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
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