Re: [PATCH] mm/damon/core: trace esz at first setup

[SeongJae Park <sj@kernel.org>]

On Thu, 21 May 2026 16:28:34 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 20 May 2026 08:03:10 -0700 SeongJae Park <sj@kernel.org> wrote:
> 
> > DAMON traces effective size quota from the second update, only if a
> > change has been made by the update.  Tracing only changed updates was an
> > intentional decision to avoid unnecessary same value tracing.  Always
> > skipping the first value is just an unintended mistake.
> > 
> > The mistake makes the tracepoint based investigation incomplete, because
> > the first effective size quota is never traced.  It is not a big issue
> > when the 'consist' quota tuner is used, because it keeps changing the
> > quota in the usual setup.
> > 
> > However, when the 'temporal' tuner is used, the quota value is not
> > changed before the goal achievement status is completely changed.  For
> > example, if the DAMOS scheme is started with an under-achieved goal, the
> > quota is set to the maximum value, and kept the same value until the
> > goal is achieved.  Because DAMON skips the first value, the user cannot
> > know what effective quota the current scheme is using.  Only after the
> > goal is achieved, the effective quota is changed to zero, and traced.
> > 
> > Unconditionally trace the initial quota value to fix this problem.
> > 
> > Note that the 'temporal' quota tuner was introduced by commit
> > af738a6a00c1 ("mm/damon/core: introduce
> > DAMOS_QUOTA_GOAL_TUNER_TEMPORAL"), which was added to 7.1-rc1.  But even
> > with the 'consist' quota tuner, the tracing is unintentionally
> > incomplete. Hence this commit marks the introduction of the trace event
> > as the broken commit.
> 
> OK, but...
> 
> > Fixes: a86d695193bf ("mm/damon: add trace event for effective size quota")
> > Cc: <stable@vger.kernel.org> # 6.17.x
> > Signed-off-by: SeongJae Park <sj@kernel.org>
> 
> The patch is marked for backporting but it assumes the presence of
> "mm/damon/core: make charge_addr_from aware of end-address
> exclusivity", which is queued for 7.2-rc1.
> 
> We can either redo this against current -linus and fix up mm.git's
> "mm/damon/core: make charge_addr_from aware of end-address exclusivity"
> or we can queue this for 7.2-rc1 and you get to deal with fallout when
> -stable maintainers hit issues backporting this.
> 
> Preferences?

Either is ok.  Because this fix is not urgent to my perspective, I'd pick the
second option.  Let me know if you need anything from my side.


Thanks,
SJ

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