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

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Date:   Wed, 8 Jan 2020 08:50:09 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     kan.liang@...ux.intel.com
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        tglx@...utronix.de, bp@...en8.de, linux-kernel@...r.kernel.org,
        eranian@...gle.com, alexey.budankov@...ux.intel.com,
        vitaly.slobodskoy@...el.com
Subject: Re: [RFC PATCH V3 2/7] perf: attach/detach PMU specific data

> +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.

> +	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.

> +		}
> +
> +		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