Messages in this thread | | | Date | Sat, 01 Aug 2009 12:38:34 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [GIT PULL] Additional x86 fixes for 2.6.31-rc5 |
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On 08/01/2009 12:28 PM, Linus Torvalds wrote: > > Hmm. > > I just noticed another issue on x86 code generation, since I was looking > at assembly language generation due to the do_sigaltstack() kernel stack > info leak thing. > > Our "get_current()" seriously sucks now that it's a per-cpu variable. > > Look at the code generated for something like > > current->sas_ss_sp = (unsigned long) ss_sp; > current->sas_ss_size = ss_size; > > and notice how the code really really sucks: > > movq %gs:per_cpu__current_task,%rcx > movq %rdx, 1152(%rcx) > movq %gs:per_cpu__current_task,%rdx > movq %rax, 1160(%rdx) > > because it reloads that silly per-cpu variable every time, because the > assembler has a constraint of > > "m" (per_cpu__current_task) > > and so gcc is worried that the stores will invalidate the result of the > load from the per-cpu variable. > > I don't know how to fix that _well_, but here's a not-so-very-pretty patch > that seems to shave off 4.5kB from my kernel, and gives gcc much better > scheduling for 'current' and 'thread_info' because now it can load them > early - and cache them - even in the presense of stores. >
This is clearly better... now the semi-obvious question becomes if there is any way we can get compiler support to do better and migrate to that as the compiler allows. In particular, if I remember right the problem with using __thread for percpu was exactly that the current cpuness can change almost anywhere, unless preemption is disabled.
I'm wondering if we could use __thread or something like it for the stable perthreads, perhaps with additional compiler hints.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
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