linux-kernel - Re: [patch V2 08/28] sched/smt: Make sched_smt

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Message-ID: <20181129154808.GT2131@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Nov 2018 16:48:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch V2 08/28] sched/smt: Make sched_smt_present track topology

On Thu, Nov 29, 2018 at 09:50:13AM -0500, Konrad Rzeszutek Wilk wrote:
> On Thu, Nov 29, 2018 at 09:42:56AM -0500, Konrad Rzeszutek Wilk wrote:
> > On Sun, Nov 25, 2018 at 07:33:36PM +0100, Thomas Gleixner wrote:
> > > Currently the 'sched_smt_present' static key is enabled when at CPU bringup
> > > SMT topology is observed, but it is never disabled. However there is demand
> > > to also disable the key when the topology changes such that there is no SMT
> > > present anymore.
> > > 
> > > Implement this by making the key count the number of cores that have SMT
> > > enabled.
> > > 
> > > In particular, the SMT topology bits are set before interrrupts are enabled
> > > and similarly, are cleared after interrupts are disabled for the last time
> > > and the CPU dies.
> > 
> > I see that the number you used is '2', but I thought that there are some
> > CPUs out there (Knights Landing?) that could have four threads?

This code is generic, Sparc, Power and I think MIPS all have parts with
SMT8.

> > Would it be better to have a generic function that would provide the
> > amount of threads the platform does expose - and use that instead
> > of a constant value? 
> 
> Nevermind - this would work even with 4 threads as we would hit the
> number '2' before '4' and the key would be turned on/off properly.

Indeed so, 2 is the point where we either have more than 1 sibling, or
will go back to 1.