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: <20150501164526.GB1548@gmail.com>
Date:	Fri, 1 May 2015 18:45:26 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Rik van Riel <riel@...hat.com>
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


* 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.

Thanks,

	Ingo
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