Messages in this thread |  | | From | Fredrik Markström <> | Date | Wed, 5 Oct 2016 18:39:32 +0200 | Subject | Re: [PATCH v2] arm: Added support for getcpu() vDSO using TPIDRURW |
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The approach I suggested below with the vDSO data page will obviously not work on smp, so suggestions are welcome.
/Fredrik
On Wed, Oct 5, 2016 at 2:25 PM, Fredrik Markström <fredrik.markstrom@gmail.com> wrote: > On Tue, Oct 4, 2016 at 7:08 PM Mark Rutland <mark.rutland@arm.com> wrote: >> >> On Tue, Oct 04, 2016 at 05:35:33PM +0200, Fredrik Markstrom wrote: >> > This makes getcpu() ~1000 times faster, this is very useful when >> > implementing per-cpu buffers in userspace (to avoid cache line >> > bouncing). As an example lttng ust becomes ~30% faster. >> > >> > The patch will break applications using TPIDRURW (which is context switched >> > since commit 4780adeefd042482f624f5e0d577bf9cdcbb760 ("ARM: 7735/2: >> >> It looks like you dropped the leading 'a' from the commit ID. For >> everyone else's benefit, the full ID is: >> >> a4780adeefd042482f624f5e0d577bf9cdcbb760 > > > Sorry for that and thanks for fixing it. > >> >> >> Please note that arm64 has done similar for compat tasks since commit: >> >> d00a3810c16207d2 ("arm64: context-switch user tls register tpidr_el0 for >> compat tasks") >> >> > Preserve the user r/w register TPIDRURW on context switch and fork")) and >> > is therefore made configurable. >> >> As you note above, this is an ABI break and *will* break some existing >> applications. That's generally a no-go. > > > Ok, I wasn't sure this was considered an ABI (but I'm not entirely > surprised ;) ). The way I was > trying to defend the breakage was by reasoning that that if it was an > ABI we broke it both with a4780ad > and with 6a1c531, and since we don't break ABI:s, it can't be one. > > But hey, I'm humble here and ready to back off. > >> >> This also leaves arm64's compat with the existing behaviour, differing >> from arm. >> >> I was under the impression that other mechanisms were being considered >> for fast userspace access to per-cpu data structures, e.g. restartable >> sequences. What is the state of those? Why is this better? >> >> If getcpu() specifically is necessary, is there no other way to >> implement it? > > If you are referring to the user space stuff can probably be > implemented other ways, > it's just convenient since the interface is there and it will speed up > stuff like lttng without > modifications (well, except glibc). It's also already implemented as a > vDSO on other > major architectures (like x86, x86_64, ppc32 and ppc64). > > If you are referring to the implementation of the vdso call, there are > other possibilities, but > I haven't found any that doesn't introduce overhead in context switching. > > But if TPIDRURW is definitely a no go, I can work on a patch that does > this with a thread notifier > and the vdso data page. Would that be a viable option ? > >> >> > +notrace int __vdso_getcpu(unsigned int *cpup, unsigned int *nodep, >> > + struct getcpu_cache *tcache) >> > +{ >> > + unsigned long node_and_cpu; >> > + >> > + asm("mrc p15, 0, %0, c13, c0, 2\n" : "=r"(node_and_cpu)); >> > + >> > + if (nodep) >> > + *nodep = cpu_to_node(node_and_cpu >> 16); >> > + if (cpup) >> > + *cpup = node_and_cpu & 0xffffUL; >> >> Given this is directly user-accessible, this format is a de-facto ABI, >> even if it's not documented as such. Is this definitely the format you >> want long-term? > > Yes, this (the interface) is indeed the important part and therefore I > tried not to invent anything > on my own. > This is the interface used by ppc32, ppc64, x86, x86_64. It's also this is > how the getcpu(2) system call is documented. > > /Fredrik > > >> >> >> Thanks, >> Mark.
-- /Fredrik
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