Re: FOR REVIEW: New x86-64 vsyscall vgetcpu()

[Steve Munroe <sjmunroe@us.ibm.com>]

Andi Kleen <ak@suse.de> wrote on 06/14/2006 02:42:31 AM:

>
> I got several requests over the years to provide a fast way to get
> the current CPU and node on x86-64.  That is useful for a couple of
things:
>
> - The kernel gets a lot of benefit from using per CPU data to get better
> cache locality and avoid cache line bouncing. This is currently
> not quite possible for user programs. With a fast way to know the current
> CPU user space can use per CPU data that is likely in cache already.
> Locking is still needed of course - after all the thread might switch
> to a different CPU - but at least the memory should be already in cache
> and locking on cached memory is much cheaper.
>
> - For NUMA optimization in user space you really need to know the current
> node to find out where to allocate memory from.
> If you allocate a fresh page from the kernel the kernel will give you
> one in the current node, but if you keep your own pools like most
programs
> do you need to know this to select the right pool.
> On single threaded programs it is usually not a big issue because they
> tend to start on one node, allocate all their memory there and then
eventually
> use it there too, but on multithreaded programs where threads can
> run on different nodes it's a bigger problem to make sure the threads
> can get node local memory for best performance.
>
PowerPC has similar issues and could use VDSO/vsyscal to implement
vgetcpu() as well. So we should get Ben Herrenschmidt involved to insure
that we have a cross platform solution.


> At first look such a call still looks like a bad idea - after all
> the kernel can
> switch a process at any time to other CPUs so any result of this call
might
> be wrong as soon as it returns.
>
> But at a closer look it really makes sense:
> - The kernel has strong thread affinity and usually keeps a process on
the
> same CPU. So switching CPUs is rare. This makes it an useful
optimization.
>
> The alternative is usually to bind the process to a specific CPU - then
it
> "know" where it is - but the problem is that this is nasty to use and
> requires user configuration. The kernel often can make better decisions
on
> where to schedule. And doing it automatically makes it just work.
>
> This cannot be done effectively in user space because only the kernel
> knows how to get this information from the CPUs because it  requires
> translating local APIC numbers to Linux CPU numbers.
>
> Doing it in a syscall is too slow so doing it in a vsyscall makes sense.
>
> I have patches now in my tree from Vojtech
> ftp://ftp.firstfloor.org/pub/ak/x86_64/quilt/patches/getcpu-vsyscall
> (note doesn't apply on its own, needs earlier patches in the quilt set)
>
> The prototype is
>
> long vgetcpu(int *cpu, int *node, unsigned long *tcache)
>
> cpu gets the current CPU number if not NULL.
> node gets the current node number if not NULL
> tcache is a pointer to a two element long array, can be also NULL.
> Described below.
> Return is always 0.
>
> [I modified the prototype a bit over Vojtech's original implementation
> to be more foolproof and add the caching mechanism]
>
> Unfortunately all ways to get this information from the CPU are
> still relatively slow:
> it supports RDTSCP on CPUs that support it and CPUID(1) otherwise.
> Unfortunately
> they both are relatively slow.
>
> They stall the pipeline and add some overhead
> so I added a special caching mechanism. The idea is that if it's a little
> slow then user space would likely cache the information anyways. The
problem
> with caching is that you need a way to find if it's out of date. User
space
> cannot do this because it doesn't have a fast way to access a time stamp.
>
> But the x86-64 vsyscall implementation happens to incidentally -
> vgettimeofday()
> already has access to jiffies, that can be just used as a timestamp to
> invalidate the cache. The vsyscall cannot cache this information by
itself
> though - it doesn't have any storage. The idea is that the user would
pass a
> TLS variable in there which is then used for storage.  With that the
> information
> can be at best a jiffie out of date, which is good enough.
>
> The contents of the cache are theoretically supposed to be opaque
> (although I'm
> sure user programs  will soon abuse that because it will such a
> convenient way
> to get at jiffies ..). I've considered xoring it with a value to make it
clear
> it's not, but that is probably overkill (?). Might be still safer because
> jiffies is unsafe to use in user space because the unit might change.
>
> The array is slightly ugly - one open possibility is to replace it with
> a structure. Shouldn't make much difference to the general semantics
> of the syscall though.
>
> Some numbers:  (the getpid is to compare syscall cost)
>
> AMD RevF (with RDTSCP support):
> getpid 162 cycles
> vgetcpu 145 cycles
> vgetcpu rdtscp 32 cycles
> vgetcpu cached 14 cycles
>
> Intel Pentium-D (Smithfield):
> getpid 719 cycles
> vgetcpu 535 cycles
> vgetcpu cached 27 cycles
>
> AMD RevE:
> getpid 162 cycles
> vgetcpu 185 cycles
> vgetcpu cached 15 cycles
>
> As you can see CPUID(1) is always very slow, but usually narrowly wins
> against the syscall still, except on AMD E stepping. The difference
> is very small there and while it would have been possible to implement
> a third mode for this that uses a real syscall I ended not too because it

> has some other implications.
>
> With the caching mechanism it really flies though and should be fast
enough
> for most uses.
>
> My eventual hope is that glibc will be start using this to implement
> a NUMA aware
> malloc() in user space that tries to allocate local memory preferably.
> I would say that's the biggest gap we still have in "general
> purpose" NUMA tuning
> on Linux. Of course it will be likely useful for a lot of other scalable
> code too.
>
> Comments on the general mechanism are welcome. If someone is
> interested in using
> this in user space for SMP or NUMA tuning please let me know.
>
> I haven't quite made of my mind yet if it's 2.6.18 material or not.
>

Steven J. Munroe
Linux on Power Toolchain Architect
IBM Corporation, Linux Technology Center

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