linux-kernel - Re: [PATCH] sched: introduce configurable delay before entering idle

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Message-ID: <CANRm+CytV7PfS++RnYU0P3HT_QBufrO=bzd6Fx-7dC2=sotvmA@mail.gmail.com>
Date:   Wed, 15 May 2019 09:42:48 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        kvm-devel <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Bandan Das <bsd@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] sched: introduce configurable delay before entering idle

On Wed, 15 May 2019 at 02:20, Marcelo Tosatti <mtosatti@...hat.com> wrote:
>
> On Tue, May 14, 2019 at 11:20:15AM -0400, Konrad Rzeszutek Wilk wrote:
> > On Tue, May 14, 2019 at 10:50:23AM -0300, Marcelo Tosatti wrote:
> > > On Mon, May 13, 2019 at 05:20:37PM +0800, Wanpeng Li wrote:
> > > > On Wed, 8 May 2019 at 02:57, Marcelo Tosatti <mtosatti@...hat.com> wrote:
> > > > >
> > > > >
> > > > > Certain workloads perform poorly on KVM compared to baremetal
> > > > > due to baremetal's ability to perform mwait on NEED_RESCHED
> > > > > bit of task flags (therefore skipping the IPI).
> > > >
> > > > KVM supports expose mwait to the guest, if it can solve this?
> > > >
> > > > Regards,
> > > > Wanpeng Li
> > >
> > > Unfortunately mwait in guest is not feasible (uncompatible with multiple
> > > guests). Checking whether a paravirt solution is possible.
> >
> > There is the obvious problem with that the guest can be malicious and
> > provide via the paravirt solution bogus data. That is it expose 0% CPU
> > usage but in reality be mining and using 100%.
>
> The idea is to have a hypercall for the guest to perform the
> need_resched=1 bit set. It can only hurt itself.

This lets me recall the patchset from aliyun
https://lkml.org/lkml/2017/6/22/296 They poll after
__current_set_polling() in do_idle() so avoid this hypercall I think.
Btw, do you get SAP HANA by 5-10% bonus even if adaptive halt-polling
is enabled?

Regards,
Wanpeng Li