Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 17 Sep 2018 14:25:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jan =?iso-8859-1?Q?H=2E_Sch=F6nherr?= <jschoenh@amazon.de>
Cc:     Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Paul Turner <pjt@google.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [RFC 00/60] Coscheduling for Linux
Message-ID: <20180917122538.GT24124@hirez.programming.kicks-ass.net>
References: <20180907214047.26914-1-jschoenh@amazon.de>
 <20180914111251.GC24106@hirez.programming.kicks-ass.net>
 <1d86f497-9fef-0b19-50d6-d46ef1c0bffa@amazon.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <1d86f497-9fef-0b19-50d6-d46ef1c0bffa@amazon.de>
User-Agent: Mutt/1.10.0 (2018-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Sep 14, 2018 at 06:25:44PM +0200, Jan H. Sch?nherr wrote:

> Assuming, there is a cgroup-less solution that can prevent simultaneous
> execution of tasks on a core, when they're not supposed to. How would you
> tell the scheduler, which tasks these are?

Specifically for L1TF I hooked into/extended KVM's preempt_notifier
registration interface, which tells us which tasks are VCPUs and to
which VM they belong.

But if we want to actually expose this to userspace, we can either do a
prctl() or extend struct sched_attr.

> >> 1. Execute parallel applications that rely on active waiting or synchronous
> >>    execution concurrently with other applications.
> >>
> >>    The prime example in this class are probably virtual machines. Here,
> >>    coscheduling is an alternative to paravirtualized spinlocks, pause loop
> >>    exiting, and other techniques with its own set of advantages and
> >>    disadvantages over the other approaches.
> > 
> > Note that in order to avoid PLE and paravirt spinlocks and paravirt
> > tlb-invalidate you have to gang-schedule the _entire_ VM, not just SMT
> > siblings.
> > 
> > Now explain to me how you're going to gang-schedule a VM with a good
> > number of vCPU threads (say spanning a number of nodes) and preserving
> > the rest of CFS without it turning into a massive trainwreck?
> 
> You probably don't -- for the same reason, why it is a bad idea to give
> an endless loop realtime priority. It's just a bad idea. As I said in the
> text you quoted: coscheduling comes with its own set of advantages and
> disadvantages. Just because you find one example, where it is a bad idea,
> doesn't make it a bad thing in general.

Well, you mentioned it as an alternative to paravirt spinlocks -- I'm
saying that co-scheduling cannot do that, you need full featured
gang-scheduling for that.