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Re: [RFC PATCH] getcpu_cache system call: caching current CPU number (x86)

On Sat, Jul 18, 2015 at 01:28:36AM +0200, OndÅej BÃlka wrote:
> On Fri, Jul 17, 2015 at 11:48:14AM -0700, Linus Torvalds wrote:
> > On Thu, Jul 16, 2015 at 12:27 PM, Andy Lutomirski <> wrote:
> > >
> > > If we actually bit the bullet and implemented per-cpu mappings
> > 
> > That's not ever going to happen.
> > 
> > The Linux VM model of "one page table per VM" is the right one.
> > Anything else sucks, and makes threading a disaster.
> > 
> > So you can try to prove me wrong, but seriously, I doubt you'll succeed.
> > 
> > On x86, if you want per-cpu memory areas, you should basically plan on
> > using segment registers instead (although other odd state has been
> > used - there's been the people who use segment limits etc rather than
> > the *pointer* itself, preferring to use "lsl" to get percpu data. You
> > could also imaging hiding things in the vector state somewhere if you
> > control your environment well enough).
> >
> Thats correct, problem is that you need some sort of hack like this on
> archs that otherwise would need syscall to get tid/access tls variable.
> On x64 and archs that have register for tls this could be implemented
> relatively easily.
> Kernel needs to allocate 
> int running_cpu_for_tid[32768];

This does not scale. You're assuming the default task ("pid") number
limit, but this can be raised up to 512k (beyond that is impossible
because of PI/robust futex ABI).

> On context switch it atomically writes to this table 
> running_cpu_for_tid[tid] = cpu;
> This table is read-only accessible from userspace as mmaped file.

There is a much simpler solution: use a per-cpu (rather than per-task)
page that contains the right value for the cpu. I believe vdso already
does something like this, no?


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