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

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Date:	Fri, 01 May 2015 13:22:21 -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 01:12 PM, Ingo Molnar wrote:
> 
> * Rik van Riel <riel@...hat.com> wrote:
> 
>> 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?
> 
> So I'd split the timer tick into two parts: just the constant-work 
> sampling bit that doesn't do much, and the variable-work part which 
> gets essentially shut off when the timeout is far into the future.
> 
> Then we could do IRQ driven sampling without introducing variable 
> amount jitter into hard-RT execution time.
> 
> I.e. much of what we do today, except that we could skip variable work 
> such as the scheduler tick or (unforced) RCU processing like the RCU 
> softirq work.

Any ideas how we could avoid that sampling timer interrupt
latency stacking up when dealing with both guest and host?

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