Date:   Thu, 3 Jun 2021 16:11:03 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Phil Auld <pauld@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Kate Carcia <kcarcia@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Clark Willaims <williams@...hat.com>,
        John Kacur <jkacur@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH V3 2/9] tracing/hwlat: Implement the mode config option

On Fri, 14 May 2021 22:51:11 +0200
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> Provides the "mode" config to the hardware latency detector. hwlatd has
> two different operation modes. The default mode is the "round-robin" one,
> in which a single hwlatd thread runs, migrating among the allowed CPUs in a
> "round-robin" fashion. This is the current behavior.
> 
> The "none" sets the allowed cpumask for a single hwlatd thread at the
> startup, but skips the round-robin, letting the scheduler handle the
> migration.
> 
> In preparation to the per-cpu mode.
> 
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Alexandre Chartre <alexandre.chartre@...cle.com>
> Cc: Clark Willaims <williams@...hat.com>
> Cc: John Kacur <jkacur@...hat.com>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> ---
>  Documentation/trace/hwlat_detector.rst |  12 +-
>  kernel/trace/trace_hwlat.c             | 171 +++++++++++++++++++++++--
>  2 files changed, 169 insertions(+), 14 deletions(-)
> 
> diff --git a/Documentation/trace/hwlat_detector.rst b/Documentation/trace/hwlat_detector.rst
> index 5739349649c8..4d952df0586a 100644
> --- a/Documentation/trace/hwlat_detector.rst
> +++ b/Documentation/trace/hwlat_detector.rst
> @@ -76,8 +76,12 @@ in /sys/kernel/tracing:
>   - tracing_cpumask	- the CPUs to move the hwlat thread across
>   - hwlat_detector/width	- specified amount of time to spin within window (usecs)
>   - hwlat_detector/window	- amount of time between (width) runs (usecs)
> + - hwlat_detector/mode	- the thread mode
>  
> -The hwlat detector's kernel thread will migrate across each CPU specified in
> -tracing_cpumask between each window. To limit the migration, either modify
> -tracing_cpumask, or modify the hwlat kernel thread (named [hwlatd]) CPU
> -affinity directly, and the migration will stop.
> +By default, the hwlat detector's kernel thread will migrate across each CPU
> +specified in cpumask at the beginning of a new window, in a round-robin
> +fashion. This behavior can be changed by changing the thread mode,
> +the available options are:
> +
> + - none:        do not force migration
> + - round-robin: migrate across each CPU specified in cpumask [default]
> diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
> index 0a5635401125..1f5d48830fd6 100644
> --- a/kernel/trace/trace_hwlat.c
> +++ b/kernel/trace/trace_hwlat.c
> @@ -59,6 +59,14 @@ static struct task_struct *hwlat_kthread;
>  
>  static struct dentry *hwlat_sample_width;	/* sample width us */
>  static struct dentry *hwlat_sample_window;	/* sample window us */
> +static struct dentry *hwlat_thread_mode;	/* hwlat thread mode */
> +
> +enum {
> +	MODE_NONE = 0,
> +	MODE_ROUND_ROBIN,
> +	MODE_MAX
> +};
> +static char *thread_mode_str[] = { "none", "round-robin" };

I usually do the above with:

#define HWLAT_MODES 			\
	C(NONE, "none"),		\
	C(ROUND_ROBIN, "round-robin")

#undef C
#define C(x,y) MODE_##x

enum {
	HWLAT_MODES,
	MODE_MAX
};

#undef C
#define C(x,y) y

static char *thread_mode_str[] = { HWLAT_MODES };

But this is small enough not to do this.

>  
>  /* Save the previous tracing_thresh value */
>  static unsigned long save_tracing_thresh;
> @@ -96,11 +104,16 @@ static struct hwlat_data {
>  	u64	sample_window;		/* total sampling window (on+off) */
>  	u64	sample_width;		/* active sampling portion of window */
>  
> +	int	thread_mode;		/* thread mode */
> +
>  } hwlat_data = {
>  	.sample_window		= DEFAULT_SAMPLE_WINDOW,
>  	.sample_width		= DEFAULT_SAMPLE_WIDTH,
> +	.thread_mode		= MODE_ROUND_ROBIN
>  };
>  
> +static bool hwlat_busy;
> +
>  static void trace_hwlat_sample(struct hwlat_sample *sample)
>  {
>  	struct trace_array *tr = hwlat_trace;
> @@ -328,7 +341,8 @@ static int kthread_fn(void *data)
>  
>  	while (!kthread_should_stop()) {
>  
> -		move_to_next_cpu();
> +		if (hwlat_data.thread_mode == MODE_ROUND_ROBIN)
> +			move_to_next_cpu();
>  
>  		local_irq_disable();
>  		get_sample();
> @@ -366,11 +380,6 @@ static int start_kthread(struct trace_array *tr)
>  	if (hwlat_kthread)
>  		return 0;
>  
> -	/* Just pick the first CPU on first iteration */
> -	get_online_cpus();
> -	cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);
> -	put_online_cpus();
> -	next_cpu = cpumask_first(current_mask);
>  
>  	kthread = kthread_create(kthread_fn, NULL, "hwlatd");
>  	if (IS_ERR(kthread)) {
> @@ -378,8 +387,19 @@ static int start_kthread(struct trace_array *tr)
>  		return -ENOMEM;
>  	}
>  
> -	cpumask_clear(current_mask);
> -	cpumask_set_cpu(next_cpu, current_mask);
> +
> +	/* Just pick the first CPU on first iteration */
> +	get_online_cpus();
> +	cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);
> +	put_online_cpus();
> +
> +	if (hwlat_data.thread_mode == MODE_ROUND_ROBIN) {
> +		next_cpu = cpumask_first(current_mask);
> +		cpumask_clear(current_mask);
> +		cpumask_set_cpu(next_cpu, current_mask);
> +
> +	}
> +
>  	sched_setaffinity(kthread->pid, current_mask);
>  
>  	hwlat_kthread = kthread;
> @@ -511,6 +531,125 @@ hwlat_window_write(struct file *filp, const char __user *ubuf,
>  	return cnt;
>  }
>  
> +static void *s_mode_start(struct seq_file *s, loff_t *pos)
> +{
> +	int mode = *pos;
> +

Probably should add:

	mutex_lock(&hwlat_data.lock);

here.

> +	if (mode >= MODE_MAX)
> +		return NULL;
> +
> +	return pos;
> +}
> +
> +static void *s_mode_next(struct seq_file *s, void *v, loff_t *pos)
> +{
> +	int mode = ++(*pos);
> +
> +	if (mode >= MODE_MAX)
> +		return NULL;
> +
> +	return pos;
> +}
> +
> +static int s_mode_show(struct seq_file *s, void *v)
> +{
> +	loff_t *pos = v;
> +	int mode = *pos;
> +
> +	if (mode == hwlat_data.thread_mode)
> +		seq_printf(s, "[%s]", thread_mode_str[mode]);
> +	else
> +		seq_printf(s, "%s", thread_mode_str[mode]);
> +
> +	if (mode != MODE_MAX)
> +		seq_puts(s, " ");
> +
> +	return 0;
> +}
> +
> +static void s_mode_stop(struct seq_file *s, void *v)
> +{
> +	seq_puts(s, "\n");

And

	mutex_unlock(&hwlat_data.lock);

here to prevent conflicts with the update?

> +}
> +
> +static const struct seq_operations thread_mode_seq_ops = {
> +	.start		= s_mode_start,
> +	.next		= s_mode_next,
> +	.show		= s_mode_show,
> +	.stop		= s_mode_stop
> +};
> +
> +static int hwlat_mode_open(struct inode *inode, struct file *file)
> +{
> +	return seq_open(file, &thread_mode_seq_ops);
> +};
> +
> +static void hwlat_tracer_start(struct trace_array *tr);
> +static void hwlat_tracer_stop(struct trace_array *tr);

Add newline here.

> +/**
> + * hwlat_mode_write - Write function for "mode" entry
> + * @filp: The active open file structure
> + * @ubuf: The user buffer that contains the value to write
> + * @cnt: The maximum number of bytes to write to "file"
> + * @ppos: The current position in @file
> + *
> + * This function provides a write implementation for the "mode" interface
> + * to the hardware latency detector. hwlatd has different operation modes.
> + * The "none" sets the allowed cpumask for a single hwlatd thread at the
> + * startup and lets the scheduler handle the migration. The default mode is
> + * the "round-robin" one, in which a single hwlatd thread runs, migrating
> + * among the allowed CPUs in a round-robin fashion.
> + */
> +static ssize_t hwlat_mode_write(struct file *filp, const char __user *ubuf,
> +				 size_t cnt, loff_t *ppos)
> +{
> +	struct trace_array *tr = hwlat_trace;
> +	const char *mode;
> +	char buf[64];
> +	int ret, i;
> +
> +	if (cnt >= sizeof(buf))
> +		return -EINVAL;
> +
> +	if (copy_from_user(buf, ubuf, cnt))
> +		return -EFAULT;
> +
> +	buf[cnt] = 0;
> +
> +	mode = strstrip(buf);
> +
> +	ret = -EINVAL;
> +
> +	/*
> +	 * trace_types_lock is taken to avoid concurrency on start/stop
> +	 * and hwlat_busy.
> +	 */
> +	mutex_lock(&trace_types_lock);
> +	if (hwlat_busy)
> +		hwlat_tracer_stop(tr);
> +
> +	mutex_lock(&hwlat_data.lock);
> +
> +	for (i = 0; i < MODE_MAX; i++) {
> +		if (strcmp(mode, thread_mode_str[i]) == 0) {
> +			hwlat_data.thread_mode = i;
> +			ret = cnt;
> +		}
> +	}
> +
> +	mutex_unlock(&hwlat_data.lock);
> +
> +	if (hwlat_busy)
> +		hwlat_tracer_start(tr);
> +	mutex_unlock(&trace_types_lock);
> +
> +	*ppos += cnt;
> +
> +
> +
> +	return ret;
> +}
> +
>  static const struct file_operations width_fops = {
>  	.open		= tracing_open_generic,
>  	.read		= hwlat_read,
> @@ -523,6 +662,13 @@ static const struct file_operations window_fops = {
>  	.write		= hwlat_window_write,
>  };
>  
> +static const struct file_operations thread_mode_fops = {
> +	.open		= hwlat_mode_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= seq_release,
> +	.write		= hwlat_mode_write
> +};
>  /**
>   * init_tracefs - A function to initialize the tracefs interface files
>   *
> @@ -558,6 +704,13 @@ static int init_tracefs(void)
>  	if (!hwlat_sample_width)
>  		goto err;
>  
> +	hwlat_thread_mode = trace_create_file("mode", 0644,
> +					      top_dir,
> +					      NULL,
> +					      &thread_mode_fops);
> +	if (!hwlat_thread_mode)
> +		goto err;
> +
>  	return 0;
>  
>   err:
> @@ -579,8 +732,6 @@ static void hwlat_tracer_stop(struct trace_array *tr)
>  	stop_kthread();
>  }
>  
> -static bool hwlat_busy;
> -
>  static int hwlat_tracer_init(struct trace_array *tr)
>  {
>  	/* Only allow one instance to enable this */


-- Steve

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