Re: [PATCH] mm/damon/stat: set last_refresh_jiffies to jiffies at startup

[Quanmin Yan <yanquanmin1@huawei.com>]

在 2025/10/28 22:19, SeongJae Park 写道:

> On Tue, 28 Oct 2025 14:19:27 +0800 Quanmin Yan <yanquanmin1@huawei.com> 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.
>>
>> Fix it by setting last_refresh_jiffies to jiffies at startup.
> Nice catch, thank you for this patch!
>
>> [1] https://lkml.kernel.org/r/20250822025057.1740854-1-ekffu200098@gmail.com
>>
>> Fixes: fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")
>> Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
>> ---
>>   mm/damon/stat.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/damon/stat.c b/mm/damon/stat.c
>> index 6c4503d2aee3..6dc3e18de910 100644
>> --- a/mm/damon/stat.c
>> +++ b/mm/damon/stat.c
>> @@ -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;
> -       static unsigned long last_refresh_jiffies;
> +       static unsigned long last_refresh_jiffies = jiffies;
>
Thank you for your suggestion. Well, I actually tried that before, but
found that jiffies is not a compile-time constant,|it| is set at runtime,
while static initializers must use constant expressions, that's why the
current patch is modified this way.


Thanks,
Quanmin Yan

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