linux-kernel - Re: [RFC PATCH 2/5] tracing/hwlat: Implement the mode config option

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Message-ID: <20210414103029.7c48b76e@gandalf.local.home>
Date:   Wed, 14 Apr 2021 10:30:29 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     linux-kernel@...r.kernel.org, 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: [RFC PATCH 2/5] tracing/hwlat: Implement the mode config option

On Thu,  8 Apr 2021 16:13:20 +0200
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> +/**
> + * 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)
> +{
> +	const char *mode;
> +	char buf[64];
> +	int ret;
> +	int i;
> +
> +	if (hwlat_busy)
> +		return -EBUSY;

So we can't switch modes while running?


Also, with this implemented, you can remove the disable_migrate variable,
and just switch the mode to NONE when it's detected that the affinity mask
of the thread has been changed.

-- Steve


> +
> +	if (cnt >= sizeof(buf))
> +		return -EINVAL;
> +
> +	if (copy_from_user(buf, ubuf, cnt))
> +		return -EFAULT;
> +
> +	buf[cnt] = 0;
> +
> +	mode = strstrip(buf);
> +
> +	ret = -EINVAL;
> +
> +	for (i = 0; i < MODE_MAX; i++) {
> +		if (strcmp(mode, thread_mode_str[i]) == 0) {
> +			hwlat_data.thread_mode = i;
> +			ret = cnt;
> +		}
> +	}
> +
> +	*ppos += cnt;
> +
> +	return cnt;
> +}
> +
> +