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: <554B891A.9040109@redhat.com>
Date:	Thu, 07 May 2015 11:47:38 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"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 05/07/2015 08:29 AM, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
>>>> We cannot take the lock_trace(task) from irq context, and we 
>>>> probably do not need to anyway, since we do not care about a 
>>>> precise stack trace for the task.
>>>
>>> So one worry with this and similar approaches of statistically 
>>> detecting user mode would be the fact that on the way out to 
>>> user-space we don't really destroy the previous call trace - we 
>>> just pop off the stack (non-destructively), restore RIPs and are 
>>> gone.
>>>
>>> We'll need that percpu flag I suspect.
>>
>> Note we have the context tracking state which tells where the 
>> current task is: user/system/guest.
> 
> Yes, but that overhead is what I'm suggesting we get rid of, I thought 
> Rik was trying to find a mechanism that would be independent of that?

One thing at a time :)

I am working on the timer sampling stuff, which should be easy
to adapt to a different user/system/guest/irq/softirq/... tracking
thing, if somebody else comes up with a more efficient way to do
that.

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