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@xxxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxx>
>>>>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
>>>>> ---
>>>>> 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...)
>> Ex:
>>
>> nr_unused/0: 0
>> nr_unused/1: 0
>>
>> dentry goes to the LRU at cpu 1:
>> nr_unused/0: 0
>> nr_unused/1: 1
>>
>> CPU 1 goes down:
>> nr_unused/0: 0
>
> why?
>
>> dentry goes out of the LRU at cpu 0:
>> nr_unused/0: 1 << 32.
>
> Sorry, where does that shift come from? Pulling from the LRU is just
> a simple subtraction. (i.e. 0 - 1 = 0xffffffff), and so
> when we sum them all up:
>
> nr_unused/0: 1
> nr_unused/0: -1 (0xffffffff)
>
> sum = 1 + 0xffffffff = 0
>
>> That would easily be fixed by using a normal signed long, and is in fact
>> what the percpu code does in its internal operations.
>
> Changing it to a long means it becomes at 64 bit value on 64 bit
> machines (doubling memory usage), and now you're summing a 64 bit
> values into a 32 bit integer. Something else to go wrong....
>
>> Any reason not to do it? Something I am not seeing?
>
> It's a direct copy of the counting code in fs/inode.c. That has not
> demonstrated any problems in all my monitoring for the past coupl
> eof years (these are userspace visible stats) so AFAICT this code
> is just fine...
>
Well, in this case I can revert that.
My main concern is us being caught by overflows and stuff like that. But
I trust the "it's working for x years" more than my eyes.
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