linux-kernel - Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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Message-ID: <5543AFDC.6010204@redhat.com>
Date:	Fri, 01 May 2015 12:54:52 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, williams@...hat.com,
	Andrew Lutomirski <luto@...nel.org>, fweisbec@...hat.com,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry

On 05/01/2015 12:45 PM, Ingo Molnar wrote:
> 
> * Rik van Riel <riel@...hat.com> wrote:
> 
>> On 05/01/2015 12:37 PM, Ingo Molnar wrote:
>>
>>> Also note that this bit in context_tracking_enter():
>>>
>>>                         if (state == CONTEXT_USER) {
>>>                                 trace_user_enter(0);
>>>                                 vtime_user_enter(current);
>>>                         }
>>>
>>>
>>> is related to precise time measurement of user/kernel execution 
>>> times, it's not needed by the scheduler at all, it's just exported 
>>> to tooling. It's not fundamental to the scheduler.
>>
>> Any objections to the idea from the other thread to simply keep the 
>> time accumulating in buffers in local_clock() units, and only update 
>> the task vtime once a second or so?
> 
> So I really think per syscall overhead is the wrong thing to do for 
> anything that a common distro enables.
> 
> I see very little use for such precise, high-freq measurements on 
> normal systems - and abnormal systems could enable it dynamically just 
> like they can enable syscall auditing.
> 
> I.e. I don't mind the occasional crazy piece of code, as long as it 
> does not hurt the innocent.

Then how should/could we keep a rough idea of user / system / guest
time when running without a periodic timer tick?

These statistics are useful, and gathering them in a low impact
way (local_clock read + variable read + subtraction + addition +
store) may be acceptable overhead.

This is a few function calls, one tsc read, a possible cache
line miss, and an unlikely branch.

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