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

[Quan Xu]

On 2017/11/13 23:08, Ingo Molnar wrote:
> * Quan Xu <quan.xu04@xxxxxxxxx> wrote:
>
> > From: Quan Xu <quan.xu0@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > Signed-off-by: Quan Xu <quan.xu0@xxxxxxxxx>
> > ---
> >   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