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 before). As a result, DAMON_STAT does not update any monitoring results during that period, which becomes more confusing when DAMON_STAT_ENABLED_DEFAULT is enabled. There is also an issue unrelated to the system’s word size[2]: if the user stops DAMON_STAT just after last_refresh_jiffies is updated and restarts it after 5 seconds or a longer delay, last_refresh_jiffies will retain an older value, causing time_before_eq() to return false and the update to happen earlier than expected. Fix these issues by making last_refresh_jiffies a global variable and initializing it each time DAMON_STAT is started. [1] https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@gmail.com [2] https://lore.kernel.org/all/20251028143250.50144-1-sj@kernel.org/ Fixes: fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth") Suggested-by: SeongJae Park Signed-off-by: Quanmin Yan --- mm/damon/stat.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/mm/damon/stat.c b/mm/damon/stat.c index 6c4503d2aee3..ed8e3629d31a 100644 --- a/mm/damon/stat.c +++ b/mm/damon/stat.c @@ -46,6 +46,8 @@ MODULE_PARM_DESC(aggr_interval_us, static struct damon_ctx *damon_stat_context; +static unsigned long damon_stat_last_refresh_jiffies; + static void damon_stat_set_estimated_memory_bandwidth(struct damon_ctx *c) { struct damon_target *t; @@ -130,13 +132,12 @@ static void damon_stat_set_idletime_percentiles(struct damon_ctx *c) static int damon_stat_damon_call_fn(void *data) { struct damon_ctx *c = data; - static unsigned long last_refresh_jiffies; /* avoid unnecessarily frequent stat update */ - if (time_before_eq(jiffies, last_refresh_jiffies + + if (time_before_eq(jiffies, damon_stat_last_refresh_jiffies + msecs_to_jiffies(5 * MSEC_PER_SEC))) return 0; - last_refresh_jiffies = jiffies; + damon_stat_last_refresh_jiffies = jiffies; aggr_interval_us = c->attrs.aggr_interval; damon_stat_set_estimated_memory_bandwidth(c); @@ -211,6 +212,8 @@ static int damon_stat_start(void) err = damon_start(&damon_stat_context, 1, true); if (err) return err; + + damon_stat_last_refresh_jiffies = jiffies; call_control.data = damon_stat_context; return damon_call(damon_stat_context, &call_control); } -- 2.43.0 In DAMON’s damon_sysfs_repeat_call_fn(), time_before() is used to compare the current jiffies with next_update_jiffies to determine whether to update the sysfs files at this moment. On 32-bit systems, the kernel initializes jiffies to "-5 minutes" to make jiffies wrap bugs appear earlier. However, this causes time_before() in damon_sysfs_repeat_call_fn() 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 before). As a result, DAMON does not update sysfs files during that period. There is also an issue unrelated to the system’s word size[2]: if the user stops DAMON just after next_update_jiffies is updated and restarts it after 'refresh_ms' or a longer delay, next_update_jiffies will retain an older value, causing time_before() to return false and the update to happen earlier than expected. Fix these issues by making next_update_jiffies a global variable and initializing it each time DAMON is started. [1] https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@gmail.com [2] https://lore.kernel.org/all/20251029013038.66625-1-sj@kernel.org/ Fixes: d809a7c64ba8 ("mm/damon/sysfs: implement refresh_ms file internal work") Suggested-by: SeongJae Park Signed-off-by: Quanmin Yan --- mm/damon/sysfs.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c index 43ee9ce4dd84..e2bd2d7becdd 100644 --- a/mm/damon/sysfs.c +++ b/mm/damon/sysfs.c @@ -1601,16 +1601,17 @@ static struct damon_ctx *damon_sysfs_build_ctx( return ctx; } +static unsigned long damon_sysfs_next_update_jiffies; + static int damon_sysfs_repeat_call_fn(void *data) { struct damon_sysfs_kdamond *sysfs_kdamond = data; - static unsigned long next_update_jiffies; if (!sysfs_kdamond->refresh_ms) return 0; - if (time_before(jiffies, next_update_jiffies)) + if (time_before(jiffies, damon_sysfs_next_update_jiffies)) return 0; - next_update_jiffies = jiffies + + damon_sysfs_next_update_jiffies = jiffies + msecs_to_jiffies(sysfs_kdamond->refresh_ms); if (!mutex_trylock(&damon_sysfs_lock)) @@ -1656,6 +1657,9 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond) } kdamond->damon_ctx = ctx; + damon_sysfs_next_update_jiffies = + jiffies + msecs_to_jiffies(kdamond->refresh_ms); + repeat_call_control->fn = damon_sysfs_repeat_call_fn; repeat_call_control->data = kdamond; repeat_call_control->repeat = true; -- 2.43.0