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

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Date:   Thu, 15 Apr 2021 15:16:04 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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 4/14/21 4:30 PM, Steven Rostedt wrote:
> 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?
> 

As you mentioned in the patch 3/5, this limitation was added because
of the running threads. But, yes, stopping and starting the tracer to
re-create the threads should work as well. I will try it for the next round.

> 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.
> 

That was my initial intention with the NONE mode, but I feared breaking
something by removing the "migrate_disable" logic. If you do not think it is
a problem, I will remove the migrate disable and just change the mode.

-- Daniel
> -- 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;
>> +}
>> +
>> +
>