Re: [PATCH v3 0/6] percpu: partial chunk depopulation
From: Pratik Sampat
Date: Sat Apr 17 2021 - 03:09:46 EST
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