Messages in this thread | | | Date | Thu, 16 Apr 2020 09:27:23 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/cpuacct: Use __this_cpu_add() instead of this_cpu_ptr() |
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On Thu, Apr 16, 2020 at 02:53:10PM +0800, Muchun Song wrote: > 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().
The patch is OK, but I can't take it with such complete nonsense for a Changelog.
The reason this_cpu_add() and __this_cpu_add() exist and are different is for different calling context. this_cpu_*() is always safe and correct, but as you notice, not always optimal. __this_cpu_*() relies on the caller already having preemption (and or IRQs disabled) to allow for better code-gen.
Now, the below call-sites have rq->lock taken, and this means preemption (and IRQs) are indeed disabled, so it is safe to use __this_cpu_*().
> Signed-off-by: Muchun Song <songmuchun@bytedance.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 >
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