Re: [PATCH v2 03/28] dcache: convert dentry_stat.nr_unused to per-cpu counters

[Dave Chinner <david@fromorbit.com>]

On Tue, Apr 09, 2013 at 12:02:21PM +0400, Glauber Costa wrote:
> On 04/09/2013 03:26 AM, Dave Chinner wrote:
> > On Mon, Apr 08, 2013 at 01:14:48PM +0400, Glauber Costa wrote:
> >> On 04/05/2013 05:15 AM, Dave Chinner wrote:
> >>> On Thu, Apr 04, 2013 at 06:09:31PM -0700, Greg Thelen wrote:
> >>>> On Fri, Mar 29 2013, Glauber Costa wrote:
> >>>>
> >>>>> From: Dave Chinner <dchinner@redhat.com>
> >>>>>
> >>>>> Before we split up the dcache_lru_lock, the unused dentry counter
> >>>>> needs to be made independent of the global dcache_lru_lock. Convert
> >>>>> it to per-cpu counters to do this.
> >>>>>
> >>>>> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> >>>>> Reviewed-by: Christoph Hellwig <hch@lst.de>
> >>>>> ---
> >>>>>  fs/dcache.c | 17 ++++++++++++++---
> >>>>>  1 file changed, 14 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/fs/dcache.c b/fs/dcache.c
> >>>>> index fbfae008..f1196f2 100644
> >>>>> --- a/fs/dcache.c
> >>>>> +++ b/fs/dcache.c
> >>>>> @@ -118,6 +118,7 @@ struct dentry_stat_t dentry_stat = {
> >>>>>  };
> >>>>>  
> >>>>>  static DEFINE_PER_CPU(unsigned int, nr_dentry);
> >>>>> +static DEFINE_PER_CPU(unsigned int, nr_dentry_unused);
> >>>>>  
> >>>>>  #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
> >>>>>  static int get_nr_dentry(void)
> >>>>> @@ -129,10 +130,20 @@ static int get_nr_dentry(void)
> >>>>>  	return sum < 0 ? 0 : sum;
> >>>>>  }
> >>>>>  
> >>>>> +static int get_nr_dentry_unused(void)
> >>>>> +{
> >>>>> +	int i;
> >>>>> +	int sum = 0;
> >>>>> +	for_each_possible_cpu(i)
> >>>>> +		sum += per_cpu(nr_dentry_unused, i);
> >>>>> +	return sum < 0 ? 0 : sum;
> >>>>> +}
> >>>>
> >>>> Just checking...  If cpu x is removed, then its per cpu nr_dentry_unused
> >>>> count survives so we don't leak nr_dentry_unused.  Right?  I see code in
> >>>> percpu_counter_sum_positive() to explicitly handle this case and I want
> >>>> to make sure we don't need it here.
> >>>
> >>> DEFINE_PER_CPU() gives a variable per possible CPU, and we sum for
> >>> all possible CPUs. Therefore online/offline CPUs just don't matter.
> >>>
> >>> The percpu_counter code uses for_each_online_cpu(), and so it has to
> >>> be aware of hotplug operations so taht it doesn't leak counts.
> >>
> >> It is an unsigned quantity, however. Can't we go negative if it becomes
> >> unused in one cpu, but used in another?
> > 
> > Sure, but it's unsigned for the purposes of summing, not for the
> > purposes of having pos/neg values - they are just delta counters.
> > 
> > I'm just copying the code from fs/inode.c. I originally implemented
> > the fs/inode.c code using generic per-cpu counters, but there was
> > a hissy fit over "too much overhead" and so someone implemented
> > their own lightweight version. I've just copied the existing code to
> > code because I don't care to revisit this....
> > 
> 
> Funny enough, we re implement per-cpu counters in memcg as well.
> This is mostly overhead/counters cache layout related. Maybe it is time
> for a better percpu counter ? (not that I have the time for it...)

Word.

I've just given up trying to convince people to use the generic code
when they are set on micro-optimising code. The "I can trim 3
instructions from every increment and decrement" argument seems to
win every time over "we know the generic counters work"....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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