Re: [PATCH v3 3/3] vfs: Use per-cpu list for superblock's inode list

[Waiman Long <waiman.long@hpe.com>]

On 02/24/2016 03:28 AM, Jan Kara wrote:
> On Tue 23-02-16 14:04:32, Waiman Long wrote:
>> When many threads are trying to add or delete inode to or from
>> a superblock's s_inodes list, spinlock contention on the list can
>> become a performance bottleneck.
>>
>> This patch changes the s_inodes field to become a per-cpu list with
>> per-cpu spinlocks. As a result, the following superblock inode list
>> (sb->s_inodes) iteration functions in vfs are also being modified:
>>
>>   1. iterate_bdevs()
>>   2. drop_pagecache_sb()
>>   3. wait_sb_inodes()
>>   4. evict_inodes()
>>   5. invalidate_inodes()
>>   6. fsnotify_unmount_inodes()
>>   7. add_dquot_ref()
>>   8. remove_dquot_ref()
>>
>> With an exit microbenchmark that creates a large number of threads,
>> attachs many inodes to them and then exits. The runtimes of that
>> microbenchmark with 1000 threads before and after the patch on a
>> 4-socket Intel E7-4820 v3 system (40 cores, 80 threads) were as
>> follows:
>>
>>    Kernel            Elapsed Time    System Time
>>    ------            ------------    -----------
>>    Vanilla 4.5-rc4      65.29s         82m14s
>>    Patched 4.5-rc4      22.81s         23m03s
>>
>> Before the patch, spinlock contention at the inode_sb_list_add()
>> function at the startup phase and the inode_sb_list_del() function at
>> the exit phase were about 79% and 93% of total CPU time respectively
>> (as measured by perf). After the patch, the percpu_list_add()
>> function consumed only about 0.04% of CPU time at startup phase. The
>> percpu_list_del() function consumed about 0.4% of CPU time at exit
>> phase. There were still some spinlock contention, but they happened
>> elsewhere.
> While looking through this patch, I have noticed that the
> list_for_each_entry_safe() iterations in evict_inodes() and
> invalidate_inodes() are actually unnecessary. So if you first apply the
> attached patch, you don't have to implement safe iteration variants at all.

Thank for the patch. I will apply that in my next update. As for the 
safe iteration variant, I think I will keep it since I had implemented 
that already just in case it may be needed in some other places.

> As a second comment, I'd note that this patch grows struct inode by 1
> pointer. It is probably acceptable for large machines given the speedup but
> it should be noted in the changelog. Furthermore for UP or even small SMP
> systems this is IMHO undesired bloat since the speedup won't be noticeable.
>
> So for these small systems it would be good if per-cpu list magic would just
> fall back to single linked list with a spinlock. Do you think that is
> reasonably doable?
>

I already have a somewhat separate code path for UP. So I can remove the 
lock pointer for that. For small SMP system, however, the only way to 
avoid the extra pointer is to add a config parameter to turn this 
feature off. That can be added as a separate patch, if necessary.

BTW, I think the current inode structure is already pretty big, adding 
one more pointer will have too much impact on its overall size.

Cheers,
Longman