linux-kernel - Re: [PATCH v6 09/10] arm64: support cpuidle-haltpoll

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Message-ID: <104d0ec31cb45477e27273e089402d4205ee4042.camel@amazon.com>
Date: Tue, 13 Aug 2024 15:26:11 +0000
From: "Okanovic, Haris" <harisokn@...zon.com>
To: "ankur.a.arora@...cle.com" <ankur.a.arora@...cle.com>
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Subject: Re: [PATCH v6 09/10] arm64: support cpuidle-haltpoll

On Mon, 2024-07-29 at 11:02 -0700, Ankur Arora wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> Okanovic, Haris <harisokn@...zon.com> writes:
> 
> > On Fri, 2024-07-26 at 13:21 -0700, Ankur Arora wrote:
> > > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> > > 
> > > 
> > > 
> > > Add architectural support for cpuidle-haltpoll driver by defining
> > > arch_haltpoll_*().
> > > 
> > > Also define ARCH_CPUIDLE_HALTPOLL to allow cpuidle-haltpoll to be
> > > selected, and given that we have an optimized polling mechanism
> > > in smp_cond_load*(), select ARCH_HAS_OPTIMIZED_POLL.
> > > 
> > > smp_cond_load*() are implemented via LDXR, WFE, with LDXR loading
> > > a memory region in exclusive state and the WFE waiting for any
> > > stores to it.
> > > 
> > > In the edge case -- no CPU stores to the waited region and there's no
> > > interrupt -- the event-stream will provide the terminating condition
> > > ensuring we don't wait forever, but because the event-stream runs at
> > > a fixed frequency (configured at 10kHz) we might spend more time in
> > > the polling stage than specified by cpuidle_poll_time().
> > > 
> > > This would only happen in the last iteration, since overshooting the
> > > poll_limit means the governor moves out of the polling stage.
> > > 
> > > Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> > > ---
> > >  arch/arm64/Kconfig                        | 10 ++++++++++
> > >  arch/arm64/include/asm/cpuidle_haltpoll.h |  9 +++++++++
> > >  arch/arm64/kernel/cpuidle.c               | 23 +++++++++++++++++++++++
> > >  3 files changed, 42 insertions(+)
> > >  create mode 100644 arch/arm64/include/asm/cpuidle_haltpoll.h
> > > 
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index 5d91259ee7b5..cf1c6681eb0a 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -35,6 +35,7 @@ config ARM64
> > >         select ARCH_HAS_MEMBARRIER_SYNC_CORE
> > >         select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
> > >         select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> > > +       select ARCH_HAS_OPTIMIZED_POLL
> > >         select ARCH_HAS_PTE_DEVMAP
> > >         select ARCH_HAS_PTE_SPECIAL
> > >         select ARCH_HAS_HW_PTE_YOUNG
> > > @@ -2376,6 +2377,15 @@ config ARCH_HIBERNATION_HEADER
> > >  config ARCH_SUSPEND_POSSIBLE
> > >         def_bool y
> > > 
> > > +config ARCH_CPUIDLE_HALTPOLL
> > > +       bool "Enable selection of the cpuidle-haltpoll driver"
> > > +       default n
> > > +       help
> > > +         cpuidle-haltpoll allows for adaptive polling based on
> > > +         current load before entering the idle state.
> > > +
> > > +         Some virtualized workloads benefit from using it.
> > > +
> > >  endmenu # "Power management options"
> > > 
> > >  menu "CPU Power Management"
> > > diff --git a/arch/arm64/include/asm/cpuidle_haltpoll.h b/arch/arm64/include/asm/cpuidle_haltpoll.h
> > > new file mode 100644
> > > index 000000000000..65f289407a6c
> > > --- /dev/null
> > > +++ b/arch/arm64/include/asm/cpuidle_haltpoll.h
> > > @@ -0,0 +1,9 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +#ifndef _ARCH_HALTPOLL_H
> > > +#define _ARCH_HALTPOLL_H
> > > +
> > > +static inline void arch_haltpoll_enable(unsigned int cpu) { }
> > > +static inline void arch_haltpoll_disable(unsigned int cpu) { }
> > > +
> > > +bool arch_haltpoll_want(bool force);
> > > +#endif
> > > diff --git a/arch/arm64/kernel/cpuidle.c b/arch/arm64/kernel/cpuidle.c
> > > index f372295207fb..334df82a0eac 100644
> > > --- a/arch/arm64/kernel/cpuidle.c
> > > +++ b/arch/arm64/kernel/cpuidle.c
> > > @@ -72,3 +72,26 @@ __cpuidle int acpi_processor_ffh_lpi_enter(struct acpi_lpi_state *lpi)
> > >                                              lpi->index, state);
> > >  }
> > >  #endif
> > > +
> > > +#if IS_ENABLED(CONFIG_HALTPOLL_CPUIDLE)
> > > +
> > > +#include <asm/cpuidle_haltpoll.h>
> > > +
> > > +bool arch_haltpoll_want(bool force)
> > > +{
> > > +       /*
> > > +        * Enabling haltpoll requires two things:
> > > +        *
> > > +        * - Event stream support to provide a terminating condition to the
> > > +        *   WFE in the poll loop.
> > > +        *
> > > +        * - KVM support for arch_haltpoll_enable(), arch_haltpoll_enable().
> > 
> > typo: "arch_haltpoll_enable" and "arch_haltpoll_enable"
> > 
> > > +        *
> > > +        * Given that the second is missing, allow haltpoll to only be force
> > > +        * loaded.
> > > +        */
> > > +       return (arch_timer_evtstrm_available() && false) || force;
> > 
> > This should always evaluate false without force. Perhaps you meant
> > something like this?
> > 
> > ```
> > -       return (arch_timer_evtstrm_available() && false) || force;
> > +       return arch_timer_evtstrm_available() || force;
> > ```
> 
> No. This was intentional. As I meniton in the comment above, right now
> the KVM support is missing. Which means that the guest has no way to
> tell the host to not poll as part of host haltpoll.
> 
> Until that is available, only allow force loading.

I see, arm64's kvm is missing the poll control mechanism.

I'll follow-up your changes with a patch for AWS Graviton; still seeing
the same performance gains.

Tested-by: Haris Okanovic <harisokn@...zon.com>

> 
> --
> ankur