Messages in this thread | | | Subject | Re: [PATCH v3 0/6] percpu: partial chunk depopulation | From | Pratik Sampat <> | Date | Sat, 17 Apr 2021 12:38:08 +0530 |
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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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