[RFC PATCH] mm/damon/core: avoid time-quota permanently disabling scheme

[RFC PATCH] mm/damon/core: avoid time-quota permanently disabling scheme

[SeongJae Park]

When the throughput of a DAMOS scheme is very slow, DAMOS time quota can
make the effective size quota smaller than damon_ctx->min_region_sz.  In
the case, damos_apply_scheme() will skip applying the action, because
the action is tried at region level, which requires >=min_region_sz
size.  That is, the quota is effectively exceeded for the quota charge
window.

Because no action will be applied, the total_charged_sz and
total_charged_ns are also not updated.  damos_set_effective_quota() will
try to update the effective size quota before starting the next charge
window.  However, because the total_charged_sz and total_charged_ns have
not updated, the throughput and effective size quota are also not
changed.  Since effective size quota can only be decreased, other
effective size quota update factors including DAMOS quota goals and size
quota cannot make any change, either.

As a result, the scheme is unexpectedly deactivated until the user
notices and mitigates the situation.  The users can mitigate this
situation by changing the time quota online or re-install the scheme.
While the mitigation is somewhat straightforward, finding the situation
would be challenging, because DAMON is not providing good
observabilities for that.  Even if such observability is provided, doing
the additional monitoring and the mitigation is somewhat cumbersome and
not aligned to the intention of the time quota.  The time quota was
intended to help reduce the user's administration overhead.

Fix the problem by setting time quota-modified effective size quota be
at least min_region_sz always.

The issue was discovered [1] by sashiko.

[1] https://lore.kernel.org/20260405192504.110014-1-sj@kernel.org

Fixes: 1cd243030059 ("mm/damon/schemes: implement time quota")
Cc: <stable@vger.kernel.org> # 5.16.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/core.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 3bc7a2bbfe7de..12544c60531d3 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2384,7 +2384,8 @@ static void damos_goal_tune_esz_bp_temporal(struct damos_quota *quota)
 /*
  * Called only if quota->ms, or quota->sz are set, or quota->goals is not empty
  */
-static void damos_set_effective_quota(struct damos_quota *quota)
+static void damos_set_effective_quota(struct damos_quota *quota,
+		struct damon_ctx *ctx)
 {
 	unsigned long throughput;
 	unsigned long esz = ULONG_MAX;
@@ -2409,6 +2410,7 @@ static void damos_set_effective_quota(struct damos_quota *quota)
 		else
 			throughput = PAGE_SIZE * 1024;
 		esz = min(throughput * quota->ms, esz);
+		esz = max(ctx->min_region_sz, esz);
 	}
 
 	if (quota->sz && quota->sz < esz)
@@ -2445,7 +2447,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	/* First charge window */
 	if (!quota->total_charged_sz && !quota->charged_from) {
 		quota->charged_from = jiffies;
-		damos_set_effective_quota(quota);
+		damos_set_effective_quota(quota, c);
 	}
 
 	/* New charge window starts */
@@ -2460,7 +2462,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 		quota->charged_sz = 0;
 		if (trace_damos_esz_enabled())
 			cached_esz = quota->esz;
-		damos_set_effective_quota(quota);
+		damos_set_effective_quota(quota, c);
 		if (trace_damos_esz_enabled() && quota->esz != cached_esz)
 			damos_trace_esz(c, s, quota);
 	}

base-commit: 8c08ea3625d4fa8c1c74b208c0c630fce76e14a9
-- 
2.47.3

Re: (sashiko review) [RFC PATCH] mm/damon/core: avoid time-quota permanently disabling scheme

[SeongJae Park]

TL; DR: sashiko found no issue in this patch.  I will repost this patch after
dropping RFC, soon.

Forwarding sashiko review in a reply format with my inline
comments below, for details of my view and doing discussions
via mails if needed.

> # review url: https://sashiko.dev/#/patchset/20260405225440.76827-1-sj@kernel.org
> # start of sashiko.dev inline review
> commit 93dc55170095d9a174cb626df2cde4fe5e49b9a6
> Author: SeongJae Park <sj@kernel.org>
> 
> mm/damon/core: avoid time-quota permanently disabling scheme
> 
> When a DAMOS scheme throughput is very slow, the time quota can make the
> effective size quota smaller than the minimum region size. This causes
> the action to be skipped, freezing the effective size quota updates.
> This patch sets the time quota-modified effective size quota to be at
> least min_region_sz.
> 
> Link: https://lore.kernel.org/20260405192504.110014-1-sj@kernel.org
> 
> > diff --git a/mm/damon/core.c b/mm/damon/core.c
> > index 3bc7a2bbfe7de..12544c60531d3 100644
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
> 
> [ ... ]
> 
> > @@ -2409,6 +2410,7 @@ static void damos_set_effective_quota(struct damos_quota *quota)
> >  		else
> >  			throughput = PAGE_SIZE * 1024;
> >  		esz = min(throughput * quota->ms, esz);
> > +		esz = max(ctx->min_region_sz, esz);
> 
> Does this minimum region size bound incorrectly override DAMOS quota goals?
> 
> In damos_set_effective_quota(), esz is first initialized based on
> quota->goals. The goal tuner can legitimately reduce esz below
> ctx->min_region_sz (for example, to 0) to completely pause the scheme if
> the system is over-achieving its target limits.
> 
> Because the max() operates on the combined esz instead of just the time
> quota component, it artificially inflates the quota back up to
> ctx->min_region_sz. This forces the scheme to process at least one region
> per charge window, overriding the goal tuner's request to pause the scheme.
> 
> If quota->ms is 0, this block is bypassed and the goal is properly
> respected, leading to inconsistent behavior depending on whether a time
> quota is configured.

That's correct, and it is an intended behavior.

It's true that it is bit inconsistent.  But I don't think that will cause any
real issue, since the difference is quite minimal (min_region_sz quota).

> 
> Could the lower bound be applied exclusively to the calculated time quota
> before it is combined with the goal-based quota?
> 
> For example:
> 
> unsigned long time_esz = max(throughput * quota->ms, ctx->min_region_sz);
> esz = min(time_esz, esz);

Maybe we can later improve the behavior by doing the time quota based esz setup
before goals based setup.  But that's not good for stable kernels hotfix in my
opinion, so it is out of the scope of this patch.

> 
> 
> # end of sashiko.dev inline review
> # review url: https://sashiko.dev/#/patchset/20260405225440.76827-1-sj@kernel.org


Thanks,
SJ

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