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

[Gautham R Shenoy]

On Thu, Apr 15, 2021 at 05:39:34PM +0530, Srikar Dronamraju wrote:
> On systems with large CPUs per node, even with the filtered matching of
> related CPUs, there can be large number of calls to cpu_to_chip_id for
> the same CPU. For example with 4096 vCPU, 1 node QEMU configuration,
> with 4 threads per core, system could be see upto 1024 calls to
> cpu_to_chip_id() for the same CPU. On a given system, cpu_to_chip_id()
> for a given CPU would always return the same. Hence cache the result in
> a lookup table for use in subsequent calls.
> 
> Since all CPUs sharing the same core will belong to the same chip, the
> lookup_table has an entry for one CPU per core.  chip_id_lookup_table is
> not being freed and would be used on subsequent CPU online post CPU
> offline.
> 
> Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: qemu-ppc@nongnu.org
> Cc: Cedric Le Goater <clg@kaod.org>
> Cc: David Gibson <david@gibson.dropbear.id.au>
> Cc: Nathan Lynch <nathanl@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>
> Reported-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/smp.h |  1 +
>  arch/powerpc/kernel/prom.c     | 19 +++++++++++++++----
>  arch/powerpc/kernel/smp.c      | 21 +++++++++++++++++++--
>  3 files changed, 35 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/smp.h b/arch/powerpc/include/asm/smp.h
> index 47081a9e13ca..03b3d010cbab 100644
> --- a/arch/powerpc/include/asm/smp.h
> +++ b/arch/powerpc/include/asm/smp.h
> @@ -31,6 +31,7 @@ extern u32 *cpu_to_phys_id;
>  extern bool coregroup_enabled;
> 
>  extern int cpu_to_chip_id(int cpu);
> +extern int *chip_id_lookup_table;
> 
>  #ifdef CONFIG_SMP
> 
> diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
> index 9a4797d1d40d..6d2e4a5bc471 100644
> --- a/arch/powerpc/kernel/prom.c
> +++ b/arch/powerpc/kernel/prom.c
> @@ -65,6 +65,8 @@
>  #define DBG(fmt...)
>  #endif
> 
> +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.

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 ?


> +             return chip_id_lookup_table[idx];
> 
>       np = of_get_cpu_node(cpu, NULL);
> -     if (!np)
> -             return -1;
> +     if (np) {
> +             ret = of_get_ibm_chip_id(np);
> +             of_node_put(np);
> +
> +             if (chip_id_lookup_table)
> +                     chip_id_lookup_table[idx] = ret;
> +     }
> 
> -     of_node_put(np);
> -     return of_get_ibm_chip_id(np);
> +     return ret;
>  }
>  EXPORT_SYMBOL(cpu_to_chip_id);
> 
> diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> index 5c7ce1d50631..50520fbea424 100644
> --- a/arch/powerpc/kernel/smp.c
> +++ b/arch/powerpc/kernel/smp.c
> @@ -1073,6 +1073,20 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
>                               cpu_smallcore_mask(boot_cpuid));
>       }
> 
> +     if (cpu_to_chip_id(boot_cpuid) != -1) {
> +             int idx = num_possible_cpus() / threads_per_core;
> +
> +             /*
> +              * All threads of a core will all belong to the same core,
> +              * chip_id_lookup_table will have one entry per core.
> +              * Assumption: if boot_cpuid doesn't have a chip-id, then no
> +              * other CPUs, will also not have chip-id.
> +              */
> +             chip_id_lookup_table = kcalloc(idx, sizeof(int), GFP_KERNEL);
> +             if (chip_id_lookup_table)
> +                     memset(chip_id_lookup_table, -1, sizeof(int) * idx);
> +     }
> +
>       if (smp_ops && smp_ops->probe)
>               smp_ops->probe();
>  }
> @@ -1468,8 +1482,8 @@ static void add_cpu_to_masks(int cpu)
>  {
>       struct cpumask *(*submask_fn)(int) = cpu_sibling_mask;
>       int first_thread = cpu_first_thread_sibling(cpu);
> -     int chip_id = cpu_to_chip_id(cpu);
>       cpumask_var_t mask;
> +     int chip_id = -1;
>       bool ret;
>       int i;
> 
> @@ -1492,7 +1506,10 @@ static void add_cpu_to_masks(int cpu)
>       if (has_coregroup_support())
>               update_coregroup_mask(cpu, &mask);
> 
> -     if (chip_id == -1 || !ret) {
> +     if (chip_id_lookup_table && ret)
> +             chip_id = cpu_to_chip_id(cpu);
> +
> +     if (chip_id == -1) {
>               cpumask_copy(per_cpu(cpu_core_map, cpu), cpu_cpu_mask(cpu));
>               goto out;
>       }
> -- 
> 2.25.1
>