STRUCT CLOCK_READ_DATA(9)

💖 struct clock_read_data(9) 💖

okay so bestie, welcome to the Kernel API Manual ✨ we're talking about struct clock_read_data today and honestly it's lowkey iconic 🎀

🌸 NAME 🌸

struct clock_read_data — the data you literally need to read from sched_clock() 🫶

✨ SYNOPSIS ✨

struct clock_read_data {
    u64 epoch_ns;
    u64 epoch_cyc;
    u64 sched_clock_mask;
    u64 (*read_sched_clock)(void);
    u32 mult;
    u32 shift;
 };

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🦋 Members 🦋

• epoch_ns — the sched_clock value at the last update, periodt ✨
• epoch_cyc — clock cycle value at the last update 💅
• sched_clock_mask — the bitmask for two's complement subtraction of non 64bit clocks, no cap 🧚
• read_sched_clock — the current clock source (or the dummy source when suspended — yes, it has a standby era 😭)
• mult — multiplier for the scaled math conversion, it ate 🔥
• shift — shift value for the scaled math conversion 🌈

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👑 Description 👑

okay so listen 👀 care must be taken when updating this structure — it's read by some very hot code paths 🔥🔥🔥 she's main character energy but she's on the fast path so be nice to her.

it occupies <=40 bytes and, when combined with the seqcount used to synchronize access, comfortably fits into a 64 byte cache line 🌸 like yes she's compact, she's efficient, she's that girl 💅

ngl this is genuinely a caution, not a vibe: get the update wrong and the hot paths feel it 😭

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🎀 SEE ALSO 🎀

• Kernel file ./include/linux/sched_clock.h 🫶

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and that's the whole page bestie, ty for coming to my ted talk 💖 this was brought to you straight out of ./include/linux, dated October 2026, and yes it's still struct clock_read_data(9) — slay 💅✨🎀🦋