Re: [PATCH V8 1/6] perf: Save PMU specific data in task_struct

[Peter Zijlstra]

I'm sorry, but since I spotted a bug in the second patch, I'm going to
reply and suggest some overall changes.

On Wed, Mar 12, 2025 at 11:25:20AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:

> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 3e270822b915..b8442047a2b6 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1021,6 +1021,36 @@ struct perf_event_context {
>  	local_t				nr_no_switch_fast;
>  };
>  
> +/**
> + * struct perf_ctx_data - PMU specific data for a task
> + * @rcu_head:  To avoid the race on free PMU specific data
> + * @refcount:  To track users
> + * @global:    To track system-wide users
> + * @ctx_cache: Kmem cache of PMU specific data
> + * @data:      PMU specific data
> + *
> + * Currently, the struct is only used in Intel LBR call stack mode to
> + * save/restore the call stack of a task on context switches.
> + * The data only be allocated when Intel LBR call stack mode is enabled.
> + * The data will be freed when the mode is disabled. The rcu_head is
> + * used to prevent the race on free the data.
> + * The content of the data will only be accessed in context switch, which
> + * should be protected by rcu_read_lock().
> + *
> + * Careful: Struct perf_ctx_data is added as a pointor in struct task_struct.

pointer

> + * When system-wide Intel LBR call stack mode is enabled, a buffer with
> + * constant size will be allocated for each task.
> + * Also, system memory consumption can further grow when the size of
> + * struct perf_ctx_data enlarges.
> + */
> +struct perf_ctx_data {
> +	struct rcu_head			rcu_head;
> +	refcount_t			refcount;
> +	int				global;
> +	struct kmem_cache		*ctx_cache;
> +	void				*data;
> +};

I can't remember why this is complicated like this. Why do we have a
kmemcache and yet another data pointer in there?

Specifically, why can't we do something like:

struct perf_ctx_data {
	struct rcu_head		rcu;
	refcount_t		refcount;
	int			global;
	char			data[];
};

and simply allocate the whole thing as a single allocation?

So then the allocation is something like:

	cd = kzalloc(sizeof(*cd) + event->pmu->task_ctx_size, GFP_KERNEL);