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 08:44:10 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Quan Xu <quan.xu0@...il.com>
Cc:     Quan Xu <quan.xu04@...il.com>, 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


* Quan Xu <quan.xu0@...il.com> wrote:

> 
> 
> 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..

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