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

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Date:   Tue, 14 May 2019 14:42:37 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Wanpeng Li <kernellwp@...il.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 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.