linux-kernel - Re: [PATCH 2/2] powerpc: Enable ASYM_SMT on interleaved big-core systems

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Message-ID: <87muwxxgei.fsf@concordia.ellerman.id.au>
Date:   Fri, 18 May 2018 23:05:57 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Michael Neuling <mikey@...ling.org>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.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>,
        Balbir Singh <bsingharora@...il.com>,
        Oliver O'Halloran <oohall@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] powerpc: Enable ASYM_SMT on interleaved big-core systems

Gautham R Shenoy <ego@...ux.vnet.ibm.com> writes:

> On Mon, May 14, 2018 at 01:22:07PM +1000, Michael Neuling wrote:
>> On Fri, 2018-05-11 at 16:47 +0530, Gautham R. Shenoy wrote:
>> > From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
>> > 
>> > Each of the SMT4 cores forming a fused-core are more or less
>> > independent units. Thus when multiple tasks are scheduled to run on
>> > the fused core, we get the best performance when the tasks are spread
>> > across the pair of SMT4 cores.
>> > 
>> > Since the threads in the pair of SMT4 cores of an interleaved big-core
>> > are numbered {0,2,4,6} and {1,3,5,7} respectively, enable ASYM_SMT on
>> > such interleaved big-cores that will bias the load-balancing of tasks
>> > on smaller numbered threads, which will automatically result in
>> > spreading the tasks uniformly across the associated pair of SMT4
>> > cores.
>> > 
>> > Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
>> > ---
>> >  arch/powerpc/kernel/smp.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> > 
>> > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
>> > index 9ca7148..0153f01 100644
>> > --- a/arch/powerpc/kernel/smp.c
>> > +++ b/arch/powerpc/kernel/smp.c
>> > @@ -1082,7 +1082,7 @@ static int powerpc_smt_flags(void)
>> >  {
>> >  	int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
>> >  
>> > -	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
>> > +	if (cpu_has_feature(CPU_FTR_ASYM_SMT) || has_interleaved_big_core) {
>> 
>> Shouldn't we just set CPU_FTR_ASYM_SMT and leave this code
> unchanged?
>
> Yes, that would have the same effect. I refrained from doing that
> since I thought CPU_FTR_ASYM_SMT has the "lower numbered threads
> expedite thread-folding" connotation from the POWER7 generation.

The above code is the only use of the feature, so I don't think we need
to worry about any other connotations.

> If it is ok to overload CPU_FTR_ASYM_SMT, we can do what you suggest
> and have all the changes in setup-common.c

Yeah let's do that.

cheers