linux-kernel - Re: [PATCH 1/2] powerpc: Detect the presence of big-core with interleaved threads

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Message-Id: <20180522043119.GA5213@in.ibm.com>
Date:   Tue, 22 May 2018 10:01:19 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Michael Neuling <mikey@...ling.org>,
        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 1/2] powerpc: Detect the presence of big-core with
 interleaved threads

Hello Michael,

On Fri, May 18, 2018 at 11:14:04PM +1000, Michael Ellerman wrote:
> Gautham R Shenoy <ego@...ux.vnet.ibm.com> writes:
> ...
> >> > @@ -565,7 +615,16 @@ void __init smp_setup_cpu_maps(void)
> >> >  	vdso_data->processorCount = num_present_cpus();
> >> >  #endif /* CONFIG_PPC64 */
> >> >  
> >> > -        /* Initialize CPU <=> thread mapping/
> >> > +	dn = of_find_node_by_type(NULL, "cpu");
> >> > +	if (dn) {
> >> > +		if (check_for_interleaved_big_core(dn)) {
> >> > +			has_interleaved_big_core = true;
> >> > +			pr_info("Detected interleaved big-cores\n");
> >> 
> >> Is there a runtime way to check this also?  If the dmesg buffer overflows, we
> >> lose this.
> >
> > Where do you suggest we put this ? Should it be a part of
> > /proc/cpuinfo ?
> 
> Hmm, it'd be nice not to pollute it with more junk.
> 
> Can you just look at the pir files in sysfs?

Sure Michael. I will explore this option.

If we add a file called l1cache_thread_group, then the siblings of the
big-core that share the L1-cache can be described as follows.

	# cd  /sys/devices/system/cpu
	# grep . cpu[0-7]/l1cache_thread_group
	cpu0/l1cache_thread_group:0,2,4,6
	cpu1/l1cache_thread_group:1,3,5,7
	cpu2/l1cache_thread_group:0,2,4,6
	cpu3/l1cache_thread_group:1,3,5,7
	cpu4/l1cache_thread_group:0,2,4,6
	cpu5/l1cache_thread_group:1,3,5,7
	cpu6/l1cache_thread_group:0,2,4,6
	cpu7/l1cache_thread_group:1,3,5,7

> 
> eg. on a normal system:
> 
>   # cd /sys/devices/system/cpu
>   # grep . cpu[0-7]/pir
>   cpu0/pir:20
>   cpu1/pir:21
>   cpu2/pir:22
>   cpu3/pir:23
>   cpu4/pir:24
>   cpu5/pir:25
>   cpu6/pir:26
>   cpu7/pir:27


> 
> 
> cheers
>