STRUCT BLK_STAT_CALLBACK(9)

💖 struct blk_stat_callback(9) 💖

okay so this is the Kernel API Manual, and today’s main character is struct blk_stat_callback — block stats callback representation, no cap ✨

✨ NAME ✨

struct blk_stat_callback — Block statistics callback. That’s the whole name section bestie, short and iconic 💅

✨ SYNOPSIS ✨

       struct blk_stat_callback {
           struct list_head list;
           struct timer_list timer;
           struct blk_rq_stat __percpu *cpu_stat;
           int (*bucket_fn)(const struct request *);
           unsigned int buckets;
           struct blk_rq_stat *stat;
           void (*timer_fn)(struct blk_stat_callback *);
           void *data;
           struct rcu_head rcu;
        };

That’s the struct, laid out exactly like that — copy it, respect it, don’t freestyle it 🫶

🫶 Members 🫶

• list — the RCU list of callbacks for a struct request_queue.
• timer — the timer for the next callback invocation ⏰
• cpu_stat — the per-cpu statistics buckets 🍓
• bucket_fn — given a request, returns which statistics bucket it should be accounted under; return -1 for no bucket for this request.
• buckets — the number of statistics buckets.
• stat — the array of statistics buckets.
• timer_fn — the callback function.
• data — a private pointer for the user.
• rcu — the rcu list head.

🌸 Description 🌸

Here’s the tea: a struct blk_stat_callback is associated with a struct request_queue 👑

While timer is active, that queue’s request completion latencies are sorted into buckets by bucket_fn and added to a per-cpu buffer, cpu_stat. When the timer fires, cpu_stat is flushed to stat and timer_fn is invoked. Literally the whole flow, and it ate 🔥

🔥 SEE ALSO 🔥

Kernel file ./block/blk-stat.h — also go read blk_stat_alloc_callback(9), blk_stat_add_callback(9), blk_stat_remove_callback(9), blk_stat_free_callback(9), blk_stat_is_active(9), blk_stat_activate_nsecs(9), and blk_stat_activate_msecs(9) 💖

source/version: ./block · date: October 2026 · page: struct blk_stat_callback(9) — signing off with maximum main character energy, mwah 💋✨