iocost: addio.cost.boost cgroup interface

[ecjtusbs]

Introduce a new per-cgroup file "io.cost.boost" that allows temporarily boosting an io.cost cgroup. Writing a duration in microseconds to the file enables boost mode for that cgroup. When a cgroup is boosted, iocost skips surplus evaluation for it and restores its inuse weight to active, ensuring the boosted cgroup keeps its full IO share during the boost period.

Summary by Sourcery

Add temporary per-cgroup IO cost boosting to preserve full IO shares for latency-sensitive workloads.

New Features:

• Add a per-cgroup io.cost.boost interface for enabling temporary IO share boosting with a duration in microseconds.
• Expose the remaining boost duration and support disabling boosts by writing zero, with a maximum duration limit of one minute.

Enhancements:

• Preserve a boosted cgroup's active IO weight by skipping surplus evaluation during the boost period.

[sourcery-ai]

Reviewer's Guide

Introduces the non-root io.cost.boost cgroup interface, backed by a bounded expiration deadline, and updates timer processing to temporarily preserve a boosted cgroup's full active IO share by bypassing surplus evaluation and restoring its in-use weight.

Sequence diagram for the io.cost.boost cgroup interface

sequenceDiagram
    actor Admin
    participant Cgroup as io.cost.boost
    participant Iocost as ioc_cgrp
    participant Timer as ioc_timer_fn

    Admin->>Cgroup: write duration_us
    Cgroup->>Iocost: kstrtou64(strim(buf), 10)
    alt duration_us <= MAX_BOOST_US
        Iocost->>Iocost: WRITE_ONCE(boost_deadline)
        Admin->>Cgroup: read remaining duration
        Cgroup->>Iocost: ktime_get_ns()
        Iocost-->>Cgroup: remaining_us
    else duration_us > MAX_BOOST_US
        Cgroup-->>Admin: -EINVAL
    end

    Timer->>Iocost: READ_ONCE(boost_deadline)
    alt boost_deadline > now.now_ns
        Iocost->>Iocost: __propagate_weights(active, active, true, &now)
        Iocost-->>Timer: skip surplus evaluation
    else boost expired
        Timer->>Timer: evaluate surplus
    end

Loading

File-Level Changes

Change	Details	Files
Add a per-cgroup boost deadline and expose it through a new io.cost control file.	Store the boost expiration timestamp in cgroup policy data. Add a non-root cost.boost interface that reports remaining microseconds and accepts a duration in microseconds. Cap requested boosts at 60 seconds and support zero to cancel an active boost.	block/blk-iocost.c
Modify iocost timer accounting to preserve full active weight while a cgroup is boosted.	Skip surplus evaluation for cgroups whose boost deadline has not expired. Restore in-use weight to active weight when applicable and avoid propagation for cgroups with absolute virtual debt.	block/blk-iocost.c

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[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="block/blk-iocost.c" line_range="2319-2331" />
<code_context>
 		usage_us = iocg->usage_delta_us;
 		usage_us_sum += usage_us;

+		/* skip surplus evaluation if this cgroup is boosted */
+		{
+			struct blkcg_gq *blkg = iocg_to_blkg(iocg);
+			struct ioc_cgrp *iocc = blkcg_to_iocc(blkg->blkcg);
+
+			if (READ_ONCE(iocc->boost_deadline) > now.now_ns) {
+				if (iocg->inuse != iocg->active &&
+				    !iocg->abs_vdebt) {
+					__propagate_weights(iocg, iocg->active,
+							    iocg->active, true,
+							    &now);
+				}
+				continue;
+			}
+		}
</code_context>
<issue_to_address>
**issue (broader_impact):** Boosting a non-leaf cgroup does not preserve the cgroup's full hierarchical IO share: the timer skips surplus evaluation only for the boosted cgroup's own `iocg`, while its active child `iocg`s continue donating weight. When the parent is restored, `__propagate_weights()` derives an internal node's in-use weight from its children's reduced weights, so the boosted parent remains throttled.

**Triggers:** When the boosted cgroup has active child cgroups that have donated surplus weight.

**Suggested fix:** Propagate the boost to the active descendants, or restore the entire boosted subtree's in-use weights rather than only evaluating the cgroup whose `boost_deadline` is set.
</issue_to_address>

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[sourcery-ai]

issue (broader_impact): Boosting a non-leaf cgroup does not preserve the cgroup's full hierarchical IO share: the timer skips surplus evaluation only for the boosted cgroup's own iocg, while its active child iocgs continue donating weight. When the parent is restored, __propagate_weights() derives an internal node's in-use weight from its children's reduced weights, so the boosted parent remains throttled.

Triggers: When the boosted cgroup has active child cgroups that have donated surplus weight.

Suggested fix: Propagate the boost to the active descendants, or restore the entire boosted subtree's in-use weights rather than only evaluating the cgroup whose boost_deadline is set.