Re: [RFC, patch] i386: vgetcpu(), take 2

[Rohit Seth <rohitseth@google.com>]

On Fri, 2006-06-23 at 00:14 +0200, Andi Kleen wrote:
> > Would sgdt not be sufficient?  I agree that we will have to end up
> > giving RO access to user for the gdt page.
> 
> I meant exporting the GDT page
> 

Yes indeed.  That shouldn't be an issue though.

> > I agree that we should not overload a single call (though cpu, package
> > and node numbers do belong in one category IMO).  We can have multiple
> > calls if that is required as long as there is an efficient mechanism to
> > provide that information.
> 
> The current mechanism doesn't scale to much more calls, but I guess
> i'll have to do a vDSO sooner or later.
>  
> > Why maintain that extra logic in user space when kernel can easily give
> > that information.
> 
> It already does.
>  

I'm missing your point here.  How and where?

> > > I've been pondering to put some more information about that
> > > in the ELF aux vector, but exporting might work too. I suppose
> > > exporting would require the vDSO first to give a sane interface.
> > > 
> > Can you please tell me what more information you are thinking of putting
> > in aux vector?
> 
> One proposal (not fully fleshed out was) number of siblings / sockets / nodes 
> I don't think bitmaps would work well there (and if someone really needs
> those they can read cpuinfo again) 
> 

This is exactly the point, why do that expensive /proc operation when
you can do a quick vsyscall and get all of that information.  I'm not
sure if Aux is the right direction.

> This is mostly for OpenMP and tuning of a few functions (e.g. on AMD
> the memory latencies varies with the number of nodes so some functions
> can be tuned in different ways based on that) 
> 
> > You are absolutely right that the mechanism I'm proposing makes sense
> > only if we have more fields AND if any of those fields are dynamically
> > changing.  But this is a generic mechanism that could be extended to
> > share any user visible information in efficient way.  Once we have this
> > in place then information like whole cpuinfo, percpu interrupts etc. can
> > be retrieved easily.
> 
> The problem with exposing too much is that it might be a nightmare
> to guarantee a stable ABI for this. At least it would
> constrain the kernel internally. Probably less is better here. 
> 

There will be (in all probability) requests to include as much as
possible, but I think that should be manageable with sensible API.

> Also I'm still not sure why user space should care about interrupts?
> 
Okay. I just cooked that example for some monitoring process to find out
the interrupts /sec on that CPU.  But as you mentioned above sibling,
sockets, nodes, flags, and even other characteristics like current
p-state are all important information that will help applications
sitting in user land (even if some of them will be used only couple of
times in the life of a process).

Side note: I don't want to delay the vgetcpu call into mainline because
of this discussion (as long as there is no cpuid and tcache in that
call).

-rohit