linux-kernel - Re: [PATCH 6/7] printk: Avoid scheduling printing threads on the same CPU

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Message-ID: <20151210151906.GE540@swordfish>
Date:	Fri, 11 Dec 2015 00:19:06 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Jan Kara <jack@...e.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Petr Mladek <pmladek@...e.cz>,
	KY Srinivasan <kys@...rosoft.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH 6/7] printk: Avoid scheduling printing threads on the
 same CPU

>+static void distribute_printing_kthreads(void)
>+{
>+	int i;
>+	unsigned int cpus_per_thread;
>+	unsigned int cpu, seen_cpu;
>+
>+	for (i = 0; i < PRINTING_TASKS; i++)
>+		cpumask_clear(printing_kthread_mask[i]);
>+
>+	cpus_per_thread = DIV_ROUND_UP(num_online_cpus(), PRINTING_TASKS);
>+	seen_cpu = 0;
>+	for_each_online_cpu(cpu) {
>+		cpumask_set_cpu(cpu,
>+			printing_kthread_mask[seen_cpu / cpus_per_thread]);
>+		seen_cpu++;
>+	}
>+
>+	for (i = 0; i < PRINTING_TASKS; i++)
>+		if (!cpumask_empty(printing_kthread_mask[i]))
>+			set_cpus_allowed_ptr(printing_kthread[i],
>+					     printing_kthread_mask[i]);
>+}


I certainly understand what are you trying to do here, but I'm a bit concerned.
This may result in 'bad' affinities on big.LITTLE platforms, for example. So I
think that printk is not quite good place for that type of decisions. Just my 5
cents.

	-ss
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