linux-kernel - Re: [PATCH -tip 14/32] sched: migration changes for core scheduling

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Date:   Tue, 24 Nov 2020 16:42:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Balbir Singh <bsingharora@...il.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
        pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...el.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 14/32] sched: migration changes for core scheduling

On Mon, Nov 23, 2020 at 12:36:10PM +0800, Li, Aubrey wrote:
> >> +#ifdef CONFIG_SCHED_CORE
> >> +		/*
> >> +		 * Skip this cpu if source task's cookie does not match
> >> +		 * with CPU's core cookie.
> >> +		 */
> >> +		if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
> >> +			continue;
> >> +#endif
> >> +
> > 
> > Any reason this is under an #ifdef? In sched_core_cookie_match() won't
> > the check for sched_core_enabled() do the right thing even when
> > CONFIG_SCHED_CORE is not enabed?> 
> Yes, sched_core_enabled works properly when CONFIG_SCHED_CORE is not
> enabled. But when CONFIG_SCHED_CORE is not enabled, it does not make
> sense to leave a core scheduler specific function here even at compile
> time. Also, for the cases in hot path, this saves CPU cycles to avoid
> a judgment.

No, that's nonsense. If it works, remove the #ifdef. Less (#ifdef) is
more.

> >> +static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
> >> +{
> >> +	bool idle_core = true;
> >> +	int cpu;
> >> +
> >> +	/* Ignore cookie match if core scheduler is not enabled on the CPU. */
> >> +	if (!sched_core_enabled(rq))
> >> +		return true;
> >> +
> >> +	for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
> >> +		if (!available_idle_cpu(cpu)) {
> > 
> > I was looking at this snippet and comparing this to is_core_idle(), the
> > major difference is the check for vcpu_is_preempted(). Do we want to
> > call the core as non idle if any vcpu was preempted on this CPU?
> 
> Yes, if there is a VCPU was preempted on this CPU, better not place task
> on this core as the VCPU may be holding a spinlock and wants to be executed
> again ASAP.

If you're doing core scheduling on vcpus, you deserve all the pain
possible.