linux-kernel - Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched

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Date:   Thu, 30 Mar 2017 15:46:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paul Turner <pjt@...gle.com>
Cc:     Yuyang Du <yuyang.du@...el.com>, Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        umgwanakikbuti@...il.com
Subject: Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched_avg()

On Thu, Mar 30, 2017 at 02:16:58PM +0200, Peter Zijlstra wrote:
> On Thu, Mar 30, 2017 at 04:21:08AM -0700, Paul Turner wrote:

> > - The naming here is really ambiguous:
> >     "__accumulate_sum" -> "__accumulate_pelt_segments"?
> 
> OK, I did struggle with that a bit too but failed to improve, I'll change it.
> 
> > - Passing in "remainder" seems irrelevant to the sum accumulation.  It would be
> >   more clear to handle it from the caller.
> 
> Well, this way we have all 3 delta parts in one function. I'll try it
> and see what it looks like though.

> > This is super confusing.  It only works because remainder already had
> > period_contrib aggregated _into_ it.  We're literally computing:
> >   remainder + period_contrib - period_contrib
> 
> Correct; although I didn't find it too confusing. Could be because I'd
> been staring at this code for a few hours though.
> 
> > We should just not call this in the !periods case and handle the remainder
> > below.
> 
> I'll change it see what it looks like.

How's this?

---
 kernel/sched/fair.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 76f67b3e34d6..10d34498b5fe 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2795,12 +2795,9 @@ static u64 decay_load(u64 val, u64 n)
 	return val;
 }
 
-static u32 __accumulate_sum(u64 periods, u32 period_contrib, u32 remainder)
+static u32 __accumulate_pelt_segments(u64 periods, u32 d1, u32 d3)
 {
-	u32 c1, c2, c3 = remainder; /* y^0 == 1 */
-
-	if (!periods)
-		return remainder - period_contrib;
+	u32 c1, c2, c3 = d3; /* y^0 == 1 */
 
 	if (unlikely(periods >= LOAD_AVG_MAX_N))
 		return LOAD_AVG_MAX;
@@ -2861,8 +2858,8 @@ accumulate_sum(u64 delta, int cpu, struct sched_avg *sa,
 	       unsigned long weight, int running, struct cfs_rq *cfs_rq)
 {
 	unsigned long scale_freq, scale_cpu;
+	u32 contrib = delta;
 	u64 periods;
-	u32 contrib;
 
 	scale_freq = arch_scale_freq_capacity(NULL, cpu);
 	scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
@@ -2880,13 +2877,14 @@ accumulate_sum(u64 delta, int cpu, struct sched_avg *sa,
 				decay_load(cfs_rq->runnable_load_sum, periods);
 		}
 		sa->util_sum = decay_load((u64)(sa->util_sum), periods);
-	}
 
-	/*
-	 * Step 2
-	 */
-	delta %= 1024;
-	contrib = __accumulate_sum(periods, sa->period_contrib, delta);
+		/*
+		 * Step 2
+		 */
+		delta %= 1024;
+		contrib = __accumulate_pelt_segments(periods,
+				1024 - sa->period_contrib, delta);
+	}
 	sa->period_contrib = delta;
 
 	contrib = cap_scale(contrib, scale_freq);