linux-kernel - [PATCH] sched/cpuacct: Use __this_cpu_add() instead of this_cpu

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Message-Id: <20200416065310.37539-1-songmuchun@bytedance.com>
Date:   Thu, 16 Apr 2020 14:53:10 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: [PATCH] sched/cpuacct: Use __this_cpu_add() instead of this_cpu_ptr()

There seems to be no difference between the two, but on some
architectures(e.g. x86_64), there will be optimizations for
__this_cpu_add(). We can disassemble the code for you to see
the difference between them on x86_64.

  1) this_cpu_ptr(ca->cpuusage)->usages[index] += cputime;

     ffffffff810d7227: add %gs:0x7ef37fa9(%rip),%rax # f1d8 <this_cpu_off>
     ffffffff810d722f: add %rsi,(%rax)               # %rsi is @cputime

This result in two add instructions emitted by the compiler.

  2) __this_cpu_add(ca->cpuusage->usages[index], cputime);

     ffffffff810d7227: add %rsi,%gs:(%rax)           # %rsi is @cputime

This result in only one add instruction emitted by the compiler.

So we have enough reasons to use the __this_cpu_add().

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 kernel/sched/cpuacct.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
index 9fbb103834345..6448b0438ffb2 100644
--- a/kernel/sched/cpuacct.c
+++ b/kernel/sched/cpuacct.c
@@ -347,7 +347,7 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
 	rcu_read_lock();
 
 	for (ca = task_ca(tsk); ca; ca = parent_ca(ca))
-		this_cpu_ptr(ca->cpuusage)->usages[index] += cputime;
+		__this_cpu_add(ca->cpuusage->usages[index], cputime);
 
 	rcu_read_unlock();
 }
@@ -363,7 +363,7 @@ void cpuacct_account_field(struct task_struct *tsk, int index, u64 val)
 
 	rcu_read_lock();
 	for (ca = task_ca(tsk); ca != &root_cpuacct; ca = parent_ca(ca))
-		this_cpu_ptr(ca->cpustat)->cpustat[index] += val;
+		__this_cpu_add(ca->cpustat->cpustat[index], val);
 	rcu_read_unlock();
 }
 
-- 
2.11.0