Messages in this thread Patch in this message | | | Date | Fri, 27 Feb 2015 19:53:23 +0000 | From | Will Deacon <> | Subject | Re: [RFC PATCH] arm64: Implement cpu_relax as yield |
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On Fri, Feb 27, 2015 at 07:43:27PM +0000, Peter Crosthwaite wrote: > On Wed, Feb 25, 2015 at 5:24 AM, Will Deacon <will.deacon@arm.com> wrote: > > On Tue, Feb 24, 2015 at 11:07:37PM +0000, Peter Crosthwaite wrote: > >> ARM64 has the yield nop hint which has the intended semantics of > >> cpu_relax. Implement. > >> > >> The immediate application is ARM CPU emulators. An emulator can take > >> advantage of the yield hint to de-prioritise an emulated CPU in favor > >> of other emulation tasks. QEMU A64 SMP emulation has yield awareness, > >> and sees a significant boot time performance increase with this change. > > > > Could you elaborate on the QEMU SMP boot case please? Usually SMP pens > > for booting make use of wfe/sev to minimise the spinning overhead. > > > > So I did some follow up experiments. With my patch applied I started > trapping instances of cpu_relax (now yielding) in gdb. I then > commented out the cpu_relax's one by one. > > This one seems to be the key: > > diff --git a/kernel/smp.c b/kernel/smp.c > index f38a1e6..1c692be 100644 > --- a/kernel/smp.c > +++ b/kernel/smp.c > @@ -108,7 +108,8 @@ void __init call_function_init(void) > static void csd_lock_wait(struct call_single_data *csd) > { > while (csd->flags & CSD_FLAG_LOCK) > - cpu_relax(); > + ; > + //cpu_relax(); > } > > Hack above causes boot time delay even with my patch applied, so this > is causing my issue it seems.
Ok; I was wondering whether this was going to be part of the bootloader, but as it's not and this feature is useful to you, then feel free to add my:
Acked-by: Will Deacon <will.deacon@arm.com>
to your original patch.
Thanks,
Will
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