blk-iocost: publish hweights as one atomic value

current_hweight() publishes the cache generation and both weights with
separate stores.  Refreshers running on different CPUs can interleave
their stores and leave a new generation paired with weights from an older
calculation.  The compiler can also reorder the generation store before
the weight stores, so a reader that sees the new generation can accept
stale weights.

A stale weight is not only a reporting artifact.  It feeds the cost
calculation and the donation logic, where an inconsistent combination can
lead to a zero divisor in transfer_surpluses().

In addition, the slow path rereads iocg->hweight_active/inuse after
publishing them, so the value it returns can be the one written by another
refresher instead of the locally computed one.

Pack the generation and both weights into a single atomic64_t and publish
them with one atomic write.  Readers take one atomic snapshot and return
the local values.  Weights are in [1, WEIGHT_ONE]; storing each minus one
preserves the full range in 16 bits and leaves 32 bits for the generation.

The two weight fields remain as the donation workspace used under
ioc->lock.  Tracepoints decode the packed cache without refreshing it and
iocost_monitor.py is updated accordingly.

Developed with assistance from Claude Code (Anthropic): the assistant
drafted the packed representation, the helpers and the tracepoint/monitor
changes, and ran the checks listed below.  The author reviewed the result
and remains responsible for it.

Fixes: 7caa47151ab2 ("blkcg: implement blk-iocost")
Signed-off-by: Kunhai Dai <daikunhai@didiglobal.com>
Tested-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Tao Cui <cuitao@kylinos.cn>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://patch.msgid.link/20261019125412.3123895-3-daikunhai@didiglobal.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions