From: Marcelo Tosatti <mtosatti@redhat.com>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
"Radim Krčmář" <rkrcmar@redhat.com>,
"Linux PM" <linux-pm@vger.kernel.org>
Subject: Re: [PATCH] cpuidle-haltpoll: Enable kvm guest polling when dedicated physical CPUs are available
Date: Mon, 26 Aug 2019 17:40:50 -0300 [thread overview]
Message-ID: <20190826204045.GA24697@amt.cnet> (raw)
In-Reply-To: <CANRm+CwtHBOVWFcn+6Z3Ds7dEcNL2JP+b6hLRS=oeUW98A24MQ@mail.gmail.com>
On Tue, Aug 13, 2019 at 08:55:29AM +0800, Wanpeng Li wrote:
> On Sun, 4 Aug 2019 at 04:21, Marcelo Tosatti <mtosatti@redhat.com> wrote:
> >
> > On Thu, Aug 01, 2019 at 06:54:49PM +0200, Paolo Bonzini wrote:
> > > On 01/08/19 18:51, Rafael J. Wysocki wrote:
> > > > On 8/1/2019 9:06 AM, Wanpeng Li wrote:
> > > >> From: Wanpeng Li <wanpengli@tencent.com>
> > > >>
> > > >> The downside of guest side polling is that polling is performed even
> > > >> with other runnable tasks in the host. However, even if poll in kvm
> > > >> can aware whether or not other runnable tasks in the same pCPU, it
> > > >> can still incur extra overhead in over-subscribe scenario. Now we can
> > > >> just enable guest polling when dedicated pCPUs are available.
> > > >>
> > > >> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > >> Cc: Radim Krčmář <rkrcmar@redhat.com>
> > > >> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > > >> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > > >
> > > > Paolo, Marcelo, any comments?
> > >
> > > Yes, it's a good idea.
> > >
> > > Acked-by: Paolo Bonzini <pbonzini@redhat.com>
> > >
> > > Paolo
> >
>
> Hi Marcelo,
>
> Sorry for the late response.
>
> > I think KVM_HINTS_REALTIME is being abused somewhat.
> > It has no clear meaning and used in different locations
> > for different purposes.
>
> ================== ============ =================================
> KVM_HINTS_REALTIME 0 guest checks this feature bit to
>
> determine that vCPUs are never
>
> preempted for an unlimited time
Unlimited time means infinite time, or unlimited time means
10s ? 1s ?
The previous definition was much better IMO: HINTS_DEDICATED.
> allowing optimizations
> ================== ============ =================================
>
> Now it disables pv queued spinlock,
OK.
> pv tlb shootdown,
OK.
> pv sched yield
"The idea is from Xen, when sending a call-function IPI-many to vCPUs,
yield if any of the IPI target vCPUs was preempted. 17% performance
increasement of ebizzy benchmark can be observed in an over-subscribe
environment. (w/ kvm-pv-tlb disabled, testing TLB flush call-function
IPI-many since call-function is not easy to be trigged by userspace
workload)."
This can probably hurt if vcpus are rarely preempted.
> which are not expected present in vCPUs are never preempted for an
> unlimited time scenario.
>
> >
> > For example, i think that using pv queued spinlocks and
> > haltpoll is a desired scenario, which the patch below disallows.
>
> So even if dedicated pCPU is available, pv queued spinlocks should
> still be chose if something like vhost-kthreads are used instead of
> DPDK/vhost-user.
Can't you enable the individual features you need for optimizing
the overcommitted case? This is how things have been done historically:
If a new feature is available, you enable it to get the desired
performance. x2apic, invariant-tsc, cpuidle haltpoll...
So in your case: enable pv schedyield, enable pv tlb shootdown.
> kvm adaptive halt-polling will compete with
> vhost-kthreads, however, poll in guest unaware other runnable tasks in
> the host which will defeat vhost-kthreads.
It depends on how much work vhost-kthreads needs to do, how successful
halt-poll in the guest is, and what improvement halt-polling brings.
The amount of polling will be reduced to zero if polling
is not successful.
next prev parent reply other threads:[~2019-08-26 20:42 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1564643196-7797-1-git-send-email-wanpengli@tencent.com>
2019-08-01 16:51 ` [PATCH] cpuidle-haltpoll: Enable kvm guest polling when dedicated physical CPUs are available Rafael J. Wysocki
2019-08-01 16:54 ` Paolo Bonzini
2019-08-03 20:21 ` Marcelo Tosatti
2019-08-13 0:55 ` Wanpeng Li
2019-08-13 7:55 ` Paolo Bonzini
2019-08-26 20:40 ` Marcelo Tosatti [this message]
2019-08-27 0:43 ` Wanpeng Li
2019-08-28 8:35 ` Wanpeng Li
2019-08-28 8:45 ` Rafael J. Wysocki
2019-08-28 8:48 ` Wanpeng Li
2019-08-28 14:39 ` Marcelo Tosatti
2019-08-28 23:37 ` Rafael J. Wysocki
2019-08-29 12:04 ` Marcelo Tosatti
2019-08-29 12:16 ` Wanpeng Li
2019-08-29 12:53 ` Marcelo Tosatti
2019-08-29 14:39 ` Marcelo Tosatti
2019-08-28 14:48 ` Marcelo Tosatti
2019-08-28 14:52 ` Marcelo Tosatti
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190826204045.GA24697@amt.cnet \
--to=mtosatti@redhat.com \
--cc=kernellwp@gmail.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-pm@vger.kernel.org \
--cc=pbonzini@redhat.com \
--cc=rafael.j.wysocki@intel.com \
--cc=rkrcmar@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).