linux-kernel - Re: [PATCH] lib/percpu_counter.c: disable local irq when updating percpu couter

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Date:	Wed, 8 Jan 2014 09:12:19 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Shaohua Li <shli@...ionio.com>, Jens Axboe <axboe@...nel.dk>,
	Fan Du <fan.du@...driver.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] lib/percpu_counter.c: disable local irq when updating
 percpu couter

Hi Andrew,

On Wed, Jan 8, 2014 at 6:27 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Tue,  7 Jan 2014 18:29:27 +0800 Ming Lei <tom.leiming@...il.com> wrote:
>
>> __percpu_counter_add() may be called in softirq/hardirq handler
>> (such as, blk_mq_queue_exit() is typically called in hardirq/softirq
>> handler), so we need to disable local irq when updating the percpu
>> counter, otherwise counts may be lost.
>
> OK.
>
>> The patch fixes problem that 'rmmod null_blk' may hang in blk_cleanup_queue()
>> because of miscounting of request_queue->mq_usage_counter.
>>
>> ...
>>
>> --- a/lib/percpu_counter.c
>> +++ b/lib/percpu_counter.c
>> @@ -75,19 +75,19 @@ EXPORT_SYMBOL(percpu_counter_set);
>>  void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
>>  {
>>       s64 count;
>> +     unsigned long flags;
>>
>> -     preempt_disable();
>> +     raw_local_irq_save(flags);
>>       count = __this_cpu_read(*fbc->counters) + amount;
>>       if (count >= batch || count <= -batch) {
>> -             unsigned long flags;
>> -             raw_spin_lock_irqsave(&fbc->lock, flags);
>> +             raw_spin_lock(&fbc->lock);
>>               fbc->count += count;
>> -             raw_spin_unlock_irqrestore(&fbc->lock, flags);
>> +             raw_spin_unlock(&fbc->lock);
>>               __this_cpu_write(*fbc->counters, 0);
>>       } else {
>>               __this_cpu_write(*fbc->counters, count);
>>       }
>> -     preempt_enable();
>> +     raw_local_irq_restore(flags);
>>  }
>>  EXPORT_SYMBOL(__percpu_counter_add);
>
> Can this be made more efficient?
>
> The this_cpu_foo() documentation is fairly dreadful, but way down at
> the end of Documentation/this_cpu_ops.txt we find "this_cpu ops are
> interrupt safe".  So I think this is a more efficient fix:
>
> --- a/lib/percpu_counter.c~a
> +++ a/lib/percpu_counter.c
> @@ -82,10 +82,10 @@ void __percpu_counter_add(struct percpu_
>                 unsigned long flags;
>                 raw_spin_lock_irqsave(&fbc->lock, flags);
>                 fbc->count += count;
> +               __this_cpu_sub(*fbc->counters, count);
>                 raw_spin_unlock_irqrestore(&fbc->lock, flags);
> -               __this_cpu_write(*fbc->counters, 0);
>         } else {
> -               __this_cpu_write(*fbc->counters, count);
> +               this_cpu_add(*fbc->counters, amount);
>         }
>         preempt_enable();
>  }
>
> It avoids the local_irq_disable() in the common case, when the CPU
> supports efficient this_cpu_add().  It will in rare race situations
> permit the cpu-local counter to exceed `batch', but that should be
> harmless.

I am wondering if the above patch is more efficient, because:

- raw_local_irq_save()/raw_local_irq_restore() should be cheaper
than preempt_enable() in theory

- except for x86 and s390, other ARCHs have not their own implementation
of  this_cpu_foo(), and the generic one just disables local interrupt
when operating the percpu variable.

So I suggest to fix it by replacing preempt_* with raw_local_irq_*.


Thanks,
-- 
Ming Lei
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