mm/damon/stat: set last_refresh_jiffies to jiffies at startup

In DAMON_STAT's damon_stat_damon_call_fn(), time_before_eq() is used to avoid unnecessarily frequent stat update.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make jiffies wrap bugs appear earlier. However, this causes time_before_eq() in DAMON_STAT to unexpectedly return true during the first 5 minutes after boot on 32-bit systems (see [1] for more explanation, which fixes another jiffies-related issue in DAMON). As a result, DAMON_STAT does not update any monitoring results during that period, which can be more confusing when DAMON_STAT_ENABLED_DEFAULT is enabled.

Fix it by setting last_refresh_jiffies to jiffies at startup.

[1] https://lkml.kernel.org/r/[email protected]

Fixes fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")

Signed-off-by Quanmin Yan <[email protected]>

---
 mm/damon/stat.c | 3 +++
 1 file changed, 3 insertions(+)

mm/damon/stat.c +0 -0

index 6c4503d2aee3..6dc3e18de910 100644

Quoted 11 lines · On Tue, 28 Oct 2025 14:19:27 +0800 Quanmin Yan <[email protected]> wrote:

In DAMON_STAT's damon_stat_damon_call_fn(), time_before_eq() is used to avoid unnecessarily frequent stat update.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make jiffies wrap bugs appear earlier. However, this causes time_before_eq() in DAMON_STAT to unexpectedly return true during the first 5 minutes after boot on 32-bit systems (see [1] for more explanation, which fixes another jiffies-related issue in DAMON). As a result, DAMON_STAT does not update any monitoring results during that period, which can be more confusing when DAMON_STAT_ENABLED_DEFAULT is enabled.

Quoted 1 line

Fix it by setting last_refresh_jiffies to jiffies at startup.

Nice catch, thank you for this patch!

Quoted 14 lines

[1] https://lkml.kernel.org/r/[email protected]

Fixes fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")

Signed-off-by Quanmin Yan <[email protected]>

---
 mm/damon/stat.c | 3 +++
 1 file changed, 3 insertions(+)

mm/damon/stat.c +0 -0

index 6c4503d2aee3..6dc3e18de910 100644

mm/damon/stat.c static int damon_stat_damon_call_fn(void *data) · line 137

Quoted 6 lines

mm/damon/stat.c +3 -0

@@ -132,6 +132,9 @@ static int damon_stat_damon_call_fn(void *data)
 	struct damon_ctx *c = data;
 	static unsigned long last_refresh_jiffies;
 
+	if (unlikely(!last_refresh_jiffies))
+		last_refresh_jiffies = jiffies;
+

How about doing the initialization together with the declaration? E.g.,

static int damon_stat_damon_call_fn(void *data)

 {
        struct damon_ctx *c = data;

Quoted 40 lines · On Tue, 28 Oct 2025 07:19:14 -0700 SeongJae Park <[email protected]> wrote:

Quoted 11 lines · On Tue, 28 Oct 2025 14:19:27 +0800 Quanmin Yan <[email protected]> wrote:

In DAMON_STAT's damon_stat_damon_call_fn(), time_before_eq() is used to avoid unnecessarily frequent stat update.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make jiffies wrap bugs appear earlier. However, this causes time_before_eq() in DAMON_STAT to unexpectedly return true during the first 5 minutes after boot on 32-bit systems (see [1] for more explanation, which fixes another jiffies-related issue in DAMON). As a result, DAMON_STAT does not update any monitoring results during that period, which can be more confusing when DAMON_STAT_ENABLED_DEFAULT is enabled.

Quoted 1 line

Fix it by setting last_refresh_jiffies to jiffies at startup.

Nice catch, thank you for this patch!

Quoted 14 lines

[1] https://lkml.kernel.org/r/[email protected]

Fixes fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")

Signed-off-by Quanmin Yan <[email protected]>

---
 mm/damon/stat.c | 3 +++
 1 file changed, 3 insertions(+)

mm/damon/stat.c +0 -0

index 6c4503d2aee3..6dc3e18de910 100644

mm/damon/stat.c static int damon_stat_damon_call_fn(void *data) · line 137

Quoted 6 lines

mm/damon/stat.c +3 -0

@@ -132,6 +132,9 @@ static int damon_stat_damon_call_fn(void *data)
 	struct damon_ctx *c = data;
 	static unsigned long last_refresh_jiffies;
 
+	if (unlikely(!last_refresh_jiffies))
+		last_refresh_jiffies = jiffies;
+

How about doing the initialization together with the declaration? E.g.,

Review

Quoted 5 lines

static int damon_stat_damon_call_fn(void *data)

 {
        struct damon_ctx *c = data;

(quoted diff) +1 -1

-       static unsigned long last_refresh_jiffies;
+       static unsigned long last_refresh_jiffies = jiffies;

Actually, a similar issue can happen again if DAMON_STAT is stopped and restarted by user. That is, if user stops DAMON_STAT just after last_refresh_jiffies is updated, and restart it after 5 seconds or more, the time_before_eq() on damon_call_fn() will return true, so stat updates will happen earlier than expected. Shouldn't be a real problem, but better to avoid if possible.

How about making last_refresh_jiffies a global variable and initialize it on damon_stat_start()? To avoid unnecessary name conflicts, the variable name would also better to be changed, e.g., damon_stat_last_refresh_jiffies.

Quoted 39 lines · On Tue, 28 Oct 2025 07:32:49 -0700 SeongJae Park <[email protected]> wrote:

Quoted 37 lines · On Tue, 28 Oct 2025 07:19:14 -0700 SeongJae Park <[email protected]> wrote:

Quoted 11 lines · On Tue, 28 Oct 2025 14:19:27 +0800 Quanmin Yan <[email protected]> wrote:

In DAMON_STAT's damon_stat_damon_call_fn(), time_before_eq() is used to avoid unnecessarily frequent stat update.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make jiffies wrap bugs appear earlier. However, this causes time_before_eq() in DAMON_STAT to unexpectedly return true during the first 5 minutes after boot on 32-bit systems (see [1] for more explanation, which fixes another jiffies-related issue in DAMON). As a result, DAMON_STAT does not update any monitoring results during that period, which can be more confusing when DAMON_STAT_ENABLED_DEFAULT is enabled.

Quoted 1 line

Fix it by setting last_refresh_jiffies to jiffies at startup.

Nice catch, thank you for this patch!

Quoted 13 lines

[1] https://lkml.kernel.org/r/[email protected]

Fixes fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")

Signed-off-by Quanmin Yan <[email protected]>

---
  mm/damon/stat.c | 3 +++
  1 file changed, 3 insertions(+)

mm/damon/stat.c +0 -0

index 6c4503d2aee3..6dc3e18de910 100644

mm/damon/stat.c static int damon_stat_damon_call_fn(void *data) · line 137

Quoted 6 lines

mm/damon/stat.c +3 -0

@@ -132,6 +132,9 @@ static int damon_stat_damon_call_fn(void *data)
  	struct damon_ctx *c = data;
  	static unsigned long last_refresh_jiffies;
 
+	if (unlikely(!last_refresh_jiffies))
+		last_refresh_jiffies = jiffies;
+

How about doing the initialization together with the declaration? E.g.,

Quoted 5 lines

Review

Quoted 5 lines

  static int damon_stat_damon_call_fn(void *data)
  {
         struct damon_ctx *c = data;

(quoted diff) +1 -1

-       static unsigned long last_refresh_jiffies;
+       static unsigned long last_refresh_jiffies = jiffies;

Please ignore the above suggestion. It will even not build, like below...

.../mm/damon/stat.c: In function ‘damon_stat_damon_call_fn’:
.../mm/damon/stat.c:133:53: error: initializer element is not constant
133 | static unsigned long last_refresh_jiffies = jiffies;
| ^~~~~~~

Quoted 7 lines

Actually, a similar issue can happen again if DAMON_STAT is stopped and restarted by user. That is, if user stops DAMON_STAT just after last_refresh_jiffies is updated, and restart it after 5 seconds or more, the time_before_eq() on damon_call_fn() will return true, so stat updates will happen earlier than expected. Shouldn't be a real problem, but better to avoid if possible.

Quoted 3 lines

How about making last_refresh_jiffies a global variable and initialize it on damon_stat_start()? To avoid unnecessary name conflicts, the variable name would also better to be changed, e.g., damon_stat_last_refresh_jiffies.

But, please consider the above one.

And I just realized a similar issue exist for next_update_jiffies in mm/damon/sysfs.c file. Please feel free to send a patch for that if you willing to.

Quoted 11 lines · On Tue, 28 Oct 2025 14:19:27 +0800 Quanmin Yan <[email protected]> wrote:

In DAMON_STAT's damon_stat_damon_call_fn(), time_before_eq() is used to avoid unnecessarily frequent stat update.

On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make jiffies wrap bugs appear earlier. However, this causes time_before_eq() in DAMON_STAT to unexpectedly return true during the first 5 minutes after boot on 32-bit systems (see [1] for more explanation, which fixes another jiffies-related issue in DAMON). As a result, DAMON_STAT does not update any monitoring results during that period, which can be more confusing when DAMON_STAT_ENABLED_DEFAULT is enabled.

Quoted 1 line

Fix it by setting last_refresh_jiffies to jiffies at startup.

Nice catch, thank you for this patch!

Quoted 13 lines

[1] https://lkml.kernel.org/r/[email protected]

Fixes fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")

Signed-off-by Quanmin Yan <[email protected]>

---
  mm/damon/stat.c | 3 +++
  1 file changed, 3 insertions(+)

mm/damon/stat.c +0 -0

index 6c4503d2aee3..6dc3e18de910 100644

mm/damon/stat.c static int damon_stat_damon_call_fn(void *data) · line 137

Quoted 6 lines

mm/damon/stat.c +3 -0

@@ -132,6 +132,9 @@ static int damon_stat_damon_call_fn(void *data)
  	struct damon_ctx *c = data;
  	static unsigned long last_refresh_jiffies;
 
+	if (unlikely(!last_refresh_jiffies))
+		last_refresh_jiffies = jiffies;
+

How about doing the initialization together with the declaration? E.g.,

  static int damon_stat_damon_call_fn(void *data)
  {
         struct damon_ctx *c = data;