linux-kernel - Re: [PATCH V3 5/9] tracing/trace: Add a generic function to read/write u64 values from tracefs

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Date:   Fri, 4 Jun 2021 18:05:06 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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 5/9] tracing/trace: Add a generic function to
 read/write u64 values from tracefs

On 6/3/21 11:22 PM, Steven Rostedt wrote:
> On Fri, 14 May 2021 22:51:14 +0200
> Daniel Bristot de Oliveira <bristot@...hat.com> wrote:
> 
>> Provides a generic read and write implementation to save/read u64 values
>> from a file on tracefs. The trace_ull_config structure defines where to
>> read/write the value, the min and the max acceptable values, and a lock
>> to protect the write.
> 
> This states what the patch is doing, but does not say why it is doing it.

Yeah...

>>
>> 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>
>> ---
>>  kernel/trace/trace.c | 87 ++++++++++++++++++++++++++++++++++++++++++++
>>  kernel/trace/trace.h | 19 ++++++++++
>>  2 files changed, 106 insertions(+)
>>
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index 560e4c8d3825..b4cd89010813 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -7516,6 +7516,93 @@ static const struct file_operations snapshot_raw_fops = {
>>  
>>  #endif /* CONFIG_TRACER_SNAPSHOT */
>>  
>> +/*
>> + * trace_ull_config_write - Generic write function to save u64 value
> 
> 
> That is a horrible name. What the hell is the "config"?
> 
>> + * @filp: The active open file structure
>> + * @ubuf: The userspace provided buffer to read value into
>> + * @cnt: The maximum number of bytes to read
>> + * @ppos: The current "file" position
>> + *
>> + * This function provides a generic write implementation to save u64 values
>> + * from a file on tracefs. The filp->private_data must point to a
>> + * trace_ull_config structure that defines where to write the value, the
>> + * min and the max acceptable values, and a lock to protect the write.
> 
> This doesn't seem to be a generic way to save 64 bit values (which I still
> don't understand, because unsigned long long should work too). But it looks
> like the rational is for having some kind of generic way to read 64 bit
> values giving them a min and a max.
> 
> I see this is used later, but this patch needs to be rewritten. It makes no
> sense.

The reason for this patch is that hwlat, osnoise, and timerlat have "u64 config"
options that are read/write via tracefs "files." In the previous version, I had
multiple functions doing basically the same thing:

A write function that:
	read a u64 from user-space
	get a lock,
	check for min/max acceptable values
		save the value
	release the lock.

and a read function that:
	write the config value to the "read" buffer.

And so, I tried to come up with a way to avoid code duplication.

question: are only the names that are bad? (I agree that they are bad) or do you
think that the overall idea is bad? :-)

Suggestions?

-- Daniel

> -- Steve