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

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

[Ming Lei]

__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.

The patch fixes problem that 'rmmod null_blk' may hang in blk_cleanup_queue()
because of miscounting of request_queue->mq_usage_counter.

Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shaohua Li <shli@fusionio.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Fan Du <fan.du@windriver.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 lib/percpu_counter.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index 7473ee3..2b87bc1 100644
--- 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);
 
-- 
1.7.9.5

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

[Andrew Morton]

On Tue,  7 Jan 2014 18:29:27 +0800 Ming Lei <tom.leiming@gmail.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.

What do you think?

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

[Ming Lei]

Hi Andrew,

On Wed, Jan 8, 2014 at 6:27 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Tue,  7 Jan 2014 18:29:27 +0800 Ming Lei <tom.leiming@gmail.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

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

[Andrew Morton]

On Wed, 8 Jan 2014 09:12:19 +0800 Ming Lei <tom.leiming@gmail.com> wrote:

> Hi Andrew,
> 
> >> --- 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

Don't think so - local_irq_disable() requires quite some internal
synchronization in the CPU and is expensive.  preempt_disable() is just
an add+barrier, minus the add if the kernel is non-preemptable.

> - 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.

Yup.  But other CPUs should and will optimise their this_cpu
implementations over time.

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

[Ming Lei]

Hi Andrew,

On Wed, Jan 8, 2014 at 9:36 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
>> 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
>
> Don't think so - local_irq_disable() requires quite some internal
> synchronization in the CPU and is expensive.  preempt_disable() is just

Yes, it might be a little expensive on some CPUs, but should be
arch-dependent(CPU inside things are involved)

> an add+barrier, minus the add if the kernel is non-preemptable.

IMO, generally, from software view, local_irq_save() only save the
CPU's interrupt mask to the local variable 'flag', and sets irq mask
to register, considered local variable can be thought to be in cache,
so I think it might be cheaper than preempt_enable() because
preempt counter may not be in cache.

Also this_cpu_add() won't work in batch path(slow path), we still
need to avoid interrupt coming between reading the percpu counter
and resetting it, otherwise counts might be lost too, :-)

Thanks,
-- 
Ming Lei

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

[Ming Lei]

On Wed, Jan 8, 2014 at 11:29 AM, Ming Lei <tom.leiming@gmail.com> wrote:
> Hi Andrew,
>
> On Wed, Jan 8, 2014 at 9:36 AM, Andrew Morton <akpm@linux-foundation.org> wrote:
>>> 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
>>
>> Don't think so - local_irq_disable() requires quite some internal
>> synchronization in the CPU and is expensive.  preempt_disable() is just
>
> Yes, it might be a little expensive on some CPUs, but should be
> arch-dependent(CPU inside things are involved)
>
>> an add+barrier, minus the add if the kernel is non-preemptable.
>
> IMO, generally, from software view, local_irq_save() only save the
> CPU's interrupt mask to the local variable 'flag', and sets irq mask
> to register, considered local variable can be thought to be in cache,
> so I think it might be cheaper than preempt_enable() because
> preempt counter may not be in cache.
>
> Also this_cpu_add() won't work in batch path(slow path), we still
> need to avoid interrupt coming between reading the percpu counter
> and resetting it, otherwise counts might be lost too, :-)

Sorry, I miss the __this_cpu_sub() in slow path, so it is correct, and
even preempt_enable() and preempt_disable() can be removed.


Thanks,
-- 
Ming Lei