Re: [PATCH 3/3] powerpc/smp: Cache CPU to chip lookup

From: Gautham R Shenoy <ego@linux.vnet.ibm.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Nathan Lynch <nathanl@linux.ibm.com>,
	Gautham R Shenoy <ego@linux.vnet.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Daniel Henrique Barboza <danielhb413@gmail.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	qemu-ppc@nongnu.org, Cedric Le Goater <clg@kaod.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Ingo Molnar <mingo@kernel.org>,
	David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [PATCH 3/3] powerpc/smp: Cache CPU to chip lookup
Date: Fri, 16 Apr 2021 21:27:48 +0530	[thread overview]
Message-ID: <20210416155748.GA26496@in.ibm.com> (raw)
In-Reply-To: <20210415175110.GE2633526@linux.vnet.ibm.com>

On Thu, Apr 15, 2021 at 11:21:10PM +0530, Srikar Dronamraju wrote:
> * Gautham R Shenoy <ego@linux.vnet.ibm.com> [2021-04-15 22:49:21]:
> 
> > > 
> > > +int *chip_id_lookup_table;
> > > +
> > >  #ifdef CONFIG_PPC64
> > >  int __initdata iommu_is_off;
> > >  int __initdata iommu_force_on;
> > > @@ -914,13 +916,22 @@ EXPORT_SYMBOL(of_get_ibm_chip_id);
> > >  int cpu_to_chip_id(int cpu)
> > >  {
> > >  	struct device_node *np;
> > > +	int ret = -1, idx;
> > > +
> > > +	idx = cpu / threads_per_core;
> > > +	if (chip_id_lookup_table && chip_id_lookup_table[idx] != -1)
> > 
> 
> > The value -1 is ambiguous since we won't be able to determine if
> > it is because we haven't yet made a of_get_ibm_chip_id() call
> > or if of_get_ibm_chip_id() call was made and it returned a -1.
> > 
> 
> We don't allocate chip_id_lookup_table unless cpu_to_chip_id() return
> !-1 value for the boot-cpuid. So this ensures that we dont
> unnecessarily allocate chip_id_lookup_table. Also I check for
> chip_id_lookup_table before calling cpu_to_chip_id() for other CPUs.
> So this avoids overhead of calling cpu_to_chip_id() for platforms that
> dont support it.  Also its most likely that if the
> chip_id_lookup_table is initialized then of_get_ibm_chip_id() call
> would return a valid value.
> 
> + Below we are only populating the lookup table, only when the
> of_get_cpu_node is valid.
> 
> So I dont see any drawbacks of initializing it to -1. Do you see
any?

Only if other callers of cpu_to_chip_id() don't check for whether the
chip_id_lookup_table() has been allocated or not. From a code
readability point of view, it is easier to have that check  this inside
cpu_to_chip_id() instead of requiring all its callers to make that
check.

> 
> > Thus, perhaps we can initialize chip_id_lookup_table[idx] with a
> > different unique negative value. How about S32_MIN ? and check
> > chip_id_lookup_table[idx] is different here ?
> > 
> 
> I had initially initialized to -2, But then I thought we adding in
> more confusion than necessary and it was not solving any issues.
> 
> 
> -- 
> Thanks and Regards
> Srikar Dronamraju