Re: [patch 19/38] kcsan: Replace get_cycles() usage

[Marco Elver <elver@google.com>]

On Fri, 10 Apr 2026 at 14:20, Thomas Gleixner <tglx@kernel.org> wrote:
>
> KCSAN uses get_cycles() for two purposes:
>
>   1) Seeding the random state with get_cycles() is a historical leftover.
>
>   2) The microbenchmark uses get_cycles(), which provides an unit less
>      counter value and is not guaranteed to be functional on all
>      systems/platforms.
>
> Use random_get_entropy() for seeding the random state and ktime_get() which
> is universaly functional and provides at least a comprehensible unit.
>
> This is part of a larger effort to remove get_cycles() usage from
> non-architecture code.
>
> Signed-off-by: Thomas Gleixner <tglx@kernel.org>
> Cc: Marco Elver <elver@google.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: kasan-dev@googlegroups.com

Reviewed-by: Marco Elver <elver@google.com>

> ---
>  kernel/kcsan/core.c    |    2 +-
>  kernel/kcsan/debugfs.c |    8 ++++----
>  2 files changed, 5 insertions(+), 5 deletions(-)
>
> --- a/kernel/kcsan/core.c
> +++ b/kernel/kcsan/core.c
> @@ -798,7 +798,7 @@ void __init kcsan_init(void)
>         BUG_ON(!in_task());
>
>         for_each_possible_cpu(cpu)
> -               per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();
> +               per_cpu(kcsan_rand_state, cpu) = (u32)random_get_entropy();
>
>         /*
>          * We are in the init task, and no other tasks should be running;
> --- a/kernel/kcsan/debugfs.c
> +++ b/kernel/kcsan/debugfs.c
> @@ -58,7 +58,7 @@ static noinline void microbenchmark(unsi
>  {
>         const struct kcsan_ctx ctx_save = current->kcsan_ctx;
>         const bool was_enabled = READ_ONCE(kcsan_enabled);
> -       u64 cycles;
> +       ktime_t nsecs;
>
>         /* We may have been called from an atomic region; reset context. */
>         memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
> @@ -70,16 +70,16 @@ static noinline void microbenchmark(unsi
>
>         pr_info("%s begin | iters: %lu\n", __func__, iters);
>
> -       cycles = get_cycles();
> +       nsecs = ktime_get();
>         while (iters--) {
>                 unsigned long addr = iters & ((PAGE_SIZE << 8) - 1);
>                 int type = !(iters & 0x7f) ? KCSAN_ACCESS_ATOMIC :
>                                 (!(iters & 0xf) ? KCSAN_ACCESS_WRITE : 0);
>                 __kcsan_check_access((void *)addr, sizeof(long), type);
>         }
> -       cycles = get_cycles() - cycles;
> +       nsecs = ktime_get() - nsecs;
>
> -       pr_info("%s end   | cycles: %llu\n", __func__, cycles);
> +       pr_info("%s end   | nsecs: %llu\n", __func__, nsecs);
>
>         WRITE_ONCE(kcsan_enabled, was_enabled);
>         /* restore context */
>