Messages in this thread | | | From | Muchun Song <> | Date | Thu, 16 Apr 2020 16:17:29 +0800 | Subject | Re: [External] Re: [PATCH] sched/cpuacct: Use __this_cpu_add() instead of this_cpu_ptr() |
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On Thu, Apr 16, 2020 at 3:27 PM Peter Zijlstra <peterz@infradead.org> wrote: > > 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_*().
Thanks Peter. I will update the changelog.
> > > 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 > >
-- Yours, Muchun
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