Re: [PATCH 0/7] x86 vdso32 cleanups

From: Andy Lutomirski
Date: Tue Sep 01 2015 - 17:50:26 EST

On Mon, Aug 31, 2015 at 6:37 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> I got random errors from perf kvm, but I think I found at least part
> of the issue. The two irqs_disabled() calls in common.c are kind of
> expensive. I should disable them on non-lockdep kernels.
> The context tracking hooks are also too expensive, even when disabled.
> I should do something to optimize those. Hello, static keys? This
> doesn't affect syscalls, though.
> With context tracking off and the irqs_disabled checks commented out,
> we're probably doing well enough. We can always tweak the C code and
> aggressively force inlining if we want a few cycles back.

Currently, a compat AT_SYSINFO syscall (getpid) is 171 cycles for me.
With my patches, it's 196 cycles, so it's really not that bad. The
impact will probably be slightly worse on native 32-bit because of
increased register pressure and because one of the micro-optimizations
I threw in are 64-bit specific. We could probably tune the C code a
bit more to get a few of the cycles back.

On the flip side, the rewrite is *far* faster in some of the slow path
cases because the slow path no longer forces IRET.

On 32-bit, there's the added benefit that we could drop asmlinkage
from the syscall bodies on top of the rewrite.


> --Andy

Andy Lutomirski
AMA Capital Management, LLC
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