linux-kernel - Re: [RFC][PATCH 00/16] sched: Core scheduling

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Date:   Thu, 21 Feb 2019 15:03:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Subhra Mazumdar <subhra.mazumdar@...cle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>, kerrnel@...gle.com
Subject: Re: [RFC][PATCH 00/16] sched: Core scheduling

On Wed, Feb 20, 2019 at 06:53:08PM -0800, Subhra Mazumdar wrote:
> 
> On 2/18/19 9:49 AM, Linus Torvalds wrote:
> > On Mon, Feb 18, 2019 at 9:40 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > However; whichever way around you turn this cookie; it is expensive and nasty.
> > Do you (or anybody else) have numbers for real loads?
> > 
> > Because performance is all that matters. If performance is bad, then
> > it's pointless, since just turning off SMT is the answer.
> > 
> >                    Linus
> I tested 2 Oracle DB instances running OLTP on a 2 socket 44 cores system.
> This is on baremetal, no virtualization.  

I'm thinking oracle schedules quite a bit, right? Then you get massive
overhead (as shown).

The thing with virt workloads is that if they don't VMEXIT lots, they
also don't schedule lots (the vCPU stays running, nested scheduler
etc..).

Also; like I wrote, it is quite possible there is some sibling rivalry
here, which can cause excessive rescheduling. Someone would have to
trace a workload and check.

My older patches had a condition that would not preempt a task for a
little while, such that it might make _some_ progress, these patches
don't have that (yet).