[Xen-changelog] [xen master] x86/smp: use APIC ALLBUT destination shorthand when possible

[patchbot@xxxxxxx]

commit 5500d265a2a8fa63d60c08beb549de8ec82ff7a5
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Wed Jan 22 16:38:39 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Jan 22 16:38:39 2020 +0100

    x86/smp: use APIC ALLBUT destination shorthand when possible
    
    If the IPI destination mask matches the mask of online CPUs use the
    APIC ALLBUT destination shorthand in order to send an IPI to all CPUs
    on the system except the current one. This can only be safely used
    when no CPU hotplug or unplug operations are taking place, no
    offline CPUs or those have been onlined and parked, all CPUs in the
    system have been accounted for (ie: the number of CPUs doesn't exceed
    NR_CPUS and APIC IDs are below MAX_APICS) and there's no possibility
    of CPU hotplug (ie: no disabled CPUs have been reported by the
    firmware tables).
    
    This is specially beneficial when using the PV shim, since using the
    shorthand avoids performing an APIC register write (or multiple ones
    if using xAPIC mode) for each destination when doing a global TLB
    flush.
    
    The lock time of flush_lock on a 32 vCPU guest using the shim in
    x2APIC mode without the shorthand is:
    
    Global lock flush_lock: addr=ffff82d0804b21c0, lockval=f602f602, not locked
      lock:228455938(79406065573135), block:205908580(556416605761539)
    
    Average lock time: 347577ns
    
    While the same guest using the shorthand:
    
    Global lock flush_lock: addr=ffff82d0804b41c0, lockval=d9c4d9bc, cpu=12
      lock:1890775(416719148054), block:1663958(2500161282949)
    
    Average lock time: 220395ns
    
    Approximately a 1/3 improvement in the lock time.
    
    Note that this requires locking the CPU maps (get_cpu_maps) which uses
    a trylock. This is currently safe as all users of cpu_add_remove_lock
    do a trylock, but will need reevaluating if non-trylock users appear.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/acpi/boot.c  |  1 +
 xen/arch/x86/mpparse.c    |  7 ++++++-
 xen/arch/x86/smp.c        | 35 ++++++++++++++++++++++++++++++++++-
 xen/include/asm-x86/smp.h |  3 +++
 4 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/acpi/boot.c b/xen/arch/x86/acpi/boot.c
index 15542a9bdf..afc6ed9d99 100644
--- a/xen/arch/x86/acpi/boot.c
+++ b/xen/arch/x86/acpi/boot.c
@@ -103,6 +103,7 @@ acpi_parse_x2apic(struct acpi_subtable_header *header, 
const unsigned long end)
                               processor->lapic_flags & ACPI_MADT_ENABLED
                               ? KERN_WARNING "WARNING: " : KERN_INFO,
                               processor->local_apic_id, processor->uid);
+               unaccounted_cpus = true;
                /*
                 * Must not return an error here, to prevent
                 * acpi_table_parse_entries() from terminating early.
diff --git a/xen/arch/x86/mpparse.c b/xen/arch/x86/mpparse.c
index f057d9162f..d532575fee 100644
--- a/xen/arch/x86/mpparse.c
+++ b/xen/arch/x86/mpparse.c
@@ -61,11 +61,14 @@ unsigned int __read_mostly boot_cpu_physical_apicid = 
BAD_APICID;
 
 /* Internal processor count */
 static unsigned int num_processors;
-static unsigned int __initdata disabled_cpus;
+unsigned int __read_mostly disabled_cpus;
 
 /* Bitmask of physically existing CPUs */
 physid_mask_t phys_cpu_present_map;
 
+/* Record whether CPUs haven't been added due to overflows. */
+bool __read_mostly unaccounted_cpus;
+
 void __init set_nr_cpu_ids(unsigned int max_cpus)
 {
        unsigned int tot_cpus = num_processors + disabled_cpus;
@@ -160,6 +163,7 @@ static int MP_processor_info_x(struct mpc_config_processor 
*m,
                printk_once(XENLOG_WARNING
                            "WARNING: NR_CPUS limit of %u reached - ignoring 
further processors\n",
                            nr_cpu_ids);
+               unaccounted_cpus = true;
                return -ENOSPC;
        }
 
@@ -167,6 +171,7 @@ static int MP_processor_info_x(struct mpc_config_processor 
*m,
            && genapic.name == apic_default.name) {
                printk_once(XENLOG_WARNING
                            "WARNING: CPUs limit of 8 reached - ignoring futher 
processors\n");
+               unaccounted_cpus = true;
                return -ENOSPC;
        }
 
diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
index c14f304c09..65eb7cbda8 100644
--- a/xen/arch/x86/smp.c
+++ b/xen/arch/x86/smp.c
@@ -8,6 +8,7 @@
  *     later.
  */
 
+#include <xen/cpu.h>
 #include <xen/irq.h>
 #include <xen/sched.h>
 #include <xen/delay.h>
@@ -64,7 +65,39 @@ static void send_IPI_shortcut(unsigned int shortcut, int 
vector,
 
 void send_IPI_mask(const cpumask_t *mask, int vector)
 {
-    alternative_vcall(genapic.send_IPI_mask, mask, vector);
+    bool cpus_locked = false;
+    cpumask_t *scratch = this_cpu(scratch_cpumask);
+
+    /*
+     * This can only be safely used when no CPU hotplug or unplug operations
+     * are taking place, there are no offline CPUs (unless those have been
+     * onlined and parked), there are no disabled CPUs and all possible CPUs in
+     * the system have been accounted for.
+     */
+    if ( system_state > SYS_STATE_smp_boot &&
+         !unaccounted_cpus && !disabled_cpus &&
+         /* NB: get_cpu_maps lock requires enabled interrupts. */
+         local_irq_is_enabled() && (cpus_locked = get_cpu_maps()) &&
+         (park_offline_cpus ||
+          cpumask_equal(&cpu_online_map, &cpu_present_map)) )
+        cpumask_or(scratch, mask, cpumask_of(smp_processor_id()));
+    else
+    {
+        if ( cpus_locked )
+        {
+            put_cpu_maps();
+            cpus_locked = false;
+        }
+        cpumask_clear(scratch);
+    }
+
+    if ( cpumask_equal(scratch, &cpu_online_map) )
+        send_IPI_shortcut(APIC_DEST_ALLBUT, vector, APIC_DEST_PHYSICAL);
+    else
+        alternative_vcall(genapic.send_IPI_mask, mask, vector);
+
+    if ( cpus_locked )
+        put_cpu_maps();
 }
 
 void send_IPI_self(int vector)
diff --git a/xen/include/asm-x86/smp.h b/xen/include/asm-x86/smp.h
index dbeed2fd41..1aa55d41e1 100644
--- a/xen/include/asm-x86/smp.h
+++ b/xen/include/asm-x86/smp.h
@@ -84,6 +84,9 @@ extern cpumask_t **socket_cpumask;
 #define get_cpu_current(cpu) \
     (get_cpu_info_from_stack((unsigned long)stack_base[cpu])->current_vcpu)
 
+extern unsigned int disabled_cpus;
+extern bool unaccounted_cpus;
+
 #endif /* !__ASSEMBLY__ */
 
 #endif
--
generated by git-patchbot for /home/xen/git/xen.git#master

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