Re: [PATCH v3 0/6] percpu: partial chunk depopulation

From: Roman Gushchin
Date: Fri Apr 16 2021 - 14:34:33 EST

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!