, this version of the implementation defers such
> sophisticated goals as a future work.
> 
> Reviewed-by: Juergen Gross <jgross@suse.com>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks, Roger.

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[Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

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, host administrators can cause memory pressure in blkback by
attaching a large number of block devices and inducing I/O.  Such
problematic situations can be avoided by limiting the maximum number of
devices that can be attached, but 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 that are not mapped with granted pages.  Because
this commit is changing only the shrink limit but still uses the same
freeing mechanism it does not touch pages which are currently mapping
grants.

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 allows users to
set the duration as a 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 (pswpin) and out (pswpout)
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.  In this
test, I 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 run a simple `dd` command 5 times
as below and collect the 'MB/s' results.

    $ for i in {1..5}; do dd if=/dev/zero of=file \
                             bs=4k count=$((256*512)); sync; done

If the underlying block device is slow enough, the squeezing overhead
could be hidden.  For the reason, I do this test for both a slow block
device and a fast block device.  I use a popular cloud block storage
service, ebs[1] as a slow device and the ramdisk block device[2] for the
fast device.

The results are as below.  'max_pgs' represents the value of the
`blkback.max_buffer_pages` parameter.

On the slow block device
------------------------

    max_pgs   Min       Max       Median     Avg    Stddev
    0         38.7      45.8      38.7       40.12  3.1752165
    1024      38.7      45.8      38.7       40.12  3.1752165
    No difference proven at 95.0% confidence

On the fast block device
------------------------

    max_pgs   Min       Max       Median     Avg    Stddev
    0         417       423       420        419.4  2.5099801
    1024      414       425       416        417.8  4.4384682
    No difference proven at 95.0% confidence

In short, even worst case squeezing on ramdisk based fast block device
makes no visible performance degradation.  Please note that this is just
a very simple and minimal test.  On systems using super-fast block
devices and a special I/O workload, the results might be different.  If
you have any doubt, test on your machine for your workload to find the
optimal squeezing duration for you.

[1] https://aws.amazon.com/ebs/
[2] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html

Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/block/xen-blkback/blkback.c | 22 ++++++++++++++++++++--
 drivers/block/xen-blkback/common.h  |  1 +
 drivers/block/xen-blkback/xenbus.c  |  3 ++-
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index fd1e19f1a49f..98823d150905 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
+module_param_named(buffer_squeeze_duration_ms,
+		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 buffer_squeeze_end;
+
+void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
+{
+	buffer_squeeze_end = jiffies +
+		msecs_to_jiffies(buffer_squeeze_duration_ms);
+}
+
 static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
 {
 	unsigned long flags;
@@ -656,8 +671,11 @@ 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, 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..1e0df86cb941 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);
+void xen_blkbk_reclaim_memory(struct xenbus_device *dev);
 
 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..0477f910b018 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_memory = xen_blkbk_reclaim_memory,
 };
 
 int xen_blkif_xenbus_init(void)
-- 
2.17.1


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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[Roger Pau Monné]

Hello,

Please make sure you Cc me in blkback related patches.

On Wed, Dec 11, 2019 at 06:10:15PM +0000, SeongJae Park wrote:
> 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
                                ^ is
> 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, host administrators can cause memory pressure in blkback by
> attaching a large number of block devices and inducing I/O.  Such
> problematic situations can be avoided by limiting the maximum number of
> devices that can be attached, but finding the optimal limit is not so
> easy.  Improper set of the limit can results in the memory pressure or a
                                                  ^ s/the//
> 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.
                ^ s/a//
> 
> 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

I think you can remove both comas in the above sentence.

> other words, the pages that are not mapped with granted pages.  Because
> this commit is changing only the shrink limit but still uses the same
> freeing mechanism it does not touch pages which are currently mapping
> grants.
> 
> Once a memory pressure is detected, this commit keeps the squeezing
       ^ s/a//
> 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 allows users to
> set the duration as a 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 (pswpin) and out (pswpout)
> 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.  In this
> test, I 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 run a simple `dd` command 5 times
> as below and collect the 'MB/s' results.
> 
>     $ for i in {1..5}; do dd if=/dev/zero of=file \
>                              bs=4k count=$((256*512)); sync; done

I think it would be better if you could skip the filesystem overhead
by writing directly to a block device, ie:

Attach a null_blk based block device to the guest (on dom0):
# xl block-attach guest phy:/dev/null_blk0 xvdb w

Run a workload against the device (inside the guest):
# fio --filename=/dev/xvdb --direct=1 --rw=randrw --bs=4k --ioengine=libaio \
      --iodepth=64 --runtime=120 --numjobs=4 --time_based --group_reporting \
      --name=throughput-test-job --eta-newline=1

You should run this on a multi-vcpu guest so that multiple queues are
used, and adjust the numjobs to (at least) match the number of queues.

> 
> If the underlying block device is slow enough, the squeezing overhead
> could be hidden.  For the reason, I do this test for both a slow block
> device and a fast block device.  I use a popular cloud block storage
> service, ebs[1] as a slow device and the ramdisk block device[2] for the
> fast device.
> 
> The results are as below.  'max_pgs' represents the value of the
> `blkback.max_buffer_pages` parameter.
> 
> On the slow block device
> ------------------------
> 
>     max_pgs   Min       Max       Median     Avg    Stddev
>     0         38.7      45.8      38.7       40.12  3.1752165
>     1024      38.7      45.8      38.7       40.12  3.1752165
>     No difference proven at 95.0% confidence
> 
> On the fast block device
> ------------------------
> 
>     max_pgs   Min       Max       Median     Avg    Stddev
>     0         417       423       420        419.4  2.5099801
>     1024      414       425       416        417.8  4.4384682
>     No difference proven at 95.0% confidence

This is intriguing, as it seems to prove that the usage of a cache of
free pages is irrelevant performance wise.

The pool of free pages was introduced long ago, and it's possible that
recent improvements to the balloon driver had made such pool useless,
at which point it could be removed instead of worked around.

Do you think you could perform some more tests (as pointed out above
against the block device to skip the fs overhead) and report back the
results?

> 
> In short, even worst case squeezing on ramdisk based fast block device
> makes no visible performance degradation.  Please note that this is just
> a very simple and minimal test.  On systems using super-fast block
> devices and a special I/O workload, the results might be different.  If
> you have any doubt, test on your machine for your workload to find the
> optimal squeezing duration for you.
> 
> [1] https://aws.amazon.com/ebs/
> [2] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html
> 
> Reviewed-by: Juergen Gross <jgross@suse.com>
> Signed-off-by: SeongJae Park <sjpark@amazon.de>
> ---
>  drivers/block/xen-blkback/blkback.c | 22 ++++++++++++++++++++--
>  drivers/block/xen-blkback/common.h  |  1 +
>  drivers/block/xen-blkback/xenbus.c  |  3 ++-
>  3 files changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index fd1e19f1a49f..98823d150905 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
> +module_param_named(buffer_squeeze_duration_ms,
> +		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");

You should add a description about this parameter to
Documentation/ABI/testing/sysfs-driver-xen-blkback

Thanks, Roger.

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

On Thu, 12 Dec 2019 12:42:47 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> 
> Please make sure you Cc me in blkback related patches.

Sorry for forgotting you!  I will never forget again.

> 
> On Wed, Dec 11, 2019 at 06:10:15PM +0000, SeongJae Park wrote:
> > 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
>                                 ^ is
> > 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, host administrators can cause memory pressure in blkback by
> > attaching a large number of block devices and inducing I/O.  Such
> > problematic situations can be avoided by limiting the maximum number of
> > devices that can be attached, but finding the optimal limit is not so
> > easy.  Improper set of the limit can results in the memory pressure or a
>                                                   ^ s/the//
> > 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.
>                 ^ s/a//
> > 
> > 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
> 
> I think you can remove both comas in the above sentence.
> 
> > other words, the pages that are not mapped with granted pages.  Because
> > this commit is changing only the shrink limit but still uses the same
> > freeing mechanism it does not touch pages which are currently mapping
> > grants.
> > 
> > Once a memory pressure is detected, this commit keeps the squeezing
>        ^ s/a//

Thank you for corrections, will apply!

> > 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 allows users to
> > set the duration as a 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 (pswpin) and out (pswpout)
> > 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.  In this
> > test, I 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 run a simple `dd` command 5 times
> > as below and collect the 'MB/s' results.
> > 
> >     $ for i in {1..5}; do dd if=/dev/zero of=file \
> >                              bs=4k count=$((256*512)); sync; done
> 
> I think it would be better if you could skip the filesystem overhead
> by writing directly to a block device, ie:
> 
> Attach a null_blk based block device to the guest (on dom0):
> # xl block-attach guest phy:/dev/null_blk0 xvdb w
> 
> Run a workload against the device (inside the guest):
> # fio --filename=/dev/xvdb --direct=1 --rw=randrw --bs=4k --ioengine=libaio \
>       --iodepth=64 --runtime=120 --numjobs=4 --time_based --group_reporting \
>       --name=throughput-test-job --eta-newline=1
> 
> You should run this on a multi-vcpu guest so that multiple queues are
> used, and adjust the numjobs to (at least) match the number of queues.

I forgot to update the `dd` command.  I used the command for the slow block
device test, but directly induced the I/O towards the block device for the fast
block device test as below:

    # xl block-attach guest phy:/dev/ram0 xvdb w
 
    $ for i in {1..5}; do dd if=/dev/zero of=file \
                             bs=4k count=$((256*512)); sync; done

Nevertheless, I agree that you suggested test will provide much more accurate.
As stated before, my test is only designed for a minimal proof-of-concept.

> 
> 
> > 
> > If the underlying block device is slow enough, the squeezing overhead
> > could be hidden.  For the reason, I do this test for both a slow block
> > device and a fast block device.  I use a popular cloud block storage
> > service, ebs[1] as a slow device and the ramdisk block device[2] for the
> > fast device.
> > 
> > The results are as below.  'max_pgs' represents the value of the
> > `blkback.max_buffer_pages` parameter.
> > 
> > On the slow block device
> > ------------------------
> > 
> >     max_pgs   Min       Max       Median     Avg    Stddev
> >     0         38.7      45.8      38.7       40.12  3.1752165
> >     1024      38.7      45.8      38.7       40.12  3.1752165
> >     No difference proven at 95.0% confidence
> > 
> > On the fast block device
> > ------------------------
> > 
> >     max_pgs   Min       Max       Median     Avg    Stddev
> >     0         417       423       420        419.4  2.5099801
> >     1024      414       425       416        417.8  4.4384682
> >     No difference proven at 95.0% confidence
> 
> This is intriguing, as it seems to prove that the usage of a cache of
> free pages is irrelevant performance wise.
> 
> The pool of free pages was introduced long ago, and it's possible that
> recent improvements to the balloon driver had made such pool useless,
> at which point it could be removed instead of worked around.

I guess the grant page allocation overhead in this test scenario is really
small.  In an absence of memory pressure, fragmentation, and NUMA imbalance,
the latency of the page allocation ('get_page()') is very short, as it will
success in the fast path.

Few years ago, I once measured the page allocation latency on my machine.
Roughly speaking, it was about 1us in best case, 100us in worst case, and 5us
in average.  Please keep in mind that the measurement was not designed and
performed in serious way.  Thus the results could have profile overhead in it,
though.  While keeping that in mind, let's simply believe the number and ignore
the latency of the block layer, blkback itself (including the grant
mapping), and anything else including context switch, cache miss, but the
allocation.  In other words, suppose that the grant page allocation is only one
source of the overhead.  It will be able to achieve 1 million IOPS (4KB *
1MIOPS = 4 GB/s) in the best case, 200 thousand IOPS (800 MB/s) in average, and
10 thousand IOPS (40 MB/s) in worst case.  Based on this coarse calculation, I
think the test results is reasonable.

This also means that the effect of the blkback's free pages pool might be
visible under page allocation fast path failure situation.  Nevertheless, it
would be also hard to measure that in micro level unless the measurement is
well designed and controlled.

> 
> Do you think you could perform some more tests (as pointed out above
> against the block device to skip the fs overhead) and report back the
> results?

To be honest, I'm not sure whether additional tests are really necessary,
because I think the `dd` test and the results explanation already makes some
sense and provide the minimal proof of the concept.  Also, this change is a
fallback for the memory pressure situation, which is an error path in some
point of view.  Such errorneous situation might not happen frequently and if
the situation is not solved in short time, something much worse (e.g., OOM kill
of the user space xen control processes) than temporal I/O performance
degradation could happen.  Thus, I'm not sure whether such detailed performance
measurement is necessary for this rare error handling change.  The comment of
'xen_blkbk_unmap()' also says, "This could accumulate ops up to the batch size
to reduce the number of hypercalls, but since this is only used in error paths
there's no real need.".

That said, if you still want me to do the test, I will gladly do it.  Note that
since I'm now traveling US without the power cable of my now discharged laptop
which is the only way to connect to my test environments in my office, I would
not be able to do the additional test quickly.  I assume I could do the test
and give you the result within a couple of week.  If you want me to post next
version with minor changes such as commit message update meanwhile, please just
let me know.  Again, if you want it, I will gladly do it :)

> 
> > 
> > In short, even worst case squeezing on ramdisk based fast block device
> > makes no visible performance degradation.  Please note that this is just
> > a very simple and minimal test.  On systems using super-fast block
> > devices and a special I/O workload, the results might be different.  If
> > you have any doubt, test on your machine for your workload to find the
> > optimal squeezing duration for you.
> > 
> > [1] https://aws.amazon.com/ebs/
> > [2] https://www.kernel.org/doc/html/latest/admin-guide/blockdev/ramdisk.html
> > 
> > Reviewed-by: Juergen Gross <jgross@suse.com>
> > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > ---
> >  drivers/block/xen-blkback/blkback.c | 22 ++++++++++++++++++++--
> >  drivers/block/xen-blkback/common.h  |  1 +
> >  drivers/block/xen-blkback/xenbus.c  |  3 ++-
> >  3 files changed, 23 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> > index fd1e19f1a49f..98823d150905 100644
> > --- a/drivers/block/xen-blkback/blkback.c
> > +++ b/drivers/block/xen-blkback/blkback.c
> > @@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
> > +module_param_named(buffer_squeeze_duration_ms,
> > +		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");
> 
> You should add a description about this parameter to
> Documentation/ABI/testing/sysfs-driver-xen-blkback

Good point, I will.


Thanks,
SeongJae Park

> 
> Thanks, Roger.
> 

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[Roger Pau Monné]

On Thu, Dec 12, 2019 at 02:39:05PM +0100, SeongJae Park wrote:
> On Thu, 12 Dec 2019 12:42:47 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
> > > On the slow block device
> > > ------------------------
> > > 
> > >     max_pgs   Min       Max       Median     Avg    Stddev
> > >     0         38.7      45.8      38.7       40.12  3.1752165
> > >     1024      38.7      45.8      38.7       40.12  3.1752165
> > >     No difference proven at 95.0% confidence
> > > 
> > > On the fast block device
> > > ------------------------
> > > 
> > >     max_pgs   Min       Max       Median     Avg    Stddev
> > >     0         417       423       420        419.4  2.5099801
> > >     1024      414       425       416        417.8  4.4384682
> > >     No difference proven at 95.0% confidence
> > 
> > This is intriguing, as it seems to prove that the usage of a cache of
> > free pages is irrelevant performance wise.
> > 
> > The pool of free pages was introduced long ago, and it's possible that
> > recent improvements to the balloon driver had made such pool useless,
> > at which point it could be removed instead of worked around.
> 
> I guess the grant page allocation overhead in this test scenario is really
> small.  In an absence of memory pressure, fragmentation, and NUMA imbalance,
> the latency of the page allocation ('get_page()') is very short, as it will
> success in the fast path.

The allocation of the pool of free pages involves more than get_page,
it uses gnttab_alloc_pages which in the worse case will allocate a
page and balloon it out issuing one hypercall.

> Few years ago, I once measured the page allocation latency on my machine.
> Roughly speaking, it was about 1us in best case, 100us in worst case, and 5us
> in average.  Please keep in mind that the measurement was not designed and
> performed in serious way.  Thus the results could have profile overhead in it,
> though.  While keeping that in mind, let's simply believe the number and ignore
> the latency of the block layer, blkback itself (including the grant
> mapping), and anything else including context switch, cache miss, but the
> allocation.  In other words, suppose that the grant page allocation is only one
> source of the overhead.  It will be able to achieve 1 million IOPS (4KB *
> 1MIOPS = 4 GB/s) in the best case, 200 thousand IOPS (800 MB/s) in average, and
> 10 thousand IOPS (40 MB/s) in worst case.  Based on this coarse calculation, I
> think the test results is reasonable.
> 
> This also means that the effect of the blkback's free pages pool might be
> visible under page allocation fast path failure situation.  Nevertheless, it
> would be also hard to measure that in micro level unless the measurement is
> well designed and controlled.
> 
> > 
> > Do you think you could perform some more tests (as pointed out above
> > against the block device to skip the fs overhead) and report back the
> > results?
> 
> To be honest, I'm not sure whether additional tests are really necessary,
> because I think the `dd` test and the results explanation already makes some
> sense and provide the minimal proof of the concept.  Also, this change is a
> fallback for the memory pressure situation, which is an error path in some
> point of view.  Such errorneous situation might not happen frequently and if
> the situation is not solved in short time, something much worse (e.g., OOM kill
> of the user space xen control processes) than temporal I/O performance
> degradation could happen.  Thus, I'm not sure whether such detailed performance
> measurement is necessary for this rare error handling change.

Right, my main concern is that we seem to be adding duck tape so
things don't fall apart, but if such cache is really not beneficial
from a performance PoV I would rather see it go away than adding more
stuff to it in order to workaround corner cases like memory
starvation.

Anyway, I guess we can take such change, but long term we need to look
into fixing grants to not use ballooned pages, and figure out if the
blkback free page cache is really useful or not.

Thanks, Roger.

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

On Thu, 12 Dec 2019 16:23:17 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> > On Thu, 12 Dec 2019 12:42:47 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
> > > > On the slow block device
> > > > ------------------------
> > > > 
> > > >     max_pgs   Min       Max       Median     Avg    Stddev
> > > >     0         38.7      45.8      38.7       40.12  3.1752165
> > > >     1024      38.7      45.8      38.7       40.12  3.1752165
> > > >     No difference proven at 95.0% confidence
> > > > 
> > > > On the fast block device
> > > > ------------------------
> > > > 
> > > >     max_pgs   Min       Max       Median     Avg    Stddev
> > > >     0         417       423       420        419.4  2.5099801
> > > >     1024      414       425       416        417.8  4.4384682
> > > >     No difference proven at 95.0% confidence
> > > 
> > > This is intriguing, as it seems to prove that the usage of a cache of
> > > free pages is irrelevant performance wise.
> > > 
> > > The pool of free pages was introduced long ago, and it's possible that
> > > recent improvements to the balloon driver had made such pool useless,
> > > at which point it could be removed instead of worked around.
> > 
> > I guess the grant page allocation overhead in this test scenario is really
> > small.  In an absence of memory pressure, fragmentation, and NUMA imbalance,
> > the latency of the page allocation ('get_page()') is very short, as it will
> > success in the fast path.
> 
> The allocation of the pool of free pages involves more than get_page,
> it uses gnttab_alloc_pages which in the worse case will allocate a
> page and balloon it out issuing one hypercall.
> 
> > Few years ago, I once measured the page allocation latency on my machine.
> > Roughly speaking, it was about 1us in best case, 100us in worst case, and 5us
> > in average.  Please keep in mind that the measurement was not designed and
> > performed in serious way.  Thus the results could have profile overhead in it,
> > though.  While keeping that in mind, let's simply believe the number and ignore
> > the latency of the block layer, blkback itself (including the grant
> > mapping), and anything else including context switch, cache miss, but the
> > allocation.  In other words, suppose that the grant page allocation is only one
> > source of the overhead.  It will be able to achieve 1 million IOPS (4KB *
> > 1MIOPS = 4 GB/s) in the best case, 200 thousand IOPS (800 MB/s) in average, and
> > 10 thousand IOPS (40 MB/s) in worst case.  Based on this coarse calculation, I
> > think the test results is reasonable.
> > 
> > This also means that the effect of the blkback's free pages pool might be
> > visible under page allocation fast path failure situation.  Nevertheless, it
> > would be also hard to measure that in micro level unless the measurement is
> > well designed and controlled.
> > 
> > > 
> > > Do you think you could perform some more tests (as pointed out above
> > > against the block device to skip the fs overhead) and report back the
> > > results?
> > 
> > To be honest, I'm not sure whether additional tests are really necessary,
> > because I think the `dd` test and the results explanation already makes some
> > sense and provide the minimal proof of the concept.  Also, this change is a
> > fallback for the memory pressure situation, which is an error path in some
> > point of view.  Such errorneous situation might not happen frequently and if
> > the situation is not solved in short time, something much worse (e.g., OOM kill
> > of the user space xen control processes) than temporal I/O performance
> > degradation could happen.  Thus, I'm not sure whether such detailed performance
> > measurement is necessary for this rare error handling change.
> 
> Right, my main concern is that we seem to be adding duck tape so
> things don't fall apart, but if such cache is really not beneficial
> from a performance PoV I would rather see it go away than adding more
> stuff to it in order to workaround corner cases like memory
> starvation.

Right, if the cache is really giving no benefit, it would be much better to
simply remove it.  However, as mentioned before, I'm not sure whether it is
useless at all.  Maybe we could do some more detailed test to know that, but it
would be an out of scope of this patch.

> 
> Anyway, I guess we can take such change, but long term we need to look
> into fixing grants to not use ballooned pages, and figure out if the
> blkback free page cache is really useful or not.

Totally agreed.


Thanks,
SeongJae Park

> 
> Thanks, Roger.
> 

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[Roger Pau Monné]

On Wed, Dec 11, 2019 at 06:10:15PM +0000, SeongJae Park wrote:
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index fd1e19f1a49f..98823d150905 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
> +module_param_named(buffer_squeeze_duration_ms,
> +		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 buffer_squeeze_end;
> +
> +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
> +{
> +	buffer_squeeze_end = jiffies +
> +		msecs_to_jiffies(buffer_squeeze_duration_ms);

I'm not sure this is fully correct. This function will be called for
each blkback instance, but the timeout is stored in a global variable
that's shared between all blkback instances. Shouldn't this timeout be
stored in xen_blkif so each instance has it's own local variable?

Or else in the case you have 1k blkback instances the timeout is
certainly going to be longer than expected, because each call to
xen_blkbk_reclaim_memory will move it forward.

Thanks, Roger.

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

On Thu, 12 Dec 2019 16:27:57 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:

> > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> > index fd1e19f1a49f..98823d150905 100644
> > --- a/drivers/block/xen-blkback/blkback.c
> > +++ b/drivers/block/xen-blkback/blkback.c
> > @@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
> > +module_param_named(buffer_squeeze_duration_ms,
> > +		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 buffer_squeeze_end;
> > +
> > +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
> > +{
> > +	buffer_squeeze_end = jiffies +
> > +		msecs_to_jiffies(buffer_squeeze_duration_ms);
> 
> I'm not sure this is fully correct. This function will be called for
> each blkback instance, but the timeout is stored in a global variable
> that's shared between all blkback instances. Shouldn't this timeout be
> stored in xen_blkif so each instance has it's own local variable?
> 
> Or else in the case you have 1k blkback instances the timeout is
> certainly going to be longer than expected, because each call to
> xen_blkbk_reclaim_memory will move it forward.

Agreed that.  I think the extended timeout would not make a visible
performance, though, because the time that 1k-loop take would be short enough
to be ignored compared to the millisecond-scope duration.

I took this way because I wanted to minimize such structural changes as far as
I can, as this is just a point-fix rather than ultimate solution.  That said,
it is not fully correct and very confusing.  My another colleague also pointed
out it in internal review.  Correct solution would be to adding a variable in
the struct as you suggested or avoiding duplicated update of the variable by
initializing the variable once the squeezing duration passes.  I would prefer
the later way, as it is more straightforward and still not introducing
structural change.  For example, it might be like below:

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index f41c698dd854..6856c8ef88de 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -152,8 +152,9 @@ static unsigned long buffer_squeeze_end;
 
 void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
 {
-       buffer_squeeze_end = jiffies +
-               msecs_to_jiffies(buffer_squeeze_duration_ms);
+       if (!buffer_squeeze_end)
+               buffer_squeeze_end = jiffies +
+                       msecs_to_jiffies(buffer_squeeze_duration_ms);
 }
 
 static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
@@ -669,10 +670,13 @@ int xen_blkif_schedule(void *arg)
                }
 
                /* Shrink the free pages pool if it is too large. */
-               if (time_before(jiffies, buffer_squeeze_end))
+               if (time_before(jiffies, buffer_squeeze_end)) {
                        shrink_free_pagepool(ring, 0);
-               else
+               } else {
+                       if (unlikely(buffer_squeeze_end))
+                               buffer_squeeze_end = 0;
                        shrink_free_pagepool(ring, max_buffer_pages);
+               }
 
                if (log_stats && time_after(jiffies, ring->st_print))
                        print_stats(ring);

May I ask you what way would you prefer?


Thanks,
SeongJae Park

> 
> Thanks, Roger.
> 

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[Roger Pau Monné]

On Thu, Dec 12, 2019 at 05:06:58PM +0100, SeongJae Park wrote:
> On Thu, 12 Dec 2019 16:27:57 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
> 
> > > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> > > index fd1e19f1a49f..98823d150905 100644
> > > --- a/drivers/block/xen-blkback/blkback.c
> > > +++ b/drivers/block/xen-blkback/blkback.c
> > > @@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
> > > +module_param_named(buffer_squeeze_duration_ms,
> > > +		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 buffer_squeeze_end;
> > > +
> > > +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
> > > +{
> > > +	buffer_squeeze_end = jiffies +
> > > +		msecs_to_jiffies(buffer_squeeze_duration_ms);
> > 
> > I'm not sure this is fully correct. This function will be called for
> > each blkback instance, but the timeout is stored in a global variable
> > that's shared between all blkback instances. Shouldn't this timeout be
> > stored in xen_blkif so each instance has it's own local variable?
> > 
> > Or else in the case you have 1k blkback instances the timeout is
> > certainly going to be longer than expected, because each call to
> > xen_blkbk_reclaim_memory will move it forward.
> 
> Agreed that.  I think the extended timeout would not make a visible
> performance, though, because the time that 1k-loop take would be short enough
> to be ignored compared to the millisecond-scope duration.
> 
> I took this way because I wanted to minimize such structural changes as far as
> I can, as this is just a point-fix rather than ultimate solution.  That said,
> it is not fully correct and very confusing.  My another colleague also pointed
> out it in internal review.  Correct solution would be to adding a variable in
> the struct as you suggested or avoiding duplicated update of the variable by
> initializing the variable once the squeezing duration passes.  I would prefer
> the later way, as it is more straightforward and still not introducing
> structural change.  For example, it might be like below:
> 
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index f41c698dd854..6856c8ef88de 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -152,8 +152,9 @@ static unsigned long buffer_squeeze_end;
>  
>  void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
>  {
> -       buffer_squeeze_end = jiffies +
> -               msecs_to_jiffies(buffer_squeeze_duration_ms);
> +       if (!buffer_squeeze_end)
> +               buffer_squeeze_end = jiffies +
> +                       msecs_to_jiffies(buffer_squeeze_duration_ms);
>  }
>  
>  static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
> @@ -669,10 +670,13 @@ int xen_blkif_schedule(void *arg)
>                 }
>  
>                 /* Shrink the free pages pool if it is too large. */
> -               if (time_before(jiffies, buffer_squeeze_end))
> +               if (time_before(jiffies, buffer_squeeze_end)) {
>                         shrink_free_pagepool(ring, 0);
> -               else
> +               } else {
> +                       if (unlikely(buffer_squeeze_end))
> +                               buffer_squeeze_end = 0;
>                         shrink_free_pagepool(ring, max_buffer_pages);
> +               }
>  
>                 if (log_stats && time_after(jiffies, ring->st_print))
>                         print_stats(ring);
> 
> May I ask you what way would you prefer?

I'm not particularly found of this approach, as I think it's racy. Ie:
you would have to add some kind of lock to make sure the contents of
buffer_squeeze_end stay unmodified during the read and set cycle, or
else xen_blkif_schedule will race with xen_blkbk_reclaim_memory.

This is likely not a big deal ATM since the code will work as
expected in most cases AFAICT, but I would still prefer to have a
per-instance buffer_squeeze_end added to xen_blkif, given that the
callback is per-instance. I wouldn't call it a structural change, it's
just adding a variable to a struct instead of having a shared one, but
the code is almost the same as the current version.

Thanks, Roger.

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[Jürgen Groß]

On 13.12.19 10:27, Roger Pau Monné wrote:
> On Thu, Dec 12, 2019 at 05:06:58PM +0100, SeongJae Park wrote:
>> On Thu, 12 Dec 2019 16:27:57 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
>>
>>>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
>>>> index fd1e19f1a49f..98823d150905 100644
>>>> --- a/drivers/block/xen-blkback/blkback.c
>>>> +++ b/drivers/block/xen-blkback/blkback.c
>>>> @@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
>>>> +module_param_named(buffer_squeeze_duration_ms,
>>>> +		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 buffer_squeeze_end;
>>>> +
>>>> +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
>>>> +{
>>>> +	buffer_squeeze_end = jiffies +
>>>> +		msecs_to_jiffies(buffer_squeeze_duration_ms);
>>>
>>> I'm not sure this is fully correct. This function will be called for
>>> each blkback instance, but the timeout is stored in a global variable
>>> that's shared between all blkback instances. Shouldn't this timeout be
>>> stored in xen_blkif so each instance has it's own local variable?
>>>
>>> Or else in the case you have 1k blkback instances the timeout is
>>> certainly going to be longer than expected, because each call to
>>> xen_blkbk_reclaim_memory will move it forward.
>>
>> Agreed that.  I think the extended timeout would not make a visible
>> performance, though, because the time that 1k-loop take would be short enough
>> to be ignored compared to the millisecond-scope duration.
>>
>> I took this way because I wanted to minimize such structural changes as far as
>> I can, as this is just a point-fix rather than ultimate solution.  That said,
>> it is not fully correct and very confusing.  My another colleague also pointed
>> out it in internal review.  Correct solution would be to adding a variable in
>> the struct as you suggested or avoiding duplicated update of the variable by
>> initializing the variable once the squeezing duration passes.  I would prefer
>> the later way, as it is more straightforward and still not introducing
>> structural change.  For example, it might be like below:
>>
>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
>> index f41c698dd854..6856c8ef88de 100644
>> --- a/drivers/block/xen-blkback/blkback.c
>> +++ b/drivers/block/xen-blkback/blkback.c
>> @@ -152,8 +152,9 @@ static unsigned long buffer_squeeze_end;
>>   
>>   void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
>>   {
>> -       buffer_squeeze_end = jiffies +
>> -               msecs_to_jiffies(buffer_squeeze_duration_ms);
>> +       if (!buffer_squeeze_end)
>> +               buffer_squeeze_end = jiffies +
>> +                       msecs_to_jiffies(buffer_squeeze_duration_ms);
>>   }
>>   
>>   static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
>> @@ -669,10 +670,13 @@ int xen_blkif_schedule(void *arg)
>>                  }
>>   
>>                  /* Shrink the free pages pool if it is too large. */
>> -               if (time_before(jiffies, buffer_squeeze_end))
>> +               if (time_before(jiffies, buffer_squeeze_end)) {
>>                          shrink_free_pagepool(ring, 0);
>> -               else
>> +               } else {
>> +                       if (unlikely(buffer_squeeze_end))
>> +                               buffer_squeeze_end = 0;
>>                          shrink_free_pagepool(ring, max_buffer_pages);
>> +               }
>>   
>>                  if (log_stats && time_after(jiffies, ring->st_print))
>>                          print_stats(ring);
>>
>> May I ask you what way would you prefer?
> 
> I'm not particularly found of this approach, as I think it's racy. Ie:
> you would have to add some kind of lock to make sure the contents of
> buffer_squeeze_end stay unmodified during the read and set cycle, or
> else xen_blkif_schedule will race with xen_blkbk_reclaim_memory.
> 
> This is likely not a big deal ATM since the code will work as
> expected in most cases AFAICT, but I would still prefer to have a
> per-instance buffer_squeeze_end added to xen_blkif, given that the
> callback is per-instance. I wouldn't call it a structural change, it's
> just adding a variable to a struct instead of having a shared one, but
> the code is almost the same as the current version.

FWIW, I agree.


Juergen

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Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

[SeongJae Park]

On Fri, Dec 13, 2019 at 10:33 AM Jürgen Groß <jgross@suse.com> wrote:
>
> On 13.12.19 10:27, Roger Pau Monné wrote:
> > On Thu, Dec 12, 2019 at 05:06:58PM +0100, SeongJae Park wrote:
> >> On Thu, 12 Dec 2019 16:27:57 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
> >>
> >>>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> >>>> index fd1e19f1a49f..98823d150905 100644
> >>>> --- a/drivers/block/xen-blkback/blkback.c
> >>>> +++ b/drivers/block/xen-blkback/blkback.c
> >>>> @@ -142,6 +142,21 @@ 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 unsigned int buffer_squeeze_duration_ms = 10;
> >>>> +module_param_named(buffer_squeeze_duration_ms,
> >>>> +          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 buffer_squeeze_end;
> >>>> +
> >>>> +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
> >>>> +{
> >>>> +  buffer_squeeze_end = jiffies +
> >>>> +          msecs_to_jiffies(buffer_squeeze_duration_ms);
> >>>
> >>> I'm not sure this is fully correct. This function will be called for
> >>> each blkback instance, but the timeout is stored in a global variable
> >>> that's shared between all blkback instances. Shouldn't this timeout be
> >>> stored in xen_blkif so each instance has it's own local variable?
> >>>
> >>> Or else in the case you have 1k blkback instances the timeout is
> >>> certainly going to be longer than expected, because each call to
> >>> xen_blkbk_reclaim_memory will move it forward.
> >>
> >> Agreed that.  I think the extended timeout would not make a visible
> >> performance, though, because the time that 1k-loop take would be short enough
> >> to be ignored compared to the millisecond-scope duration.
> >>
> >> I took this way because I wanted to minimize such structural changes as far as
> >> I can, as this is just a point-fix rather than ultimate solution.  That said,
> >> it is not fully correct and very confusing.  My another colleague also pointed
> >> out it in internal review.  Correct solution would be to adding a variable in
> >> the struct as you suggested or avoiding duplicated update of the variable by
> >> initializing the variable once the squeezing duration passes.  I would prefer
> >> the later way, as it is more straightforward and still not introducing
> >> structural change.  For example, it might be like below:
> >>
> >> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> >> index f41c698dd854..6856c8ef88de 100644
> >> --- a/drivers/block/xen-blkback/blkback.c
> >> +++ b/drivers/block/xen-blkback/blkback.c
> >> @@ -152,8 +152,9 @@ static unsigned long buffer_squeeze_end;
> >>
> >>   void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
> >>   {
> >> -       buffer_squeeze_end = jiffies +
> >> -               msecs_to_jiffies(buffer_squeeze_duration_ms);
> >> +       if (!buffer_squeeze_end)
> >> +               buffer_squeeze_end = jiffies +
> >> +                       msecs_to_jiffies(buffer_squeeze_duration_ms);
> >>   }
> >>
> >>   static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
> >> @@ -669,10 +670,13 @@ int xen_blkif_schedule(void *arg)
> >>                  }
> >>
> >>                  /* Shrink the free pages pool if it is too large. */
> >> -               if (time_before(jiffies, buffer_squeeze_end))
> >> +               if (time_before(jiffies, buffer_squeeze_end)) {
> >>                          shrink_free_pagepool(ring, 0);
> >> -               else
> >> +               } else {
> >> +                       if (unlikely(buffer_squeeze_end))
> >> +                               buffer_squeeze_end = 0;
> >>                          shrink_free_pagepool(ring, max_buffer_pages);
> >> +               }
> >>
> >>                  if (log_stats && time_after(jiffies, ring->st_print))
> >>                          print_stats(ring);
> >>
> >> May I ask you what way would you prefer?
> >
> > I'm not particularly found of this approach, as I think it's racy. Ie:
> > you would have to add some kind of lock to make sure the contents of
> > buffer_squeeze_end stay unmodified during the read and set cycle, or
> > else xen_blkif_schedule will race with xen_blkbk_reclaim_memory.
> >
> > This is likely not a big deal ATM since the code will work as
> > expected in most cases AFAICT, but I would still prefer to have a
> > per-instance buffer_squeeze_end added to xen_blkif, given that the
> > callback is per-instance. I wouldn't call it a structural change, it's
> > just adding a variable to a struct instead of having a shared one, but
> > the code is almost the same as the current version.
>
> FWIW, I agree.

Agreed, will send v8 soon!


Thanks,
SeongJae Park

>
>
> Juergen

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[Xen-devel] [PATCH v7 3/3] xen/blkback: Remove unnecessary static variable name prefixes

[SeongJae Park]

A few of static variables in blkback have 'xen_blkif_' prefix, though it
is unnecessary for static variables.  This commit removes such prefixes.

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: SeongJae Park <sjpark@amazon.de>
---
 drivers/block/xen-blkback/blkback.c | 37 +++++++++++++----------------
 1 file changed, 17 insertions(+), 20 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 98823d150905..f41c698dd854 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -62,8 +62,8 @@
  * IO workloads.
  */
 
-static int xen_blkif_max_buffer_pages = 1024;
-module_param_named(max_buffer_pages, xen_blkif_max_buffer_pages, int, 0644);
+static int max_buffer_pages = 1024;
+module_param_named(max_buffer_pages, max_buffer_pages, int, 0644);
 MODULE_PARM_DESC(max_buffer_pages,
 "Maximum number of free pages to keep in each block backend buffer");
 
@@ -78,8 +78,8 @@ MODULE_PARM_DESC(max_buffer_pages,
  * algorithm.
  */
 
-static int xen_blkif_max_pgrants = 1056;
-module_param_named(max_persistent_grants, xen_blkif_max_pgrants, int, 0644);
+static int max_pgrants = 1056;
+module_param_named(max_persistent_grants, max_pgrants, int, 0644);
 MODULE_PARM_DESC(max_persistent_grants,
                  "Maximum number of grants to map persistently");
 
@@ -88,8 +88,8 @@ MODULE_PARM_DESC(max_persistent_grants,
  * use. The time is in seconds, 0 means indefinitely long.
  */
 
-static unsigned int xen_blkif_pgrant_timeout = 60;
-module_param_named(persistent_grant_unused_seconds, xen_blkif_pgrant_timeout,
+static unsigned int pgrant_timeout = 60;
+module_param_named(persistent_grant_unused_seconds, pgrant_timeout,
 		   uint, 0644);
 MODULE_PARM_DESC(persistent_grant_unused_seconds,
 		 "Time in seconds an unused persistent grant is allowed to "
@@ -137,9 +137,8 @@ module_param(log_stats, int, 0644);
 
 static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
 {
-	return xen_blkif_pgrant_timeout &&
-	       (jiffies - persistent_gnt->last_used >=
-		HZ * xen_blkif_pgrant_timeout);
+	return pgrant_timeout && (jiffies - persistent_gnt->last_used >=
+			HZ * pgrant_timeout);
 }
 
 /* Once a memory pressure is detected, squeeze free page pools for a while. */
@@ -249,7 +248,7 @@ static int add_persistent_gnt(struct xen_blkif_ring *ring,
 	struct persistent_gnt *this;
 	struct xen_blkif *blkif = ring->blkif;
 
-	if (ring->persistent_gnt_c >= xen_blkif_max_pgrants) {
+	if (ring->persistent_gnt_c >= max_pgrants) {
 		if (!blkif->vbd.overflow_max_grants)
 			blkif->vbd.overflow_max_grants = 1;
 		return -EBUSY;
@@ -412,14 +411,13 @@ static void purge_persistent_gnt(struct xen_blkif_ring *ring)
 		goto out;
 	}
 
-	if (ring->persistent_gnt_c < xen_blkif_max_pgrants ||
-	    (ring->persistent_gnt_c == xen_blkif_max_pgrants &&
+	if (ring->persistent_gnt_c < max_pgrants ||
+	    (ring->persistent_gnt_c == max_pgrants &&
 	    !ring->blkif->vbd.overflow_max_grants)) {
 		num_clean = 0;
 	} else {
-		num_clean = (xen_blkif_max_pgrants / 100) * LRU_PERCENT_CLEAN;
-		num_clean = ring->persistent_gnt_c - xen_blkif_max_pgrants +
-			    num_clean;
+		num_clean = (max_pgrants / 100) * LRU_PERCENT_CLEAN;
+		num_clean = ring->persistent_gnt_c - max_pgrants + num_clean;
 		num_clean = min(ring->persistent_gnt_c, num_clean);
 		pr_debug("Going to purge at least %u persistent grants\n",
 			 num_clean);
@@ -614,8 +612,7 @@ static void print_stats(struct xen_blkif_ring *ring)
 		 current->comm, ring->st_oo_req,
 		 ring->st_rd_req, ring->st_wr_req,
 		 ring->st_f_req, ring->st_ds_req,
-		 ring->persistent_gnt_c,
-		 xen_blkif_max_pgrants);
+		 ring->persistent_gnt_c, max_pgrants);
 	ring->st_print = jiffies + msecs_to_jiffies(10 * 1000);
 	ring->st_rd_req = 0;
 	ring->st_wr_req = 0;
@@ -675,7 +672,7 @@ int xen_blkif_schedule(void *arg)
 		if (time_before(jiffies, buffer_squeeze_end))
 			shrink_free_pagepool(ring, 0);
 		else
-			shrink_free_pagepool(ring, xen_blkif_max_buffer_pages);
+			shrink_free_pagepool(ring, max_buffer_pages);
 
 		if (log_stats && time_after(jiffies, ring->st_print))
 			print_stats(ring);
@@ -902,7 +899,7 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
 			continue;
 		}
 		if (use_persistent_gnts &&
-		    ring->persistent_gnt_c < xen_blkif_max_pgrants) {
+		    ring->persistent_gnt_c < max_pgrants) {
 			/*
 			 * We are using persistent grants, the grant is
 			 * not mapped but we might have room for it.
@@ -929,7 +926,7 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
 			pages[seg_idx]->persistent_gnt = persistent_gnt;
 			pr_debug("grant %u added to the tree of persistent grants, using %u/%u\n",
 				 persistent_gnt->gnt, ring->persistent_gnt_c,
-				 xen_blkif_max_pgrants);
+				 max_pgrants);
 			goto next;
 		}
 		if (use_persistent_gnts && !blkif->vbd.overflow_max_grants) {
-- 
2.17.1


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