[PATCH] sched/cpuacct: Use __this_cpu_add() instead of this_cpu_ptr()

From: Muchun Song
Date: Thu Apr 16 2020 - 02:54:11 EST


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@xxxxxxxxxxxxx>
---
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