linux-kernel - Re: [PATCH RFC v3 4/6] Documentation: Add three sysctls for smart idle poll

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Date:   Tue, 14 Nov 2017 12:05:16 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Ingo Molnar <mingo@...nel.org>, Quan Xu <quan.xu04@...il.com>
Cc:     kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [PATCH RFC v3 4/6] Documentation: Add three sysctls for smart
 idle poll



On 2017/11/13 23:08, Ingo Molnar wrote:
> * Quan Xu <quan.xu04@...il.com> wrote:
>
>> From: Quan Xu <quan.xu0@...il.com>
>>
>> To reduce the cost of poll, we introduce three sysctl to control the
>> poll time when running as a virtual machine with paravirt.
>>
>> Signed-off-by: Yang Zhang <yang.zhang.wz@...il.com>
>> Signed-off-by: Quan Xu <quan.xu0@...il.com>
>> ---
>>   Documentation/sysctl/kernel.txt |   35 +++++++++++++++++++++++++++++++++++
>>   arch/x86/kernel/paravirt.c      |    4 ++++
>>   include/linux/kernel.h          |    6 ++++++
>>   kernel/sysctl.c                 |   34 ++++++++++++++++++++++++++++++++++
>>   4 files changed, 79 insertions(+), 0 deletions(-)
>>
>> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
>> index 694968c..30c25fb 100644
>> --- a/Documentation/sysctl/kernel.txt
>> +++ b/Documentation/sysctl/kernel.txt
>> @@ -714,6 +714,41 @@ kernel tries to allocate a number starting from this one.
>>   
>>   ==============================================================
>>   
>> +paravirt_poll_grow: (X86 only)
>> +
>> +Multiplied value to increase the poll time. This is expected to take
>> +effect only when running as a virtual machine with CONFIG_PARAVIRT
>> +enabled. This can't bring any benifit on bare mental even with
>> +CONFIG_PARAVIRT enabled.
>> +
>> +By default this value is 2. Possible values to set are in range {2..16}.
>> +
>> +==============================================================
>> +
>> +paravirt_poll_shrink: (X86 only)
>> +
>> +Divided value to reduce the poll time. This is expected to take effect
>> +only when running as a virtual machine with CONFIG_PARAVIRT enabled.
>> +This can't bring any benifit on bare mental even with CONFIG_PARAVIRT
>> +enabled.
>> +
>> +By default this value is 2. Possible values to set are in range {2..16}.
>> +
>> +==============================================================
>> +
>> +paravirt_poll_threshold_ns: (X86 only)
>> +
>> +Controls the maximum poll time before entering real idle path. This is
>> +expected to take effect only when running as a virtual machine with
>> +CONFIG_PARAVIRT enabled. This can't bring any benifit on bare mental
>> +even with CONFIG_PARAVIRT enabled.
>> +
>> +By default, this value is 0 means not to poll. Possible values to set
>> +are in range {0..500000}. Change the value to non-zero if running
>> +latency-bound workloads in a virtual machine.
> I absolutely hate it how this hybrid idle loop polling mechanism is not
> self-tuning!

Ingo, actually it is self-tuning..
> Please make it all work fine by default, and automatically so, instead of adding
> three random parameters...
.. I will make it all fine by default. howerver cloud environment is of 
diversity,

could I only leave paravirt_poll_threshold_ns parameter (the maximum 
poll time),
which is as similar as "adaptive halt-polling" Wanpeng mentioned.. then 
user can turn
it off, or find an appropriate threshold for some odd scenario..

thanks for your comments!!
Quan
Alibaba Cloud
> And if it cannot be done automatically then we should rather not do it at all.
> Maybe the next submitter of a similar feature can think of a better approach.
>
> Thanks,
>
> 	Ingo
>