Re: [RFC PATCH v3 07/37] mm/damon/core: apply access reports to high level snapshot

[JaeJoon Jung <rgbi3307@gmail.com>]

On Sat, 13 Dec 2025 at 08:11, SeongJae Park <sj@kernel.org> wrote:
>
> On Fri, 12 Dec 2025 22:20:04 +0900 JaeJoon Jung <rgbi3307@gmail.com> wrote:
>
> > On Mon, 8 Dec 2025 at 15:35, SeongJae Park <sj@kernel.org> wrote:
> > >
> > > Now any DAMON API callers can report their observed access information.
> > > The DAMON core layer is just ignoring those, though.  Update the core to
> > > use the reported information at building the high level access pattern
> > > snapshot.
> >
> > It seems inefficient to repeatedly access the damon_access_reports[1000] array
> > using a for loop in the kdamond_check_reported_accesses() function.
> > It is inefficient to for loop through the entire
> > damon_access_reports[1000] array.
> > When CONFIG_HZ and jiffies are increased as follows and
> > damond sample_interval is 5000us (5ms), the time flow diagram is as follows.
> >
> > CONFIG_HZ 1000, jiffies == 1ms
> > damond sample_interval == 5000us (5ms)
> >
> > reports_len(==): [0 ... 5]
> >                                                      [*]
> >         0    1    2    3    4    5     6    7    8    9        997    998     999
> >         [====|====|====|====|====]-----|----|----|----|  ....  |------|-------|
> > jiffies++    1    2    3    4    5     0    0    0    0        0      0       0
> > damond_fn(sample interval)      -5[0<]
> >
> > reports_len(==): [997 ... 2]
> >                                   [*]
> >         0      1      2    3    4    5     6    7    8    9        997   998   999
> >         [======|======]----|----|----|-----|----|----|----|  ....  [=====|=====]
> > jiffies++   1001      1002 3    4    5     6    7    8    9        997   998   999
> > damond_fn(sample interval)
> >                             -5[997<]
>
> Please use fixed-length fonts for something like above, from next time.  I
> fixed the diagram with my best effort, as above.  But I still fail at
> understanding your point.  More clarification about what the diagram means
> would be nice.

Thank you for readjusting the font to fit.  The first diagram above is when
reports_len is processed normally starting from 0 to reports_len.
The second diagram shows the process where reports_len increases to its
maximum values of 997, 998, 999, and then returns to 0.
The biggest problem here is that the latter part of the array is not processed.

>
> >
> > It seems that only the section corresponding to the sample interval ([==|==])
> > can be cycled as follows.  And, how about enjoying damon_access_reports[1000]
> > as damon_access_reports[500]?  Even if it reduce the 1000ms to 500ms
> > array space, it seems that it can sufficiently report and process within
> > the sample interval of 5ms.
>
> Are you assuming the the reports can be made only once per 1 millisecond?  That
> is not true.  The design assumes any kernel API caller could make the report,
> so more than one report can be made within one millisecond.  Am I
> missingsomething?

jiffies 1ms is just to simply unfold the passage of time when
CONFIG_HZ is set to 1000.
This is a simplification to help it understand the flow of time.

>
> >
> >  static unsigned int kdamond_check_reported_accesses(struct damon_ctx *ctx)
> >  {
> > -       int i;
> > +       int i = damon_access_reports_len;
> > +       unsigned int nr = 0;
> >         struct damon_access_report *report;
> >         struct damon_target *t;
> >
> > @@ -2904,16 +2905,18 @@ static unsigned int
> > kdamond_check_reported_accesses(struct damon_ctx *ctx)
> >                 return 0;
> >
> >         mutex_lock(&damon_access_reports_lock);
> > -       for (i = 0; i < damon_access_reports_len; i++) {
> > -               report = &damon_access_reports[i];
> > -               if (time_before(report->report_jiffies,
> > -                                       jiffies -
> > -                                       usecs_to_jiffies(
> > -                                               ctx->attrs.sample_interval)))
> > -                       continue;
> > +       report = &damon_access_reports[i];
> > +       while (time_after(report->report_jiffies,
> > +               jiffies - usecs_to_jiffies(ctx->attrs.sample_interval))) {
> >                 damon_for_each_target(t, ctx)
> >                         kdamond_apply_access_report(report, t, ctx);
> > +               if (++nr >= DAMON_ACCESS_REPORTS_CAP)
> > +                       break;
> > +
> > +               i = (i == 0) ? (DAMON_ACCESS_REPORTS_CAP - 1) : (i - 1);
> > +               report = &damon_access_reports[i];
> >         }
> > +
> >         mutex_unlock(&damon_access_reports_lock);
> >         /* For nr_accesses_bp, absence of access should also be reported. */
> >         return kdamond_apply_zero_access_report(ctx);
> > }
>
> So I still don't get your points before the above code diff, but I understand
> this code diff.
>
> I agree this is more efficient.  I will consider doing something like this in
> the next spin.

What I tried above is to process the current array [1000] as
efficiently as possible.
But, if I think again, It would be better to store it in a linked-list
and process it
in FIFO mode whenever requested in damon_report_page_fault(),
damon_report_access(report)
instead of storing it in an array.  I'm also analyzing the source code
starting this week,
so I'll organize it a bit more and get back to you with my opinion.

Thanks,
JaeJoon

>
>
> Thanks,
> SJ
>
> [...]