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@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/acpi/boot.c b/xen/arch/x86/acpi/boot.c
index 15542a9bdff3448e0106336df8147e43ec612aa2..afc6ed9d99112f63db96f5c84813e4263d7a5cf0 100644 (file)
--- 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 f057d9162fefaf276dd820289ad12aaabc9cce74..d532575feeb8440d3b5bd27651846ea520188189 100644 (file)
--- 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 c14f304c09999dba154f56dea22e8c12675b27b1..65eb7cbda8ee5de5354340e936bd8236d2fa21f8 100644 (file)
--- 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 dbeed2fd4142b77429edba2b60b489e793631d6a..1aa55d41e13839405f1691e61bdbd60609800c9e 100644 (file)
--- 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