Re: [PATCH v4 2/6] powerpc/idle: Add accessor function to always read latest idle PURR

Naveen N. Rao
2020-04-01T09:43:05Z

Hi Gautham,

Gautham R. Shenoy wrote:
> From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
>
> Currently when CPU goes idle, we take a snapshot of PURR via
> pseries_idle_prolog() which is used at the CPU idle exit to compute
> the idle PURR cycles via the function pseries_idle_epilog(). Thus,
> the value of idle PURR cycle thus read before pseries_idle_prolog() and
> after pseries_idle_epilog() is always correct.
>
> However, if we were to read the idle PURR cycles from an interrupt
> context between pseries_idle_prolog() and pseries_idle_epilog() (this will
> be done in a future patch), then, the value of the idle PURR thus read
> will not include the cycles spent in the most recent idle period.
>
> This patch addresses the issue by providing accessor function to read
> the idle PURR such such that it includes the cycles spent in the most
> recent idle period, if we read it between pseries_idle_prolog() and
> pseries_idle_epilog(). In order to achieve it, the patch saves the
> snapshot of PURR in pseries_idle_prolog() in a per-cpu variable,
> instead of on the stack, so that it can be accessed from an interrupt
> context.
>
> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> ---
> arch/powerpc/include/asm/idle.h | 47 +++++++++++++++++++++++++++-------
> arch/powerpc/platforms/pseries/setup.c | 7 +++--
> drivers/cpuidle/cpuidle-pseries.c | 15 +++++------
> 3 files changed, 47 insertions(+), 22 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/idle.h b/arch/powerpc/include/asm/idle.h
> index 32064a4c..d4bfb6a 100644
> --- a/arch/powerpc/include/asm/idle.h
> +++ b/arch/powerpc/include/asm/idle.h
> @@ -5,10 +5,27 @@
> #include <asm/paca.h>
>
> #ifdef CONFIG_PPC_PSERIES
> -static inline void pseries_idle_prolog(unsigned long *in_purr)
> +DECLARE_PER_CPU(u64, idle_entry_purr_snap);
> +
> +static inline void snapshot_purr_idle_entry(void)
> +{
> + *this_cpu_ptr(&idle_entry_purr_snap) = mfspr(SPRN_PURR);
> +}
> +
> +static inline void update_idle_purr_accounting(void)
> +{
> + u64 wait_cycles;
> + u64 in_purr = *this_cpu_ptr(&idle_entry_purr_snap);
> +
> + wait_cycles = be64_to_cpu(get_lppaca()->wait_state_cycles);
> + wait_cycles += mfspr(SPRN_PURR) - in_purr;
> + get_lppaca()->wait_state_cycles = cpu_to_be64(wait_cycles);
> +}
> +
> +static inline void pseries_idle_prolog(void)
> {
> ppc64_runlatch_off();
> - *in_purr = mfspr(SPRN_PURR);
> + snapshot_purr_idle_entry();
> /*
> * Indicate to the HV that we are idle. Now would be
> * a good time to find other work to dispatch.
> @@ -16,16 +33,28 @@ static inline void pseries_idle_prolog(unsigned long *in_purr)
> get_lppaca()->idle = 1;
> }
>
> -static inline void pseries_idle_epilog(unsigned long in_purr)
> +static inline void pseries_idle_epilog(void)
> {
> - u64 wait_cycles;
> -
> - wait_cycles = be64_to_cpu(get_lppaca()->wait_state_cycles);
> - wait_cycles += mfspr(SPRN_PURR) - in_purr;
> - get_lppaca()->wait_state_cycles = cpu_to_be64(wait_cycles);
> + update_idle_purr_accounting();
> get_lppaca()->idle = 0;
> -
> ppc64_runlatch_on();
> }
> +
> +static inline u64 read_this_idle_purr(void)
> +{
> + /*
> + * If we are reading from an idle context, update the
> + * idle-purr cycles corresponding to the last idle period.
> + * Since the idle context is not yet over, take a fresh
> + * snapshot of the idle-purr.
> + */
> + if (unlikely(get_lppaca()->idle == 1)) {
> + update_idle_purr_accounting();
> + snapshot_purr_idle_entry();
> + }
> +
> + return be64_to_cpu(get_lppaca()->wait_state_cycles);
> +}
> +

I think this and read_this_idle_spurr() from the next patch should be
moved to Patch 4/6, where they are actually used.

- Naveen