linux-kernel - Re: [PATCH v3 6/9] sched: Kill select_task_rq()'s sd

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Date:   Thu, 16 Apr 2020 12:47:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [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