Hi,
On 04/02/2022 20:31, Andrew Cooper wrote:
cpumask_weight() is a horribly 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.
I looked at the archive because I was wondering why we were using
cpumask_weight here. It looks like this was a left-over of the rework in
ac3fc35d919c "x86: Fix flush_area_mask() and on_selected_cpus() to not
race updates".
Switch to using cpumask_empty() instead, which will short circuit as soon as
it find any set bit in the cpumask.
Signed-off-by: Andrew Cooper <[email protected]>
Reviewed-by: Julien Grall <[email protected]>
---
CC: Jan Beulich <[email protected]>
CC: Roger Pau Monné <[email protected]>
CC: Wei Liu <[email protected]>
CC: Juergen Gross <[email protected]>
CC: Stefano Stabellini <[email protected]>
CC: Julien Grall <[email protected]>
CC: Volodymyr Babchuk <[email protected]>
CC: Bertrand Marquis <[email protected]>
I have not done performance testing, but I would be surprised if this cannot
be measured on a busy or large box.
---
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 781bcf2c246c..a011f541f1ea 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;
--
Julien Grall
