Re: [PATCH 1/2] mm/damon/core: score an unmeasured PSI goal as not achieved for the temporal tuner

[SJ Park <sj@kernel.org>]

On Sat, 19 Sep 2026 07:44:11 +0200 Karl Mehltretter <kmehltretter@gmail.com> wrote:

> On Tue, Sep 15, 2026 at 05:13:11PM +0100, SJ Park wrote:
> > So I'd still prefer to just making no effect this round.  And my commit was
> > failed at doing that, because it didn't aware of temporal tuner.  Maybe we
> > could show whether the goal was achieved or not, using esz and let the tuner
> > show same achieveness?  What about something like below?
> > 
> 
> Thanks for explaining the two concerns. I am still new to the DAMON
> code, so I may be missing something, but your suggestion looks good
> to me and it is simpler than what I had. I have prepared a v2 this
> way and will wait for your comments before I send it.

Thank you for accepting my humble suggestion, please feel free to send it when
you are ready!

> 
> One note on the first concern: as far as I can see last_psi_total is
> updated in every tuning round, so commits in between would not make
> the measured time longer than the reset interval. But I am fine with
> your approach either way.

last_psi_total is updated in every tuning round, as you mentioned.  But the
tuning round comes if both the scheme apply interval and the quota reset
interval are passed.  damos_adjust_quota() has the quota reset interval check.
kdamond_apply_schemes() has the apply interval check before calling
damos_adjust_quota().

'''
static void kdamond_apply_schemes(struct damon_ctx *c)
{
[...]
        damon_for_each_scheme(s, c) {
                if (time_before(c->passed_sample_intervals, s->next_apply_sis))
                        continue;
[...]
                damos_adjust_quota(c, s);
'''

'''
static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
{
[...]
        /* New charge window starts */
        if (!time_in_range_open(jiffies, quota->charged_from,
                                quota->charged_from +
                                msecs_to_jiffies(quota->reset_interval))) {
[...]
                damos_set_effective_quota(c, s);
'''

And the apply interval (or, next_apply_sis) is extended for each commit, in
damon_set_attrs().  Hence, the problem can still happen.

> 
> I would also move the last_psi_total update into the new helper, so
> that the helper is the only place that touches the field and the
> unit test can check it.

Sounds good!

> 
> While testing this with damo I found a second problem. A full
> "damo tune" still gave a zero quota for one window, on the unpatched
> kernel too. damon_new_scheme() runs damos_quota_init() on the quota
> of the caller, and since b90408ef1163 damon_commit_ctx() passes the
> quota of each running scheme to it for the test context. So every
> full commit zeroes esz, esz_bp and the charge state of the running
> schemes. Please correct me if I read this wrong. I can send a
> separate patch for that.

I don't really understand the problem.  Could you please further elaborate?

> 
> In a quick QEMU test with your suggestion and that change, the quota
> stays at 1 MiB over "damo tune" and "damo tune --quota_goals_only".

Sounds good.  Looking forward to your next patch!


Thanks,
SJ 

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