linux-kernel - Re: [PATCH] x86 idle: repair large-server 50-watt idle-power regression

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Date:	Thu, 19 Dec 2013 06:40:41 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>, Len Brown <lenb@...nel.org>
CC:	x86@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>,
	stable@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86 idle: repair large-server 50-watt idle-power regression

... or just use static_cpu_has() maybe?

Ingo Molnar <mingo@...nel.org> wrote:
>
>* Len Brown <lenb@...nel.org> wrote:
>
>> From: Len Brown <len.brown@...el.com>
>> 
>> Linux 3.10 changed the timing of how thread_info->flags is touched:
>> 
>> 	x86: Use generic idle loop
>> 	(7d1a941731fabf27e5fb6edbebb79fe856edb4e5)
>> 
>> This caused Intel NHM-EX and WSM-EX servers to experience a large
>number
>> of immediate MONITOR/MWAIT break wakeups, which caused cpuidle to
>demote
>> from deep C-states to shallow C-states, which caused these platforms
>> to experience a significant increase in idle power.
>> 
>> Note that this issue was already present before the commit above,
>> however, it wasn't seen often enough to be noticed in power
>measurements.
>> 
>> Here we extend an errata workaround from the Core2 EX "Dunnington"
>> to extend to NHM-EX and WSM-EX, to prevent these immediate
>> returns from MWAIT, reducing idle power on these platforms.
>> 
>> While only acpi_idle ran on Dunnington, intel_idle
>> may also run on these two newer systems.
>> As of today, there are no other models that are known
>> to need this tweak.
>> 
>> ref: https://lkml.org/lkml/2013/12/7/22
>> Signed-off-by: Len Brown <len.brown@...el.com>
>> Cc: <stable@...r.kernel.org> # 3.12.x, 3.11.x, 3.10.x
>> ---
>>  arch/x86/kernel/cpu/intel.c | 3 ++-
>>  drivers/idle/intel_idle.c   | 3 +++
>>  2 files changed, 5 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kernel/cpu/intel.c
>b/arch/x86/kernel/cpu/intel.c
>> index dc1ec0d..ea04b34 100644
>> --- a/arch/x86/kernel/cpu/intel.c
>> +++ b/arch/x86/kernel/cpu/intel.c
>> @@ -387,7 +387,8 @@ static void init_intel(struct cpuinfo_x86 *c)
>>  			set_cpu_cap(c, X86_FEATURE_PEBS);
>>  	}
>>  
>> -	if (c->x86 == 6 && c->x86_model == 29 && cpu_has_clflush)
>> +	if (c->x86 == 6 && cpu_has_clflush &&
>> +	    (c->x86_model == 29 || c->x86_model == 46 || c->x86_model ==
>47))
>>  		set_cpu_cap(c, X86_FEATURE_CLFLUSH_MONITOR);
>>  
>>  #ifdef CONFIG_X86_64
>> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
>> index 92d1206..f80b700 100644
>> --- a/drivers/idle/intel_idle.c
>> +++ b/drivers/idle/intel_idle.c
>> @@ -377,6 +377,9 @@ static int intel_idle(struct cpuidle_device *dev,
>>  
>>  	if (!current_set_polling_and_test()) {
>>  
>> +		if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
>> +			clflush((void *)&current_thread_info()->flags);
>> +
>>  		__monitor((void *)&current_thread_info()->flags, 0, 0);
>
>I don't think either of these casts to '(void *)' is needed, both the 
>clflush() and __monitor() will take pointers.
>
>Looks good to me otherwise - except that maybe the best way to 
>represent this quirk would be for the CLFLUSH+MONITOR sequence to be a 
>single 'instruction' which is patched in dynamically during bootup, 
>using our usual alternatives framework.
>
>On non-affected CPUs a NOP would remain in place of the CLFLUSH, 
>eliminating the branch above.
>
>So the whole thing could be thought of as a slightly more complex 
>'monitor' instruction - not exposing the quirk details to actual usage 
>sites.
>
>Thanks,
>
>	Ingo

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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