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

[Rik van Riel]

On 05/07/2015 08:29 AM, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@xxxxxxxxx> 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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