Re: [PATCH tip/core/rcu 2/4] smp: Add source and destination CPUs to __call_single_data

[Boqun Feng]

Hi Paul,

On Mon, Aug 31, 2020 at 11:14:15AM -0700, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> 
> This commit adds a destination CPU to __call_single_data, and is inspired
> by an earlier commit by Peter Zijlstra.  This version adds #ifdef to
> permit use by 32-bit systems and supplying the destination CPU for all
> smp_call_function*() requests, not just smp_call_function_single().
> 
> If need be, 32-bit systems could be accommodated by shrinking the flags
> field to 16 bits (the atomic_t variant is currently unused) and by
> providing only eight bits for CPU on such systems.
> 
> It is not clear that the addition of the fields to __call_single_node
> are really needed.
> 
> Link: https://lore.kernel.org/lkml/20200615164048.GC2531@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>  include/linux/smp.h       | 3 +++
>  include/linux/smp_types.h | 3 +++
>  kernel/smp.c              | 2 ++
>  3 files changed, 8 insertions(+)
> 
> diff --git a/include/linux/smp.h b/include/linux/smp.h
> index 80d557e..9f13966 100644
> --- a/include/linux/smp.h
> +++ b/include/linux/smp.h
> @@ -26,6 +26,9 @@ struct __call_single_data {
>  		struct {
>  			struct llist_node llist;
>  			unsigned int flags;
> +#ifdef CONFIG_64BIT
> +			u16 src, dst;
> +#endif
>  		};
>  	};
>  	smp_call_func_t func;
> diff --git a/include/linux/smp_types.h b/include/linux/smp_types.h
> index 364b3ae..2e8461a 100644
> --- a/include/linux/smp_types.h
> +++ b/include/linux/smp_types.h
> @@ -61,6 +61,9 @@ struct __call_single_node {
>  		unsigned int	u_flags;
>  		atomic_t	a_flags;
>  	};
> +#ifdef CONFIG_64BIT
> +	u16 src, dst;
> +#endif
>  };
>  
>  #endif /* __LINUX_SMP_TYPES_H */
> diff --git a/kernel/smp.c b/kernel/smp.c
> index d0ae8eb..a47382d 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -375,6 +375,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
>  
>  	csd->func = func;
>  	csd->info = info;
> +	csd->dst = cpu;

Would this and the next modification cause compile errors with
CONFIG_64BIT = n? I saw you add #ifdef guard in the next patch, so maybe
move those two into next patch (of course, if they trigger compile
errors)

Regards,
Boqun

>  
>  	err = generic_exec_single(cpu, csd);
>  
> @@ -540,6 +541,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  			csd->flags |= CSD_TYPE_SYNC;
>  		csd->func = func;
>  		csd->info = info;
> +		csd->dst = cpu;
>  		if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))
>  			__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
>  	}
> -- 
> 2.9.5
>