Re: [Xen-devel] [PATCH v10 01/13] x86: add socket_cpumask

[Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>]

On Tue, Jul 07, 2015 at 06:32:55PM -0400, Boris Ostrovsky wrote:
> On 06/26/2015 04:43 AM, Chao Peng wrote:
> >Maintain socket_cpumask which contains all the HT and core siblings
> >in the same socket.
> >
> >Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> >Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> >---
> >Changes in v9:
> >* Add comments for set_nr_sockets.
> >* Move set_nr_sockets() invocation from __start_xen() to smp_prepare_cpus().
> >Changes in v8:
> >* Remove total_cpus and retrofit the algorithm for calculating nr_sockets.
> >* Change per-socket cpumask allocation as on demand.
> >* socket_to_cpumask => socket_cpumask.
> >Changes in v7:
> >* Introduce total_cpus to calculate nr_sockets.
> >* Minor code sequence improvement in set_cpu_sibling_map.
> >* Improve comments for nr_sockets.
> >---
> >  xen/arch/x86/mpparse.c    | 17 +++++++++++++++++
> >  xen/arch/x86/smpboot.c    | 26 +++++++++++++++++++++++++-
> >  xen/include/asm-x86/smp.h | 11 +++++++++++
> >  3 files changed, 53 insertions(+), 1 deletion(-)
> >
> >diff --git a/xen/arch/x86/mpparse.c b/xen/arch/x86/mpparse.c
> >index 003c56e..8609f4a 100644
> >--- a/xen/arch/x86/mpparse.c
> >+++ b/xen/arch/x86/mpparse.c
> >@@ -87,6 +87,23 @@ void __init set_nr_cpu_ids(unsigned int max_cpus)
> >  #endif
> >  }
> >+void __init set_nr_sockets(void)
> >+{
> >+    /*
> >+     * Count the actual cpus in the socket 0 and use it to calculate 
> >nr_sockets
> >+     * so that the latter will be always >= the actual socket number in the
> >+     * system even when APIC IDs from MP table are too sparse.
> >+     */
> >+    unsigned int cpus = bitmap_weight(phys_cpu_present_map.mask,
> >+                                      boot_cpu_data.x86_max_cores *
> >+                                      boot_cpu_data.x86_num_siblings);
> >+
> >+    if ( cpus == 0 )
> >+        cpus = 1;
> >+
> >+    nr_sockets = DIV_ROUND_UP(num_processors + disabled_cpus, cpus);
> >+}
> >+
> >  /*
> >   * Intel MP BIOS table parsing routines:
> >   */
> >diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
> >index 2289284..e75bbd3 100644
> >--- a/xen/arch/x86/smpboot.c
> >+++ b/xen/arch/x86/smpboot.c
> >@@ -60,6 +60,9 @@ DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_core_mask);
> >  cpumask_t cpu_online_map __read_mostly;
> >  EXPORT_SYMBOL(cpu_online_map);
> >+unsigned int __read_mostly nr_sockets;
> >+cpumask_var_t *__read_mostly socket_cpumask;
> >+
> >  struct cpuinfo_x86 cpu_data[NR_CPUS];
> >  u32 x86_cpu_to_apicid[NR_CPUS] __read_mostly =
> >@@ -245,6 +248,8 @@ static void set_cpu_sibling_map(int cpu)
> >      cpumask_set_cpu(cpu, &cpu_sibling_setup_map);
> >+    cpumask_set_cpu(cpu, socket_cpumask[cpu_to_socket(cpu)]);
> 
> This patch crashes Xen on my 32-cpu Intel box here for cpu 16, which is the
> first CPU on the second socket (i.e. on socket 1).
> 
> The reason appears to be that cpu_to_socket(16) is (correctly) 1 here, but
> ...
> 
> >+
> >      if ( c[cpu].x86_num_siblings > 1 )
> >      {
> >          for_each_cpu ( i, &cpu_sibling_setup_map )
> >@@ -649,7 +654,13 @@ void cpu_exit_clear(unsigned int cpu)
> >  static void cpu_smpboot_free(unsigned int cpu)
> >  {
> >-    unsigned int order;
> >+    unsigned int order, socket = cpu_to_socket(cpu);
> >+
> >+    if ( cpumask_empty(socket_cpumask[socket]) )
> >+    {
> >+        free_cpumask_var(socket_cpumask[socket]);
> >+        socket_cpumask[socket] = NULL;
> >+    }
> >      free_cpumask_var(per_cpu(cpu_sibling_mask, cpu));
> >      free_cpumask_var(per_cpu(cpu_core_mask, cpu));
> >@@ -694,6 +705,7 @@ static int cpu_smpboot_alloc(unsigned int cpu)
> >      nodeid_t node = cpu_to_node(cpu);
> >      struct desc_struct *gdt;
> >      unsigned long stub_page;
> >+    unsigned int socket = cpu_to_socket(cpu);
> 
> ... is zero here, meaning that socket_cpumask[1] is NULL. I suspect that
> phys_proc_id is probably not set at this point but is by the time we get to
> set_cpu_sibling_map(). I haven't looked any further yet. I might do this
> tomorrow unless Chao does it before me.

Thanks for testing.

I think I have found the reason. For AP, phys_proc_id is set in:
start_secondary()=>smp_callin()=>smp_store_cpu_info()=>identify_cpu()
which is behind cpu_smpboot_alloc() called from CPU_PREPARE.

One way would move 'zalloc_cpumask_var(socket_cpumask + socket)' to
set_cpu_sibling_map() to fix it if Jan agrees that, otherwise other
solution needs to be found.

Chao

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