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Date:   Mon, 2 Dec 2019 13:40:55 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     kan.liang@linux.intel.com
Cc:     mingo@redhat.com, acme@kernel.org, tglx@linutronix.de,
        bp@alien8.de, linux-kernel@vger.kernel.org, eranian@google.com,
        alexey.budankov@linux.intel.com, vitaly.slobodskoy@intel.com,
        ak@linux.intel.com
Subject: Re: [RFC PATCH 3/8] perf: Init/fini PMU specific data
Message-ID: <20191202124055.GC2827@hirez.programming.kicks-ass.net>
References: <1574954071-6321-1-git-send-email-kan.liang@linux.intel.com>
 <1574954071-6321-3-git-send-email-kan.liang@linux.intel.com>
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On Thu, Nov 28, 2019 at 07:14:26AM -0800, kan.liang@linux.intel.com wrote:
> +static int
> +init_system_wide_ctx_data(size_t ctx_size)
> +{
> +	struct task_struct *g, *p;
> +	int failed_alloc = 0;
> +	unsigned long flags;
> +
> +	/*
> +	 * Allocate perf_ctx_data for all existing threads by the first event.
> +	 *
> +	 * The perf_ctx_data for new thread will be allocated in
> +	 * perf_event_fork(). The perf_event_fork() is called after the thread
> +	 * is added into the tasklist. It guarantees that any new threads will
> +	 * not be missed.
> +	 */
> +	raw_spin_lock_irqsave(&task_data_events_lock, flags);
> +	if (atomic_inc_return(&nr_task_data_events) > 1)
> +		goto unlock;
> +
> +	read_lock(&tasklist_lock);
> +
> +	for_each_process_thread(g, p) {
> +		/*
> +		 * The PMU specific data may already be allocated by
> +		 * per-process event. Need to update refcounter.
> +		 * init_task_ctx_data_rcu() is called here.
> +		 * Do a quick allocation in first round with GFP_ATOMIC.
> +		 */
> +		if (init_task_ctx_data_rcu(p, ctx_size, GFP_ATOMIC))
> +			failed_alloc++;
> +	}

This is atricous crap. Also it is completely broken for -RT.