[PATCH 03/19] cpu/hotplug: Avoid busy-polling on archs where cpu_relax() is a no-op

[Jinjie Ruan]

在 2026/9/8 0:40, Will Deacon 写道:
> On some architectures (such as arm64), cpu_relax() is effectively a NOP
> and so isn't particularly efficient when used in a tight polling loop

Yes, a "yield" instruction in aarch64 is essentially a nop, with no
power optimization effect.

However, x86 "PAUSE" provides power optimization benefits.

> such as the CPU state synchronisation in cpuhp_wait_for_sync_state().
> 
> Once an incoming CPU has reached the SYNC_STATE_ALIVE state, we know
> that it is executing within the kernel and so we can use the more
> efficient polling mechanism provided by the atomic_cond_read* API.

Right! Once the secondary CPU state transitions to SYNC_STATE_ALIVE, it
is already executing kernel C code.

> 
> Extend the generic implementation of arch_cpuhp_sync_state_poll() to
> take the state details as additional parameters and polling using
> atomic_cond_read_relaxed() instead of cpu_relax() once we have reached
> the alive state. No change on x86.

Right! x86 still use cpu_relax() for atomic_cond_read_relaxed().

And we can aslo mention that this also allow RISC-V to leverage the
Zawrs extension for low-power stalling instead of busy-wasting cycles
with cpu_relax().

> 
> Signed-off-by: Will Deacon <will at kernel.org>
> ---
>  arch/x86/kernel/smpboot.c  |  2 +-
>  include/linux/cpuhotplug.h |  2 +-
>  kernel/cpu.c               | 13 +++++++++----
>  3 files changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index ba01a9e919b7..362f85cbdbaf 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1138,7 +1138,7 @@ void arch_cpuhp_cleanup_dead_cpu(unsigned int cpu)
>  		pr_info("CPU %u is now offline\n", cpu);
>  }
>  
> -void arch_cpuhp_sync_state_poll(void)
> +void arch_cpuhp_sync_state_poll(atomic_t *st, int old)
>  {
>  	if (smp_ops.poll_sync_state)
>  		smp_ops.poll_sync_state();
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index feb32949aeea..bbcee650155f 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -509,7 +509,7 @@ static inline void cpuhp_online_idle(enum cpuhp_state state) { }
>  struct task_struct;
>  
>  void cpuhp_ap_sync_alive(void);
> -void arch_cpuhp_sync_state_poll(void);
> +void arch_cpuhp_sync_state_poll(atomic_t *st, int old);
>  void arch_cpuhp_cleanup_kick_cpu(unsigned int cpu);
>  int arch_cpuhp_kick_ap_alive(unsigned int cpu, struct task_struct *tidle);
>  bool arch_cpuhp_init_parallel_bringup(void);
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 97a9bfe4edad..d9fe204f02cb 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -303,7 +303,13 @@ static inline void cpuhp_ap_update_sync_state(enum cpuhp_sync_state state)
>  	(void)atomic_xchg(st, state);
>  }
>  
> -void __weak arch_cpuhp_sync_state_poll(void) { cpu_relax(); }
> +void __weak arch_cpuhp_sync_state_poll(atomic_t *st, int old)
> +{
> +	if (old < SYNC_STATE_ALIVE)
> +		cpu_relax();
> +	else
> +		atomic_cond_read_relaxed(st, VAL != old);
> +}

With atomic_cond_read_relaxed(), we can use WFE to improve the power for
arm64.

>  
>  static bool cpuhp_wait_for_sync_state(unsigned int cpu, enum cpuhp_sync_state state,
>  				      enum cpuhp_sync_state next_state)
> @@ -328,7 +334,7 @@ static bool cpuhp_wait_for_sync_state(unsigned int cpu, enum cpuhp_sync_state st
>  			return false;
>  		} else if (now - start < NSEC_PER_MSEC) {
>  			/* Poll for one millisecond */
> -			arch_cpuhp_sync_state_poll();
> +			arch_cpuhp_sync_state_poll(st, sync);
>  		} else {
>  			usleep_range(USEC_PER_MSEC, 2 * USEC_PER_MSEC);
>  		}
> @@ -395,8 +401,7 @@ void cpuhp_ap_sync_alive(void)
>  	cpuhp_ap_update_sync_state(SYNC_STATE_ALIVE);
>  
>  	/* Wait for the control CPU to release it. */
> -	while (atomic_read(st) != SYNC_STATE_SHOULD_ONLINE)
> -		cpu_relax();
> +	atomic_cond_read_acquire(st, VAL == SYNC_STATE_SHOULD_ONLINE);

LGTM
Reviewed-by: Jinjie Ruan <ruanjinjie at huawei.com>

>  }
>  
>  static bool cpuhp_can_boot_ap(unsigned int cpu)