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

[Peter Zijlstra]

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@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
>