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Date:   Fri, 16 Apr 2021 13:03:34 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Pratik Sampat <psampat@linux.ibm.com>
CC:     Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <pratik.r.sampat@gmail.com>
Subject: Re: [PATCH v3 0/6] percpu: partial chunk depopulation
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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!