Re: [RFC PATCH V3 2/7] perf: attach/detach PMU specific data

[Liang, Kan]

On 1/8/2020 11:50 AM, Andi Kleen wrote:
> > +static int
> > +attach_system_wide_ctx_data(size_t ctx_size)
> > +{
> > +	int i, num_thread, pos, nr_failed_alloc;
> > +	unsigned long flags = GFP_ATOMIC;
> > +	struct perf_ctx_data *tsk_data;
> > +	struct perf_ctx_data **data;
> > +	struct task_struct *g, *p;
> > +	bool re_alloc = true;
> > +
> > +	/* Retrieve total number of threads */
> > +	num_thread = nr_threads;
> > +
> > +	data = kcalloc(num_thread, sizeof(*data), GFP_KERNEL);
>
> This probably needs kvcalloc for reliability and avoiding stalls.

Yes, kvcalloc looks better.

> > +	if (!data) {
> > +		printk_once(KERN_DEBUG
> > +			    "Failed to allocate space for LBR callstack. "
> > +			    "The LBR callstack for all tasks may be cutoff.\n");
> > +		return -ENOMEM;
> > +	}
> > +
> > +	atomic_inc(&nr_task_data_sys_wide_events);
> > +
> > +repeat:
> > +	/*
> > +	 * Allocate perf_ctx_data for all existing threads.
> > +	 * The perf_ctx_data for new threads will be allocated in
> > +	 * perf_event_fork().
> > +	 * Do a quick allocation in first round with GFP_ATOMIC.
> > +	 */
> > +	for (i = 0; i < num_thread; i++) {
> > +		if (alloc_perf_ctx_data(ctx_size, flags, &data[i]))
> > +			break;
> > +	}
> > +	num_thread = i;
> > +	nr_failed_alloc = 0;
> > +	pos = 0;
> > +
>
> > +	rcu_read_lock();
> > +	for_each_process_thread(g, p) {
> > +		raw_spin_lock(&p->perf_ctx_data_lock);
> > +		tsk_data = p->perf_ctx_data;
> > +		if (tsk_data) {
>
> That will be a lot of locks even for tasks that don't use perf, but I guess we
> really need it and it's bounded by the number of tasks.

Right. We don't know which tasks will be monitored later. So we have to 
attach the perf_ctx_data for all of them. The per-task lock is required 
to sync the writers of perf_ctx_data RCU pointer.


Thanks,
Kan
> > +		}
> > +
> > +		if (pos < num_thread) {
> > +			refcount_set(&data[pos]->refcount, TASK_DATA_SYS_WIDE);
> > +			rcu_assign_pointer(p->perf_ctx_data, data[pos++]);
> > +		} else {
> > +			/*
> > +			 * The quick allocation in first round may be failed.
> > +			 * Track the number in nr_failed_alloc.
> > +			 */
> > +			nr_failed_alloc++;
> > +		}
> > +		raw_spin_unlock(&p->perf_ctx_data_lock);
> > +	}
> > +	rcu_read_unlock();
>
>
> -Andi