[xen master] xen/smp: Speed up on_selected_cpus()

[patchbot@xxxxxxx]

commit f97c1abf2934e76fb69fabaf4f5ec04afa813816
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Feb 4 20:12:04 2022 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Feb 7 17:41:24 2022 +0000

    xen/smp: Speed up on_selected_cpus()
    
    cpumask_weight() is an incredibly expensive way to find if no bits are set,
    made worse by the fact that the calculation is performed with the global
    call_lock held.
    
    This appears to be a missing optimisation from c/s 433f14699d48 ("x86: Clean
    up smp_call_function handling.") in 2011 which dropped the logic requiring 
the
    count of CPUs.
    
    Switch to using cpumask_empty() instead, which will short circuit as soon as
    it finds any set bit in the cpumask.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/smp.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/xen/common/smp.c b/xen/common/smp.c
index 781bcf2c24..a011f541f1 100644
--- a/xen/common/smp.c
+++ b/xen/common/smp.c
@@ -50,8 +50,6 @@ void on_selected_cpus(
     void *info,
     int wait)
 {
-    unsigned int nr_cpus;
-
     ASSERT(local_irq_is_enabled());
     ASSERT(cpumask_subset(selected, &cpu_online_map));
 
@@ -59,8 +57,7 @@ void on_selected_cpus(
 
     cpumask_copy(&call_data.selected, selected);
 
-    nr_cpus = cpumask_weight(&call_data.selected);
-    if ( nr_cpus == 0 )
+    if ( cpumask_empty(&call_data.selected) )
         goto out;
 
     call_data.func = func;
--
generated by git-patchbot for /home/xen/git/xen.git#master