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SubjectRe: [PATCH v3 6/9] sched: Kill select_task_rq()'s sd_flag parameter
On Thu, Apr 16, 2020 at 09:42:36AM +0200, Vincent Guittot wrote:
> On Wed, 15 Apr 2020 at 23:05, Valentin Schneider
> > @@ -6622,13 +6622,25 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> > * preempt must be disabled.
> > */
> > static int
> > +select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
> > {
> > + int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> > struct sched_domain *tmp, *sd = NULL;
> > int cpu = smp_processor_id();
> > int new_cpu = prev_cpu;
> > int want_affine = 0;
> > - int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> > + int sd_flag;
> > +
> > + switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
>
> You remove a function parameter, which was directly set with the right
> flag, but then you add a switch case to recreate this sd_flag
> internally. Not sure we can say that it's real benefit
>
> > + case WF_TTWU:
> > + sd_flag = SD_BALANCE_WAKE;
> > + break;
> > + case WF_FORK:
> > + sd_flag = SD_BALANCE_FORK;
> > + break;
> > + default:
> > + sd_flag = SD_BALANCE_EXEC;
> > + }

Agreed, that's a bit yuck, how about something like so instead:


--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -11,10 +11,12 @@
*/
#ifdef CONFIG_SMP

+/* First nibble of SD_flag is shared with WF_flag */
#define SD_BALANCE_NEWIDLE 0x0001 /* Balance when about to become idle */
#define SD_BALANCE_EXEC 0x0002 /* Balance on exec */
#define SD_BALANCE_FORK 0x0004 /* Balance on fork, clone */
#define SD_BALANCE_WAKE 0x0008 /* Balance on wakeup */
+
#define SD_WAKE_AFFINE 0x0010 /* Wake task to waking CPU */
#define SD_ASYM_CPUCAPACITY 0x0020 /* Domain members have different CPU capacities */
#define SD_SHARE_CPUCAPACITY 0x0040 /* Domain members share CPU capacity */
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6635,16 +6635,8 @@ select_task_rq_fair(struct task_struct *
int want_affine = 0;
int sd_flag;

- switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) {
- case WF_TTWU:
- sd_flag = SD_BALANCE_WAKE;
- break;
- case WF_FORK:
- sd_flag = SD_BALANCE_FORK;
- break;
- default:
- sd_flag = SD_BALANCE_EXEC;
- }
+ /* SD_flags and WF_flags share the first nibble */
+ sd_flag = wake_flags & 0xf;

if (sd_flag & SD_BALANCE_WAKE) {
record_wakee(p);
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1685,11 +1685,12 @@ static inline int task_on_rq_migrating(s
/*
* Wake flags
*/
-#define WF_SYNC 0x01 /* Waker goes to sleep after wakeup */
-#define WF_TTWU 0x02 /* Regular task wakeup */
-#define WF_FORK 0x04 /* Child wakeup after fork */
-#define WF_EXEC 0x08 /* "Fake" wakeup at exec */
-#define WF_MIGRATED 0x10 /* Internal use, task got migrated */
+#define WF_EXEC 0x02 /* SD_BALANCE_EXEC */
+#define WF_FORK 0x04 /* SD_BALANCE_FORK */
+#define WF_TTWU 0x08 /* SD_BALANCE_WAKE */
+
+#define WF_SYNC 0x10 /* Waker goes to sleep after wakeup */
+#define WF_MIGRATED 0x20 /* Internal use, task got migrated */

/*
* To aid in avoiding the subversion of "niceness" due to uneven distribution
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