linux-kernel - Re: [v2 PATCH 2/2] powerpc: Enable CPU_FTR_ASYM

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Date:   Wed, 4 Jul 2018 13:45:05 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Neuling <mikey@...ling.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 2/2] powerpc: Enable CPU_FTR_ASYM_SMT for interleaved
 big-cores

Hi Murilo,

Thanks for the review.

On Tue, Jul 03, 2018 at 02:53:46PM -0300, Murilo Opsfelder Araujo wrote:
[..snip..]

> > -        /* Initialize CPU <=> thread mapping/
> > +	if (has_interleaved_big_core) {
> > +		int key = __builtin_ctzl(CPU_FTR_ASYM_SMT);
> > +
> > +		cur_cpu_spec->cpu_features |= CPU_FTR_ASYM_SMT;
> > +		static_branch_enable(&cpu_feature_keys[key]);
> > +		pr_info("Detected interleaved big-cores\n");
> > +	}
> 
> Shouldn't we use cpu_has_feature(CPU_FTR_ASYM_SMT) before setting
> > it?


Are you suggesting that we do the following?

    if (has_interleaved_big_core &&
        !cpu_has_feature(CPU_FTR_ASYM_SMT)) {
	...
    }
    
Currently CPU_FTR_ASYM_SMT is set at compile time for only POWER7
where running the tasks on lower numbered threads give us the benefit
of SMT thread folding. Interleaved big core is a feature introduced
only on POWER9. Thus, we know that CPU_FTR_ASYM_SMT is not set in
cpu_features at this point.

> 
> > +
> > +	/* Initialize CPU <=> thread mapping/
> >  	 *
> >  	 * WARNING: We assume that the number of threads is the same for
> >  	 * every CPU in the system. If that is not the case, then some code
> > -- 
> > 1.9.4
> > 
> 
> -- 
> Murilo

--
Thanks and Regards
gautham.