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Message-ID: <dfe36107-1397-4401-b8fd-86cafe871866@intel.com>
Date: Fri, 8 Nov 2024 08:03:49 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
rafael.j.wysocki@...el.com, len.brown@...el.com,
artem.bityutskiy@...ux.intel.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH v3 1/3] x86/smp: Allow calling mwait_play_dead with
arbitrary hint
On 11/8/24 04:29, Patryk Wlazlyn wrote:
> The current implementation for looking up the mwait hint for the deepest
> cstate depends on them to be continuous in range [0, NUM_SUBSTATES-1].
> While that is correct on most Intel x86 platforms, it is not documented
> and may not result in reaching the most optimized idle state on some of
> them.
First, it is not clear what code this refers to. It needs to be more clear.
Second, this is not clear about the bug. Start with being crystal clear
about the problem and the impact:
MWAIT needs different hints on different CPUs to reach the most
efficient idle states. The hint calculation* works in practice
on current hardware, but it fails on newer ones. Those newer
CPUs' battery life really suffers when the system is suspended
like when a laptop lid is closed.
* The current calculation is the for loop inspecting edx in
mwait_play_dead()
Then you can go into detail about what the bad implementation does.
> For example Intel's Sierra Forest report two C6 substates in cpuid leaf 5:
> C6S (hint 0x22)
> C6SP (hint 0x23)
>
> Hints 0x20 and 0x21 are skipped entirely,
Why does it start at 0x20? Giving the example on new, broken hardware
is also a bit weak without an example from old, fully functional hardware.
> causing the current
> implementation to compute the wrong hint, when looking for the deepest
> cstate for offlined CPU to enter. As a result, package with an offlined
> CPU can never reach PC6.
I don't think the PC6 detail matters enough to mention here.
> Allow the idle driver to call mwait_play_dead() code with the forced
> mwait hint, skipping the cpuid based computation.
>
> Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>
> ---
> arch/x86/include/asm/smp.h | 6 ++++++
> arch/x86/kernel/smpboot.c | 25 ++++++++++++++++++-------
> 2 files changed, 24 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> index ca073f40698f..fbd275d6661a 100644
> --- a/arch/x86/include/asm/smp.h
> +++ b/arch/x86/include/asm/smp.h
> @@ -114,6 +114,7 @@ void wbinvd_on_cpu(int cpu);
> int wbinvd_on_all_cpus(void);
>
> void smp_kick_mwait_play_dead(void);
> +int mwait_play_dead_with_hint(unsigned long hint);
>
> void native_smp_send_reschedule(int cpu);
> void native_send_call_func_ipi(const struct cpumask *mask);
> @@ -164,6 +165,11 @@ static inline struct cpumask *cpu_llc_shared_mask(int cpu)
> {
> return (struct cpumask *)cpumask_of(0);
> }
> +
> +static inline int mwait_play_dead_with_hint(unsigned long eax_hint)
> +{
> + return 1;
> +}
> #endif /* CONFIG_SMP */
>
> #ifdef CONFIG_DEBUG_NMI_SELFTEST
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 0c35207320cb..44c40781bad6 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1270,13 +1270,14 @@ void play_dead_common(void)
> local_irq_disable();
> }
>
> +int mwait_play_dead_with_hint(unsigned long eax_hint);
We generally prefer these get a declaration in a header or get moved
around in the file, not declared like this.
> /*
> * We need to flush the caches before going to sleep, lest we have
> * dirty data in our caches when we come back up.
> */
> -static inline void mwait_play_dead(void)
> +static inline int mwait_play_dead(void)
> {
> - struct mwait_cpu_dead *md = this_cpu_ptr(&mwait_cpu_dead);
> unsigned int eax, ebx, ecx, edx;
> unsigned int highest_cstate = 0;
> unsigned int highest_subcstate = 0;
> @@ -1284,13 +1285,13 @@ static inline void mwait_play_dead(void)
>
> if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
> boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
> - return;
> + return 1;
> if (!this_cpu_has(X86_FEATURE_MWAIT))
> - return;
> + return 1;
> if (!this_cpu_has(X86_FEATURE_CLFLUSH))
> - return;
> + return 1;
> if (__this_cpu_read(cpu_info.cpuid_level) < CPUID_MWAIT_LEAF)
> - return;
> + return 1;
If you're going to use an 'int' for a fail/nofail return type, please
just use -ERRNO's. It makes it *MUCH* more clear that these are error
returns and not something else.
That's what cpuidle_play_dead() does, for instance, Please follow its lead.
> eax = CPUID_MWAIT_LEAF;
> ecx = 0;
> @@ -1314,6 +1315,13 @@ static inline void mwait_play_dead(void)
> (highest_subcstate - 1);
> }
>
> + return mwait_play_dead_with_hint(eax);
> +}
> +
> +int mwait_play_dead_with_hint(unsigned long eax_hint)
> +{
> + struct mwait_cpu_dead *md = this_cpu_ptr(&mwait_cpu_dead);
> +
> /* Set up state for the kexec() hack below */
> md->status = CPUDEAD_MWAIT_WAIT;
> md->control = CPUDEAD_MWAIT_WAIT;
> @@ -1333,7 +1341,7 @@ static inline void mwait_play_dead(void)
> mb();
> __monitor(md, 0, 0);
> mb();
> - __mwait(eax, 0);
> + __mwait(eax_hint, 0);
>
> if (READ_ONCE(md->control) == CPUDEAD_MWAIT_KEXEC_HLT) {
> /*
> @@ -1353,6 +1361,9 @@ static inline void mwait_play_dead(void)
> native_halt();
> }
> }
> +
> + /* Never reached */
> + return 0;
> }
I think the preferred way to do this is to add a __noreturn annotation.
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