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

[Quanmin Yan]

在 2025/10/28 22:19, SeongJae Park 写道:

> On Tue, 28 Oct 2025 14:19:27 +0800 Quanmin Yan <yanquanmin1@xxxxxxxxxx> 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@xxxxxxxxx
> >
> > Fixes: fabdd1e911da ("mm/damon/stat: calculate and expose estimated memory bandwidth")
> > Signed-off-by: Quanmin Yan <yanquanmin1@xxxxxxxxxx>
> > ---
> >   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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