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Message-ID: <6843bf1b-88cd-19fd-ddb0-5052af7ef2ac@linux.ibm.com>
Date: Sat, 17 Apr 2021 12:38:08 +0530
From: Pratik Sampat <psampat@...ux.ibm.com>
To: Roman Gushchin <guro@...com>
Cc: Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
Christoph Lameter <cl@...ux.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, pratik.r.sampat@...il.com
Subject: Re: [PATCH v3 0/6] percpu: partial chunk depopulation
On 17/04/21 1:33 am, Roman Gushchin wrote:
> On Sat, Apr 17, 2021 at 01:14:03AM +0530, Pratik Sampat wrote:
>>
>> On 17/04/21 12:39 am, Roman Gushchin wrote:
>>> On Sat, Apr 17, 2021 at 12:11:37AM +0530, Pratik Sampat wrote:
>>>> On 17/04/21 12:04 am, Roman Gushchin wrote:
>>>>> On Fri, Apr 16, 2021 at 11:57:03PM +0530, Pratik Sampat wrote:
>>>>>> On 16/04/21 10:43 pm, Roman Gushchin wrote:
>>>>>>> On Fri, Apr 16, 2021 at 08:58:33PM +0530, Pratik Sampat wrote:
>>>>>>>> Hello Dennis,
>>>>>>>>
>>>>>>>> I apologize for the clutter of logs before, I'm pasting the logs of before and
>>>>>>>> after the percpu test in the case of the patchset being applied on 5.12-rc6 and
>>>>>>>> the vanilla kernel 5.12-rc6.
>>>>>>>>
>>>>>>>> On 16/04/21 7:48 pm, Dennis Zhou wrote:
>>>>>>>>> Hello,
>>>>>>>>>
>>>>>>>>> On Fri, Apr 16, 2021 at 06:26:15PM +0530, Pratik Sampat wrote:
>>>>>>>>>> Hello Roman,
>>>>>>>>>>
>>>>>>>>>> I've tried the v3 patch series on a POWER9 and an x86 KVM setup.
>>>>>>>>>>
>>>>>>>>>> My results of the percpu_test are as follows:
>>>>>>>>>> Intel KVM 4CPU:4G
>>>>>>>>>> Vanilla 5.12-rc6
>>>>>>>>>> # ./percpu_test.sh
>>>>>>>>>> Percpu: 1952 kB
>>>>>>>>>> Percpu: 219648 kB
>>>>>>>>>> Percpu: 219648 kB
>>>>>>>>>>
>>>>>>>>>> 5.12-rc6 + with patchset applied
>>>>>>>>>> # ./percpu_test.sh
>>>>>>>>>> Percpu: 2080 kB
>>>>>>>>>> Percpu: 219712 kB
>>>>>>>>>> Percpu: 72672 kB
>>>>>>>>>>
>>>>>>>>>> I'm able to see improvement comparable to that of what you're see too.
>>>>>>>>>>
>>>>>>>>>> However, on POWERPC I'm unable to reproduce these improvements with the patchset in the same configuration
>>>>>>>>>>
>>>>>>>>>> POWER9 KVM 4CPU:4G
>>>>>>>>>> Vanilla 5.12-rc6
>>>>>>>>>> # ./percpu_test.sh
>>>>>>>>>> Percpu: 5888 kB
>>>>>>>>>> Percpu: 118272 kB
>>>>>>>>>> Percpu: 118272 kB
>>>>>>>>>>
>>>>>>>>>> 5.12-rc6 + with patchset applied
>>>>>>>>>> # ./percpu_test.sh
>>>>>>>>>> Percpu: 6144 kB
>>>>>>>>>> Percpu: 119040 kB
>>>>>>>>>> Percpu: 119040 kB
>>>>>>>>>>
>>>>>>>>>> I'm wondering if there's any architectural specific code that needs plumbing
>>>>>>>>>> here?
>>>>>>>>>>
>>>>>>>>> There shouldn't be. Can you send me the percpu_stats debug output before
>>>>>>>>> and after?
>>>>>>>> I'll paste the whole debug stats before and after here.
>>>>>>>> 5.12-rc6 + patchset
>>>>>>>> -----BEFORE-----
>>>>>>>> Percpu Memory Statistics
>>>>>>>> Allocation Info:
>>>>>>> Hm, this looks highly suspicious. Here is your stats in a more compact form:
>>>>>>>
>>>>>>> Vanilla
>>>>>>>
>>>>>>> nr_alloc : 9038 nr_alloc : 97046
>>>>>>> nr_dealloc : 6992 nr_dealloc : 94237
>>>>>>> nr_cur_alloc : 2046 nr_cur_alloc : 2809
>>>>>>> nr_max_alloc : 2178 nr_max_alloc : 90054
>>>>>>> nr_chunks : 3 nr_chunks : 11
>>>>>>> nr_max_chunks : 3 nr_max_chunks : 47
>>>>>>> min_alloc_size : 4 min_alloc_size : 4
>>>>>>> max_alloc_size : 1072 max_alloc_size : 1072
>>>>>>> empty_pop_pages : 5 empty_pop_pages : 29
>>>>>>>
>>>>>>>
>>>>>>> Patched
>>>>>>>
>>>>>>> nr_alloc : 9040 nr_alloc : 97048
>>>>>>> nr_dealloc : 6994 nr_dealloc : 95002
>>>>>>> nr_cur_alloc : 2046 nr_cur_alloc : 2046
>>>>>>> nr_max_alloc : 2208 nr_max_alloc : 90054
>>>>>>> nr_chunks : 3 nr_chunks : 48
>>>>>>> nr_max_chunks : 3 nr_max_chunks : 48
>>>>>>> min_alloc_size : 4 min_alloc_size : 4
>>>>>>> max_alloc_size : 1072 max_alloc_size : 1072
>>>>>>> empty_pop_pages : 12 empty_pop_pages : 61
>>>>>>>
>>>>>>>
>>>>>>> So it looks like the number of chunks got bigger, as well as the number of
>>>>>>> empty_pop_pages? This contradicts to what you wrote, so can you, please, make
>>>>>>> sure that the data is correct and we're not messing two cases?
>>>>>>>
>>>>>>> So it looks like for some reason sidelined (depopulated) chunks are not getting
>>>>>>> freed completely. But I struggle to explain why the initial empty_pop_pages is
>>>>>>> bigger with the same amount of chunks.
>>>>>>>
>>>>>>> So, can you, please, apply the following patch and provide an updated statistics?
>>>>>> Unfortunately, I'm not completely well versed in this area, but yes the empty
>>>>>> pop pages number doesn't make sense to me either.
>>>>>>
>>>>>> I re-ran the numbers trying to make sure my experiment setup is sane but
>>>>>> results remain the same.
>>>>>>
>>>>>> Vanilla
>>>>>> nr_alloc : 9040 nr_alloc : 97048
>>>>>> nr_dealloc : 6994 nr_dealloc : 94404
>>>>>> nr_cur_alloc : 2046 nr_cur_alloc : 2644
>>>>>> nr_max_alloc : 2169 nr_max_alloc : 90054
>>>>>> nr_chunks : 3 nr_chunks : 10
>>>>>> nr_max_chunks : 3 nr_max_chunks : 47
>>>>>> min_alloc_size : 4 min_alloc_size : 4
>>>>>> max_alloc_size : 1072 max_alloc_size : 1072
>>>>>> empty_pop_pages : 4 empty_pop_pages : 32
>>>>>>
>>>>>> With the patchset + debug patch the results are as follows:
>>>>>> Patched
>>>>>>
>>>>>> nr_alloc : 9040 nr_alloc : 97048
>>>>>> nr_dealloc : 6994 nr_dealloc : 94349
>>>>>> nr_cur_alloc : 2046 nr_cur_alloc : 2699
>>>>>> nr_max_alloc : 2194 nr_max_alloc : 90054
>>>>>> nr_chunks : 3 nr_chunks : 48
>>>>>> nr_max_chunks : 3 nr_max_chunks : 48
>>>>>> min_alloc_size : 4 min_alloc_size : 4
>>>>>> max_alloc_size : 1072 max_alloc_size : 1072
>>>>>> empty_pop_pages : 12 empty_pop_pages : 54
>>>>>>
>>>>>> With the extra tracing I can see 39 entries of "Chunk (sidelined)"
>>>>>> after the test was run. I don't see any entries for "Chunk (to depopulate)"
>>>>>>
>>>>>> I've snipped the results of slidelined chunks because they went on for ~600
>>>>>> lines, if you need the full logs let me know.
>>>>> Yes, please! That's the most interesting part!
>>>> Got it. Pasting the full logs of after the percpu experiment was completed
>>> Thanks!
>>>
>>> Would you mind to apply the following patch and test again?
>>>
>>> --
>>>
>>> diff --git a/mm/percpu.c b/mm/percpu.c
>>> index ded3a7541cb2..532c6a7ebdfd 100644
>>> --- a/mm/percpu.c
>>> +++ b/mm/percpu.c
>>> @@ -2296,6 +2296,9 @@ void free_percpu(void __percpu *ptr)
>>> need_balance = true;
>>> break;
>>> }
>>> +
>>> + chunk->depopulated = false;
>>> + pcpu_chunk_relocate(chunk, -1);
>>> } else if (chunk != pcpu_first_chunk && chunk != pcpu_reserved_chunk &&
>>> !chunk->isolated &&
>>> (pcpu_nr_empty_pop_pages[pcpu_chunk_type(chunk)] >
>>>
>> Sure thing.
>>
>> I see much lower sideline chunks. In one such test run I saw zero occurrences
>> of slidelined chunks
>>
> So looking at the stats it now works properly. Do you see any savings in
> comparison to vanilla? The size of savings can significanlty depend on the exact
> size of cgroup-related objects, how many of them fit into a single chunk, etc.
> So you might want to play with numbers in the test...
>
> Anyway, thank you very much for the report and your work on testing follow-up
> patches! It helped to reveal a serious bug in the implementation (completely
> empty sidelined chunks were not released in some cases), which by pure
> coincidence wasn't triggered on x86.
>
> Thanks!
>
Unfortunately not, I don't see any savings from the test.
# ./percpu_test_roman.sh
Percpu: 6144 kB
Percpu: 122880 kB
Percpu: 122880 kB
I had assumed that because POWER has a larger page size, we would indeed also
have higher fragmentation which could possibly lead to a lot more savings.
I'll dive deeper into the patches and tweak around the setup to see if I can
understand this behavior.
Thanks for helping me understand this patchset a little better and I'm glad we
found a bug with sidelined chunks!
I'll get back to you if I do find something interesting and need help
understanding it.
Thank you again,
Pratik
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