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Date:   Fri, 26 Apr 2019 16:13:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Phil Auld <pauld@redhat.com>
Cc:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com,
        fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH v2 12/17] sched: A quick and dirty cgroup tagging
 interface
Message-ID: <20190426141307.GB2623@hirez.programming.kicks-ass.net>
References: <cover.1556025155.git.vpillai@digitalocean.com>
 <b058b74f303fe40e5822925c90a5b0161c4a0a2d.1556025155.git.vpillai@digitalocean.com>
 <20190425142652.GA979@pauld.bos.csb>
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On Thu, Apr 25, 2019 at 10:26:53AM -0400, Phil Auld wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e8e5f26db052..b312ea1e28a4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7541,6 +7541,9 @@ static int cpu_core_tag_write_u64(struct cgroup_subsys_state *css, struct cftype
>         if (val > 1)
>                 return -ERANGE;
>  
> +       if (num_online_cpus() <= 1) 
> +               return -EINVAL;

We actually know if there SMT on the system or not, which is much better
indication still:

	if (!static_branch_likely(&sched_smt_present))
		return -EINVAL;

>         if (tg->tagged == !!val)
>                 return 0;
>  
> 
> 
> 
> --