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

SeongJae Park
2025-12-12T23:11:52Z

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.

>
> 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?

>
> 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.

Thanks,
SJ …