Re: [PATCH v1 4/7] tools: Implement userspace per-cpu

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Jul 28, 2025 at 07:26:37PM -0700, Yuzhuo Jing wrote:
> Implement userspace per-cpu for imported kernel code.  Compared with
> simple thread-local definition, the kernel per-cpu provides 1) a
> guarantee of static lifetime even when thread exits, and 2) the ability
> to access other CPU's per-cpu data.
> 
> This patch adds an alternative implementation and interface for
> userspace per-cpu.  The kernel implementation uses special ELF sections
> and offset calculation.  For simplicity, this version defines a
> PERCPU_MAX length global array for each per-cpu data, and uses a
> thread-local cpu id for indexing.
> 
> Signed-off-by: Yuzhuo Jing <yuzhuo@google.com>
> ---
>  tools/include/linux/compiler_types.h  |   3 +
>  tools/include/linux/percpu-simulate.h | 128 ++++++++++++++++++++++++++
>  2 files changed, 131 insertions(+)
>  create mode 100644 tools/include/linux/percpu-simulate.h
> 
> diff --git a/tools/include/linux/compiler_types.h b/tools/include/linux/compiler_types.h
> index 9a2a2f8d7b6c..46550c500b8c 100644
> --- a/tools/include/linux/compiler_types.h
> +++ b/tools/include/linux/compiler_types.h
> @@ -31,6 +31,9 @@
>  # define __cond_lock(x,c) (c)
>  #endif /* __CHECKER__ */
>  
> +/* Per-cpu checker flag does not use address space attribute in userspace */
> +#define __percpu
> +
>  /*
>   * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
>   *			       non-scalar types unchanged.
> diff --git a/tools/include/linux/percpu-simulate.h b/tools/include/linux/percpu-simulate.h
> new file mode 100644
> index 000000000000..a6af2f2211eb
> --- /dev/null
> +++ b/tools/include/linux/percpu-simulate.h
> @@ -0,0 +1,128 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Userspace implementation of per_cpu_ptr for adapted kernel code.
> + *
> + * Userspace code does not have and does not need a per-cpu concept, but
> + * instead can declare variables as thread-local.  However, the kernel per-cpu
> + * further provides 1) the guarantee of static lifetime when thread exits, and
> + * 2) the ability to access other CPU's per-cpu data.  This file provides a
> + * simple implementation of such functionality, but with slightly different
> + * APIs and without linker script changes.
> + *
> + * 2025  Yuzhuo Jing <yuzhuo@google.com>
> + */
> +#ifndef __PERCPU_SIMULATE_H__
> +#define __PERCPU_SIMULATE_H__
> +
> +#include <assert.h>
> +
> +#include <linux/compiler.h>
> +#include <linux/types.h>
> +
> +/*
> + * The maximum supported number of CPUs.  Per-cpu variables are defined as a
> + * PERCPU_MAX length array, indexed by a thread-local cpu id.
> + */
> +#define PERCPU_MAX 4096
> +
> +#ifdef ASSERT_PERCPU
> +#define __check_cpu_id(cpu)						\
> +({									\
> +	u32 cpuid = (cpu);						\
> +	assert(cpuid < PERCPU_MAX);					\
> +	cpuid;								\
> +})
> +#else
> +#define __check_cpu_id(cpu)	(cpu)
> +#endif
> +
> +/*
> + * Use weak symbol: only define __thread_per_cpu_id variable if any perf tool
> + * includes this header file.
> + */
> +_Thread_local u32 __thread_per_cpu_id __weak;

Is there any overhead (or some indirection) when using the thread local
variable?

> +
> +static inline u32 get_this_cpu_id(void)
> +{
> +	return __thread_per_cpu_id;
> +}
> +
> +/*
> + * The user code must call this function inside of each thread that uses
> + * per-cpu data structures.  The user code can choose an id of their choice,
> + * but must ensure each thread uses a different id.
> + *
> + * Safety: asserts CPU id smaller than PERCPU_MAX if ASSERT_PERCPU is defined.
> + */
> +static inline void set_this_cpu_id(u32 id)
> +{
> +	__thread_per_cpu_id = __check_cpu_id(id);
> +}
> +
> +/*
> + * Declare a per-cpu data structure.  This only declares the data type and
> + * array length. Different per-cpu data are differentiated by a key (identifer).
> + *
> + * Different from the kernel version, this API must be called before the actual
> + * definition (i.e. DEFINE_PER_CPU_ALIGNED).
> + *
> + * Note that this implementation does not support prepending static qualifier,
> + * or appending assignment expressions.
> + */
> +#define DECLARE_PER_CPU_ALIGNED(key, type, data) \
> +	extern struct __percpu_type_##key { \
> +		type data; \
> +	} __percpu_data_##key[PERCPU_MAX]
> +
> +/*
> + * Define the per-cpu data storage for a given key.  This uses a previously
> + * defined data type in DECLARE_PER_CPU_ALIGNED.
> + *
> + * Different from the kernel version, this API only accepts a key name.
> + */
> +#define DEFINE_PER_CPU_ALIGNED(key) \
> +	struct __percpu_type_##key __percpu_data_##key[PERCPU_MAX]

How do these APIs guarantee the alignment?

Thanks,
Namhyung

> +
> +#define __raw_per_cpu_value(key, field, cpu) \
> +	(__percpu_data_##key[cpu].field)
> +
> +/*
> + * Get a pointer of per-cpu data for a given key.
> + *
> + * Different from the kernel version, users of this API don't need to pass the
> + * address of the base variable (through `&varname').
> + *
> + * Safety: asserts CPU id smaller than PERCPU_MAX if ASSERT_PERCPU is defined.
> + */
> +#define per_cpu_ptr(key, field, cpu) (&per_cpu_value(key, field, cpu))
> +#define this_cpu_ptr(key, field) (&this_cpu_value(key, field))
> +
> +/*
> + * Additional APIs for direct value access.  Effectively, `*per_cpu_ptr(...)'.
> + *
> + * Safety: asserts CPU id smaller than PERCPU_MAX if ASSERT_PERCPU is defined.
> + */
> +#define per_cpu_value(key, field, cpu) \
> +	(__raw_per_cpu_value(key, field, __check_cpu_id(cpu)))
> +#define this_cpu_value(key, field) \
> +	(__raw_per_cpu_value(key, field, __thread_per_cpu_id))
> +
> +/*
> + * Helper functions of simple per-cpu operations.
> + *
> + * The kernel version differentiates __this_cpu_* from this_cpu_* for
> + * preemption/interrupt-safe contexts, but the userspace version defines them
> + * as the same.
> + */
> +
> +#define __this_cpu_add(key, field, val)	(this_cpu_value(key, field) += (val))
> +#define __this_cpu_sub(key, field, val)	(this_cpu_value(key, field) -= (val))
> +#define __this_cpu_inc(key, field)	(++this_cpu_value(key, field))
> +#define __this_cpu_dec(key, field)	(--this_cpu_value(key, field))
> +
> +#define this_cpu_add	__this_cpu_add
> +#define this_cpu_sub	__this_cpu_sub
> +#define this_cpu_inc	__this_cpu_inc
> +#define this_cpu_dec	__this_cpu_dec
> +
> +#endif /* __PERCPU_SIMULATE_H__ */
> -- 
> 2.50.1.487.gc89ff58d15-goog
>