linux-kernel - Re: [PATCH v2 03/11] smp: eliminate SCF_WAIT and SCF_RUN

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Date:   Mon, 25 Apr 2022 11:10:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Donghai Qiao <dqiao@...hat.com>
Cc:     akpm@...ux-foundation.org, sfr@...b.auug.org.au, arnd@...db.de,
        heying24@...wei.com, andriy.shevchenko@...ux.intel.com,
        axboe@...nel.dk, rdunlap@...radead.org, tglx@...utronix.de,
        gor@...ux.ibm.com, donghai.w.qiao@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/11] smp: eliminate SCF_WAIT and SCF_RUN_LOCAL

On Fri, Apr 22, 2022 at 04:00:32PM -0400, Donghai Qiao wrote:
> The commit a32a4d8a815c ("smp: Run functions concurrently in
> smp_call_function_many_cond()") was to improve the concurrent
> of the cross call execution between local and remote. The change
> in smp_call_function_many_cond() did what was intended, but the
> new macro SCF_WAIT and SCF_RUN_LOCAL and the code around them
> to handle local call were not unnecessary because the modified
> function smp_call_function_many() was able to handle the local
> cross call. So these two macros can be eliminated and the code
> implemented around that can be removed as well.

Maybe I've not had enough wake-up-juice, but I can't parse the above,
what?!


> @@ -787,23 +787,13 @@ int smp_call_function_any(const struct cpumask *mask,
>  }
>  EXPORT_SYMBOL_GPL(smp_call_function_any);
>  
> -/*
> - * Flags to be used as scf_flags argument of smp_call_function_many_cond().
> - *
> - * %SCF_WAIT:		Wait until function execution is completed
> - * %SCF_RUN_LOCAL:	Run also locally if local cpu is set in cpumask
> - */
> -#define SCF_WAIT	(1U << 0)
> -#define SCF_RUN_LOCAL	(1U << 1)
> -
>  static void smp_call_function_many_cond(const struct cpumask *mask,
>  					smp_call_func_t func, void *info,
> -					unsigned int scf_flags,
> +					bool wait,
>  					smp_cond_func_t cond_func)
>  {
>  	int cpu, last_cpu, this_cpu = smp_processor_id();
>  	struct call_function_data *cfd;
> -	bool wait = scf_flags & SCF_WAIT;
>  	bool run_remote = false;
>  	bool run_local = false;
>  	int nr_cpus = 0;
> @@ -829,14 +819,14 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  	WARN_ON_ONCE(!in_task());
>  
>  	/* Check if we need local execution. */
> -	if ((scf_flags & SCF_RUN_LOCAL) && cpumask_test_cpu(this_cpu, mask))
> +	if (cpumask_test_cpu(this_cpu, mask))
>  		run_local = true;
>  
>  	/* Check if we need remote execution, i.e., any CPU excluding this one. */
>  	cpu = cpumask_first_and(mask, cpu_online_mask);
>  	if (cpu == this_cpu)
>  		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> -	if (cpu < nr_cpu_ids)
> +	if ((unsigned int)cpu < nr_cpu_ids)
>  		run_remote = true;
>  
>  	if (run_remote) {
> @@ -911,12 +901,8 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>   * @mask: The set of cpus to run on (only runs on online subset).
>   * @func: The function to run. This must be fast and non-blocking.
>   * @info: An arbitrary pointer to pass to the function.
> - * @wait: Bitmask that controls the operation. If %SCF_WAIT is set, wait
> - *        (atomically) until function has completed on other CPUs. If
> - *        %SCF_RUN_LOCAL is set, the function will also be run locally
> - *        if the local CPU is set in the @cpumask.
> - *
> - * If @wait is true, then returns once @func has returned.
> + * @wait: If wait is true, the call will not return until func()
> + *        has completed on other CPUs.
>   *
>   * You must not call this function with disabled interrupts or from a
>   * hardware interrupt handler or from a bottom half handler. Preemption
> @@ -925,7 +911,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  void smp_call_function_many(const struct cpumask *mask,
>  			    smp_call_func_t func, void *info, bool wait)
>  {
> -	smp_call_function_many_cond(mask, func, info, wait * SCF_WAIT, NULL);
> +	smp_call_function_many_cond(mask, func, info, wait, NULL);
>  }
>  EXPORT_SYMBOL(smp_call_function_many);

This changes the semantics of smp_call_function_many(), before this, if
self was in the mask it wouldn't call @func, now it will.

I appreciate you want to clean that up, but you can't do that without
auditing all callers.