Re: [PATCH] mm/damon/core: set initial quota->charged_from to INITIAL_JIFFIES

[SeongJae Park <sj@kernel.org>]

On Tue, 19 Aug 2025 10:52:02 +0900 Sang-Heon Jeon <ekffu200098@gmail.com> wrote:

> Hello, SeongJae
> 
> On Tue, Aug 19, 2025 at 8:01 AM SeongJae Park <sj@kernel.org> wrote:
> >
> > Hello Sang-Heon,
> >
> > On Tue, 19 Aug 2025 03:38:03 +0900 Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> >
> > > Kernel initialize "jiffies" timer as 5 minutes below zero, as shown in
> > > include/linux/jiffies.h
> > >
> > >  /*
> > >   * Have the 32 bit jiffies value wrap 5 minutes after boot
> > >   * so jiffies wrap bugs show up earlier.
> > >   */
> > >   #define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))
> > >
> > > In 32bit system, if quota->charged_from is initialized to 0 as it now,
> > > it will not adjust event if reset_interval_ms passes for the first 5
> > > minutes.
> >
> > jiffies is unsigned.  Hence the initial invocation of time_after_eq() in
> > damos_adjust_quota(), which is the only place reading quota->charged_from, will
> > return 'true' and the quota adjustment would be done without delay, unless the
> > scheme apply interval is unrealistically big.
> 
> Yeah, jiffies is unsigned but to cover wraparound in 32bit,
> `time_after_eq` casts intermediate value to signed value.
> 
> #define time_after_eq(a,b)    \
>     (typecheck(unsigned long, a) && \
>      typecheck(unsigned long, b) && \
>      ((long)((a) - (b)) >= 0))

Oops, I missed this part.  Thank you for clarifying.  I still have a question
about the fix, though.  I will reply to the original mail.


Thanks,
SJ

[...]