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

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Date:   Wed, 01 Apr 2020 15:12:53 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 2/6] powerpc/idle: Add accessor function to always read
 latest idle PURR

Hi Gautham,

Gautham R. Shenoy wrote:
> From: "Gautham R. Shenoy" <ego@...ux.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@...ux.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