Linux Virtualization - Nov 2017 - [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 at gmail.com> wrote:
>
>> From: Quan Xu <quan.xu0 at gmail.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 at gmail.com>
>> Signed-off-by: Quan Xu <quan.xu0 at gmail.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
>

* Quan Xu <quan.xu0 at gmail.com> wrote:
> 
> 
> On 2017/11/13 23:08, Ingo Molnar wrote:
> > * Quan Xu <quan.xu04 at gmail.com> wrote:
> > 
> > > From: Quan Xu <quan.xu0 at gmail.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 at gmail.com>
> > > Signed-off-by: Quan Xu <quan.xu0 at gmail.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..
Then why the hell does it touch the syscall ABI?
> 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..
That way lies utter madness. Maybe add it as a debugfs knob, but exposing it to 
userspace: NAK.

Thanks,

	Ingo