Re: [PATCH v2 2/2] powerpc: add running_clock for powerpc to prevent spurious softlockup warnings

[Paul Bolle <pebolle@tiscali.nl>]

On Fri, 2015-01-09 at 14:34 +1100, Cyril Bur wrote:
> On POWER8 virtualised kernels the VTB register can be read to have a view of
> time that only increases while the guest is running. This will prevent guests
> from seeing time jump if a guest is paused for significant amounts of time.
> 
> On POWER7 and below virtualised kernels stolen time is subtracted from
> local_clock as a best effort approximation. This will not eliminate spurious
> warnings in the case of a suspended guest but may reduce the occurance in the
> case of softlockups due to host over commit.
> 
> Bare metal kernels should avoid reading the VTB as KVM does not restore sane
> values when not executing, the approxmation is fine as host kernels won't
> observe any stolen time.
> 
> Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
> ---
> V2:
>    Replaced the use of sched_clock_with local_clock it was used originally in
> the softlockup detector.
>    Added #ifdef CONFIG_PPC_PSERIES and optimised the non lpar + vtb cases.

This became commit 3e5aba51e929 ("powerpc: add running_clock for powerpc
to prevent spurious softlockup warnings") in today's linux-next (ie,
next-20150204). I noticed because a script I use to check linux-next
spotted a trivial issues with it.

> ---
>  arch/powerpc/kernel/time.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index fa7c4f1..fd35e5b 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -621,6 +621,38 @@ unsigned long long sched_clock(void)
>  	return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;
>  }
>  
> +
> +#ifdef CONFIG_PPC_PSERIES
> +
> +/*
> + * Running clock - attempts to give a view of time passing for a virtualised
> + * kernels.
> + * Uses the VTB register if available otherwise a next best guess.
> + */
> +unsigned long long running_clock(void)
> +{
> +	/*
> +	 * Don't read the VTB as a host since KVM does not switch in host timebase
> +	 * into the VTB when it takes a guest off the CPU, reading the VTB would
> +	 * result in reading 'last switched out' guest VTB.
> +	 *
> +	 * Host kernels are often compiled with CONFIG_PSERIES checked, it would be

You obviously wanted to use CONFIG_PPC_PSERIES here.

Should I submit a trivial patch to fix that typo or do you prefer to do
that yourself?

Thanks,


Paul Bolle