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

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Date:   Wed, 4 Jul 2018 14:36:29 -0300
From:   Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
To:     Gautham R Shenoy <ego@...ux.vnet.ibm.com>
Cc:     Michael Neuling <mikey@...ling.org>, linux-kernel@...r.kernel.org,
        Nicholas Piggin <npiggin@...il.com>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Akshay Adiga <akshay.adiga@...ux.vnet.ibm.com>
Subject: Re: [v2 PATCH 2/2] powerpc: Enable CPU_FTR_ASYM_SMT for interleaved
 big-cores

On Wed, Jul 04, 2018 at 01:45:05PM +0530, Gautham R Shenoy wrote:
> 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.

Since we're setting CPU_FTR_ASYM_SMT, it doesn't make sense to use
cpu_has_feature(CPU_FTR_ASYM_SMT).  I thought cpu_has_feature() held all
available features (not necessarily enabled) that we could check before
setting or enabling such feature.  I think I misread it.  Sorry.

> 
> > 
> > > +
> > > +	/* 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.
> 

-- 
Murilo