linux-kernel - Re: [PATCH v3 2/2] proc: add /proc/<pid>/arch

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Date:   Fri, 16 Nov 2018 08:32:18 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        mingo@...hat.com, peterz@...radead.org, hpa@...or.com
Cc:     ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] proc: add /proc/<pid>/arch_state

On 2018/11/15 23:18, Dave Hansen wrote:
> On 11/14/18 3:00 PM, Aubrey Li wrote:
>> +void arch_thread_state(struct seq_file *m, struct task_struct *task)
>> +{
>> +	/*
>> +	 * Report AVX-512 Hi16_ZMM registers usage
>> +	 */
>> +	if (task->thread.fpu.hi16zmm_usage)
>> +		seq_putc(m, '1');
>> +	else
>> +		seq_putc(m, '0');
>> +	seq_putc(m, '\n');
>> +}
> 
> Am I reading this right that this file just dumps out a plain 0 or 1
> based on the internal kernel state?  BTW, there's no example of the
> output in the changelog, so it's rather hard to tell if my guess is
> right.  (Hint, hint).
> 
> If so, I'd really prefer you not do this.  We have /proc/$pid/stat to
> stand as a disaster in this regard.  It is essentially
> non-human-readable gibberish because it's impossible to tell what the
> values mean without a decoder ring.

Yes, I'm following /proc/$pid/stat format, as I think this interface is
not for the end user, but for developer and user space job scheduler. So
I guess this style might be okay.

> 
> If we go down this road, we need a file along the lines of
> /proc/$pid/status.

I checked /proc/$pid/status, all common information to architectures.
That's why I want to open a new interface to CPU specific state.

> 
> But, either way, this is a new ABI that we need to consider carefully.
> It needs documentation.  For instance, will this really mean "Hi16_ZMM
> user" from now until the end of time?  Or, does it just mean "group me
> with other tasks that have this bit set"?
> 
I'm open to this interface. Let's wait to see if there are more comments
and suggestions.

Thanks,
-Aubrey