On 30/10/2024 20:48, Barry Song wrote:
> On Thu, Oct 31, 2024 at 9:41 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>>
>>
>>
>> On 30/10/2024 20:27, Barry Song wrote:
>>> On Thu, Oct 31, 2024 at 3:51 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>> On 28/10/2024 22:03, Barry Song wrote:
>>>>> On Mon, Oct 28, 2024 at 8:07 PM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On 27/10/2024 01:14, Barry Song wrote:
>>>>>>> From: Barry Song <v-songbaohua@xxxxxxxx>
>>>>>>>
>>>>>>> In a memcg where mTHP is always utilized, even at full capacity, it
>>>>>>> might not be the best option. Consider a system that uses only small
>>>>>>> folios: after each reclamation, a process has at least SWAP_CLUSTER_MAX
>>>>>>> of buffer space before it can initiate the next reclamation. However,
>>>>>>> large folios can quickly fill this space, rapidly bringing the memcg
>>>>>>> back to full capacity, even though some portions of the large folios
>>>>>>> may not be immediately needed and used by the process.
>>>>>>>
>>>>>>> Usama and Kanchana identified a regression when building the kernel in
>>>>>>> a memcg with memory.max set to a small value while enabling large
>>>>>>> folio swap-in support on zswap[1].
>>>>>>>
>>>>>>> The issue arises from an edge case where the memory cgroup remains
>>>>>>> nearly full most of the time. Consequently, bringing in mTHP can
>>>>>>> quickly cause a memcg overflow, triggering a swap-out. The subsequent
>>>>>>> swap-in then recreates the overflow, resulting in a repetitive cycle.
>>>>>>>
>>>>>>> We need a mechanism to stop the cup from overflowing continuously.
>>>>>>> One potential solution is to slow the filling process when we identify
>>>>>>> that the cup is nearly full.
>>>>>>>
>>>>>>> Usama reported an improvement when we mitigate mTHP swap-in as the
>>>>>>> memcg approaches full capacity[2]:
>>>>>>>
>>>>>>> int mem_cgroup_swapin_charge_folio(...)
>>>>>>> {
>>>>>>> ...
>>>>>>> if (folio_test_large(folio) &&
>>>>>>> mem_cgroup_margin(memcg) < max(MEMCG_CHARGE_BATCH, folio_nr_pages(folio)))
>>>>>>> ret = -ENOMEM;
>>>>>>> else
>>>>>>> ret = charge_memcg(folio, memcg, gfp);
>>>>>>> ...
>>>>>>> }
>>>>>>>
>>>>>>> AMD 16K+32K THP=always
>>>>>>> metric mm-unstable mm-unstable + large folio zswapin series mm-unstable + large folio zswapin + no swap thrashing fix
>>>>>>> real 1m23.038s 1m23.050s 1m22.704s
>>>>>>> user 53m57.210s 53m53.437s 53m52.577s
>>>>>>> sys 7m24.592s 7m48.843s 7m22.519s
>>>>>>> zswpin 612070 999244 815934
>>>>>>> zswpout 2226403 2347979 2054980
>>>>>>> pgfault 20667366 20481728 20478690
>>>>>>> pgmajfault 385887 269117 309702
>>>>>>>
>>>>>>> AMD 16K+32K+64K THP=always
>>>>>>> metric mm-unstable mm-unstable + large folio zswapin series mm-unstable + large folio zswapin + no swap thrashing fix
>>>>>>> real 1m22.975s 1m23.266s 1m22.549s
>>>>>>> user 53m51.302s 53m51.069s 53m46.471s
>>>>>>> sys 7m40.168s 7m57.104s 7m25.012s
>>>>>>> zswpin 676492 1258573 1225703
>>>>>>> zswpout 2449839 2714767 2899178
>>>>>>> pgfault 17540746 17296555 17234663
>>>>>>> pgmajfault 429629 307495 287859
>>>>>>>
>>>>>>> I wonder if we can extend the mitigation to do_anonymous_page() as
>>>>>>> well. Without hardware like AMD and ARM with hardware TLB coalescing
>>>>>>> or CONT-PTE, I conducted a quick test on my Intel i9 workstation with
>>>>>>> 10 cores and 2 threads. I enabled one 12 GiB zRAM while running kernel
>>>>>>> builds in a memcg with memory.max set to 1 GiB.
>>>>>>>
>>>>>>> $ echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
>>>>>>> $ echo always > /sys/kernel/mm/transparent_hugepage/hugepages-32kB/enabled
>>>>>>> $ echo always > /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled
>>>>>>> $ echo never > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
>>>>>>>
>>>>>>> $ time systemd-run --scope -p MemoryMax=1G make ARCH=arm64 \
>>>>>>> CROSS_COMPILE=aarch64-linux-gnu- Image -10 1>/dev/null 2>/dev/null
>>>>>>>
>>>>>>> disable-mTHP-swapin mm-unstable with-this-patch
>>>>>>> Real: 6m54.595s 7m4.832s 6m45.811s
>>>>>>> User: 66m42.795s 66m59.984s 67m21.150s
>>>>>>> Sys: 12m7.092s 15m18.153s 12m52.644s
>>>>>>> pswpin: 4262327 11723248 5918690
>>>>>>> pswpout: 14883774 19574347 14026942
>>>>>>> 64k-swpout: 624447 889384 480039
>>>>>>> 32k-swpout: 115473 242288 73874
>>>>>>> 16k-swpout: 158203 294672 109142
>>>>>>> 64k-swpin: 0 495869 159061
>>>>>>> 32k-swpin: 0 219977 56158
>>>>>>> 16k-swpin: 0 223501 81445
>>>>>>>
>>>>>>
>>>>>
>>>>> Hi Usama,
>>>>>
>>>>>> hmm, both the user and sys time are worse with the patch compared to
>>>>>> disable-mTHP-swapin. I wonder if the real time is an anomaly and if you
>>>>>> repeat the experiment the real time might be worse as well?
>>>>>
>>>>> Well, I've improved my script to include a loop:
>>>>>
>>>>> echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
>>>>> echo always > /sys/kernel/mm/transparent_hugepage/hugepages-32kB/enabled
>>>>> echo always > /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled
>>>>> echo never > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
>>>>>
>>>>> for ((i=1; i<=100; i++))
>>>>> do
>>>>> echo "Executing round $i"
>>>>> make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- clean 1>/dev/null 2>/dev/null
>>>>> echo 3 > /proc/sys/vm/drop_caches
>>>>> time systemd-run --scope -p MemoryMax=1G make ARCH=arm64 \
>>>>> CROSS_COMPILE=aarch64-linux-gnu- vmlinux -j15 1>/dev/null 2>/dev/null
>>>>> cat /proc/vmstat | grep pswp
>>>>> echo -n 64k-swpout: ; cat
>>>>> /sys/kernel/mm/transparent_hugepage/hugepages-64kB/stats/swpout
>>>>> echo -n 32k-swpout: ; cat
>>>>> /sys/kernel/mm/transparent_hugepage/hugepages-32kB/stats/swpout
>>>>> echo -n 16k-swpout: ; cat
>>>>> /sys/kernel/mm/transparent_hugepage/hugepages-16kB/stats/swpout
>>>>> echo -n 64k-swpin: ; cat
>>>>> /sys/kernel/mm/transparent_hugepage/hugepages-64kB/stats/swpin
>>>>> echo -n 32k-swpin: ; cat
>>>>> /sys/kernel/mm/transparent_hugepage/hugepages-32kB/stats/swpin
>>>>> echo -n 16k-swpin: ; cat
>>>>> /sys/kernel/mm/transparent_hugepage/hugepages-16kB/stats/swpin
>>>>> done
>>>>>
>>>>> I've noticed that the user/sys/real time on my i9 machine fluctuates
>>>>> constantly, could be things
>>>>> like:
>>>>> real 6m52.087s
>>>>> user 67m12.463s
>>>>> sys 13m8.281s
>>>>> ...
>>>>>
>>>>> real 7m42.937s
>>>>> user 66m55.250s
>>>>> sys 12m56.330s
>>>>> ...
>>>>>
>>>>> real 6m49.374s
>>>>> user 66m37.040s
>>>>> sys 12m44.542s
>>>>> ...
>>>>>
>>>>> real 6m54.205s
>>>>> user 65m49.732s
>>>>> sys 11m33.078s
>>>>> ...
>>>>>
>>>>> likely due to unstable temperatures and I/O latency. As a result, my
>>>>> data doesn’t seem
>>>>> reference-worthy.
>>>>>
>>>>
>>>> So I had suggested retrying the experiment to see how reproducible it is,
>>>> but had not done that myself!
>>>> Thanks for sharing this. I tried many times on the AMD server and I see
>>>> varying numbers as well.
>>>>
>>>> AMD 16K THP always, cgroup = 4G, large folio zswapin patches
>>>> real 1m28.351s
>>>> user 54m14.476s
>>>> sys 8m46.596s
>>>> zswpin 811693
>>>> zswpout 2137310
>>>> pgfault 27344671
>>>> pgmajfault 290510
>>>> ..
>>>> real 1m24.557s
>>>> user 53m56.815s
>>>> sys 8m10.200s
>>>> zswpin 571532
>>>> zswpout 1645063
>>>> pgfault 26989075
>>>> pgmajfault 205177
>>>> ..
>>>> real 1m26.083s
>>>> user 54m5.303s
>>>> sys 9m55.247s
>>>> zswpin 1176292
>>>> zswpout 2910825
>>>> pgfault 27286835
>>>> pgmajfault 419746
>>>>
>>>>
>>>> The sys time can especially vary by large numbers. I think you see the same.
>>>>
>>>>
>>>>> As a phone engineer, we never use phones to run kernel builds. I'm also
>>>>> quite certain that phones won't provide stable and reliable data for this
>>>>> type of workload. Without access to a Linux server to conduct the test,
>>>>> I really need your help.
>>>>>
>>>>> I used to work on optimizing the ARM server scheduler and memory
>>>>> management, and I really miss that machine I had until three years ago :-)
>>>>>
>>>>>>
>>>>>>> I need Usama's assistance to identify a suitable patch, as I lack
>>>>>>> access to hardware such as AMD machines and ARM servers with TLB
>>>>>>> optimization.
>>>>>>>
>>>>>>> [1] https://lore.kernel.org/all/b1c17b5e-acd9-4bef-820e-699768f1426d@xxxxxxxxx/
>>>>>>> [2] https://lore.kernel.org/all/7a14c332-3001-4b9a-ada3-f4d6799be555@xxxxxxxxx/
>>>>>>>
>>>>>>> Cc: Kanchana P Sridhar <kanchana.p.sridhar@xxxxxxxxx>
>>>>>>> Cc: Usama Arif <usamaarif642@xxxxxxxxx>
>>>>>>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>>>>>>> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>>>>>>> Cc: Chris Li <chrisl@xxxxxxxxxx>
>>>>>>> Cc: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>>>>>>> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>>>>>> Cc: Kairui Song <kasong@xxxxxxxxxxx>
>>>>>>> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>>>>> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>>>>>>> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
>>>>>>> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
>>>>>>> Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
>>>>>>> Cc: Muchun Song <muchun.song@xxxxxxxxx>
>>>>>>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
>>>>>>> ---
>>>>>>> include/linux/memcontrol.h | 9 ++++++++
>>>>>>> mm/memcontrol.c | 45 ++++++++++++++++++++++++++++++++++++++
>>>>>>> mm/memory.c | 17 ++++++++++++++
>>>>>>> 3 files changed, 71 insertions(+)
>>>>>>>
>>>>>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>>>>>> index 524006313b0d..8bcc8f4af39f 100644
>>>>>>> --- a/include/linux/memcontrol.h
>>>>>>> +++ b/include/linux/memcontrol.h
>>>>>>> @@ -697,6 +697,9 @@ static inline int mem_cgroup_charge(struct folio *folio, struct mm_struct *mm,
>>>>>>> int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
>>>>>>> long nr_pages);
>>>>>>>
>>>>>>> +int mem_cgroup_precharge_large_folio(struct mm_struct *mm,
>>>>>>> + swp_entry_t *entry);
>>>>>>> +
>>>>>>> int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm,
>>>>>>> gfp_t gfp, swp_entry_t entry);
>>>>>>>
>>>>>>> @@ -1201,6 +1204,12 @@ static inline int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg,
>>>>>>> return 0;
>>>>>>> }
>>>>>>>
>>>>>>> +static inline int mem_cgroup_precharge_large_folio(struct mm_struct *mm,
>>>>>>> + swp_entry_t *entry)
>>>>>>> +{
>>>>>>> + return 0;
>>>>>>> +}
>>>>>>> +
>>>>>>> static inline int mem_cgroup_swapin_charge_folio(struct folio *folio,
>>>>>>> struct mm_struct *mm, gfp_t gfp, swp_entry_t entry)
>>>>>>> {
>>>>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>>>>> index 17af08367c68..f3d92b93ea6d 100644
>>>>>>> --- a/mm/memcontrol.c
>>>>>>> +++ b/mm/memcontrol.c
>>>>>>> @@ -4530,6 +4530,51 @@ int mem_cgroup_hugetlb_try_charge(struct mem_cgroup *memcg, gfp_t gfp,
>>>>>>> return 0;
>>>>>>> }
>>>>>>>
>>>>>>> +static inline bool mem_cgroup_has_margin(struct mem_cgroup *memcg)
>>>>>>> +{
>>>>>>> + for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
>>>>>>> + if (mem_cgroup_margin(memcg) < HPAGE_PMD_NR)
>>>>>>
>>>>>> There might be 3 issues with the approach:
>>>>>>
>>>>>> Its a very big margin, lets say you have ARM64_64K_PAGES, and you have
>>>>>> 256K THP set to always. As HPAGE_PMD is 512M for 64K page, you are
>>>>>> basically saying you need 512M free memory to swapin just 256K?
>>>>>
>>>>> Right, sorry for the noisy code. I was just thinking about 4KB pages
>>>>> and wondering
>>>>> if we could simplify the code.
>>>>>
>>>>>>
>>>>>> Its an uneven margin for different folio sizes.
>>>>>> For 16K folio swapin, you are checking if there is margin for 128 folios,
>>>>>> but for 1M folio swapin, you are checking there is margin for just 2 folios.
>>>>>>
>>>>>> Maybe it might be better to make this dependent on some factor of folio_nr_pages?
>>>>>
>>>>> Agreed. This is similar to what we discussed regarding your zswap mTHP
>>>>> swap-in series:
>>>>>
>>>>> int mem_cgroup_swapin_charge_folio(...)
>>>>> {
>>>>> ...
>>>>> if (folio_test_large(folio) &&
>>>>> mem_cgroup_margin(memcg) < max(MEMCG_CHARGE_BATCH,
>>>>> folio_nr_pages(folio)))
>>>>> ret = -ENOMEM;
>>>>> else
>>>>> ret = charge_memcg(folio, memcg, gfp);
>>>>> ...
>>>>> }
>>>>>
>>>>> As someone focused on phones, my challenge is the absence of stable platforms to
>>>>> benchmark this type of workload. If possible, Usama, I would greatly
>>>>> appreciate it if
>>>>> you could take the lead on the patch.
>>>>>
>>>>>>
>>>>>> As Johannes pointed out, the charging code already does the margin check.
>>>>>> So for 4K, the check just checks if there is 4K available, but for 16K it checks
>>>>>> if a lot more than 16K is available. Maybe there should be a similar policy for
>>>>>> all? I guess this is similar to my 2nd point, but just considers 4K folios as
>>>>>> well.
>>>>>
>>>>> I don't think the charging code performs a margin check. It simply
>>>>> tries to charge
>>>>> the specified nr_pages (whether 1 or more). If nr_pages are available,
>>>>> the charge
>>>>> proceeds; otherwise, if GFP allows blocking, it triggers memory reclamation to
>>>>> reclaim max(SWAP_CLUSTER_MAX, nr_pages) base pages.
>>>>>
>>>>
>>>> So if you have defrag not set to always, it will not trigger reclamation.
>>>> I think that is a bigger usecase, i.e. defrag=madvise,defer,etc is probably
>>>> used much more then always.
>>>>
>>>> In the current code in that case try_charge_memcg will return -ENOMEM all
>>>> the way to mem_cgroup_swapin_charge_folio and alloc_swap_folio will then
>>>> try the next order. So eventhough it might not be calling the mem_cgroup_margin
>>>> function, it is kind of is doing the same?
>>>>
>>>>> If, after reclamation, we have exactly SWAP_CLUSTER_MAX pages available, a
>>>>> large folio with nr_pages == SWAP_CLUSTER_MAX will successfully charge,
>>>>> immediately filling the memcg.
>>>>>
>>>>> Shortly after, smaller folios—typically with blockable GFP—will quickly trigger
>>>>> additional reclamation. While nr_pages - 1 subpages of the large folio may not
>>>>> be immediately needed, they still occupy enough space to fill the memcg to
>>>>> capacity.
>>>>>
>>>>> My second point about the mitigation is as follows: For a system (or
>>>>> memcg) under severe memory pressure, especially one without hardware TLB
>>>>> optimization, is enabling mTHP always the right choice? Since mTHP operates at
>>>>> a larger granularity, some internal fragmentation is unavoidable, regardless
>>>>> of optimization. Could the mitigation code help in automatically tuning
>>>>> this fragmentation?
>>>>>
>>>>
>>>> I agree with the point that enabling mTHP always is not the right thing to do
>>>> on all platforms. I also think it might be the case that enabling mTHP
>>>> might be a good thing for some workloads, but enabling mTHP swapin along with
>>>> it might not.
>>>>
>>>> As you said when you have apps switching between foreground and background
>>>> in android, it probably makes sense to have large folio swapping, as you
>>>> want to bringin all the pages from background app as quickly as possible.
>>>> And also all the TLB optimizations and smaller lru overhead you get after
>>>> you have brought in all the pages.
>>>> Linux kernel build test doesnt really get to benefit from the TLB optimization
>>>> and smaller lru overhead, as probably the pages are very short lived. So I
>>>> think it doesnt show the benefit of large folio swapin properly and
>>>> large folio swapin should probably be disabled for this kind of workload,
>>>> eventhough mTHP should be enabled.
>>>
>>> I'm not entirely sure if this applies to platforms without TLB
>>> optimization, especially
>>> in the absence of swap. In a memory-limited cgroup without swap, would
>>> mTHP still
>>> cause significant thrashing of file-backed folios? When a large swap
>>> file is present,
>>> the inability to swap in mTHP seems to act as a workaround for fragmentation,
>>> allowing fragmented pages of the original mTHP from do_anonymous_page() to
>>> remain in swap.
>>>
>>>>
>>>> I am not sure that the approach we are trying in this patch is the right way:
>>>> - This patch makes it a memcg issue, but you could have memcg disabled and
>>>> then the mitigation being tried here wont apply.
>>>> - Instead of this being a large folio swapin issue, is it more of a readahead
>>>> issue? If we zswap (without the large folio swapin series) and change the window
>>>> to 1 in swap_vma_readahead, we might see an improvement in linux kernel build time
>>>> when cgroup memory is limited as readahead would probably cause swap thrashing as
>>>> well.
>>>> - Instead of looking at cgroup margin, maybe we should try and look at
>>>> the rate of change of workingset_restore_anon? This might be a lot more complicated
>>>> to do, but probably is the right metric to determine swap thrashing. It also means
>>>> that this could be used in both the synchronous swapcache skipping path and
>>>> swapin_readahead path.
>>>> (Thanks Johannes for suggesting this)
>>>>
>>>> With the large folio swapin, I do see the large improvement when considering only
>>>> swapin performance and latency in the same way as you saw in zram.
>>>> Maybe the right short term approach is to have
>>>> /sys/kernel/mm/transparent_hugepage/swapin
>>>> and have that disabled by default to avoid regression.
>>>
>>> A crucial component is still missing—managing the compression and decompression
>>> of multiple pages as a larger block. This could significantly reduce
>>> system time and
>>> potentially resolve the kernel build issue within a small memory
>>> cgroup, even with
>>> swap thrashing.
>>>
>>> I’ll send an update ASAP so you can rebase for zswap.
>>
>> Did you mean https://lore.kernel.org/all/20241021232852.4061-1-21cnbao@xxxxxxxxx/?
>> Thats wont benefit zswap, right?
>
> That's right. I assume we can also make it work with zswap?
Hopefully yes. Thats mainly why I was looking at that series, to try and find
a way to do something similar for zswap.
>
>> I actually had a few questions about it. Mainly that the benefit comes if the
>> pagefault happens on page 0 of the large folio. But if the page fault happens
>> on any other page, lets say page 1 of a 64K folio. then it will decompress the
>> entire 64K chunk and just copy page 1? (memcpy in zram_bvec_read_multi_pages_partial).
>> Could that cause a regression as you have to decompress a large chunk for just
>> getting 1 4K page?
>> If we assume uniform distribution of page faults, maybe it could make things worse?
>>
>> I probably should ask all of this in that thread.
>
> With mTHP swap-in, a page fault on any page behaves the same as a fault on
> page 0. Without mTHP swap-in, there’s also no difference between
> faults on page 0
> and other pages.
Ah ok, its because of the ALIGN_DOWN in
https://elixir.bootlin.com/linux/v6.12-rc5/source/mm/memory.c#L4158,
right?
>
> A fault on any page means that the entire block is decompressed. The
> only difference
> is that we don’t partially copy one page when mTHP swap-in is present.
>
Ah so zram_bvec_read_multi_pages_partial would be called only
if someone swaps out mTHP, disables it and then tries to do swapin?
Thanks
>>
>>>
>>>> If the workload owner sees a benefit, they can enable it.
>>>> I can add this when sending the next version of large folio zswapin if that makes
>>>> sense?
>>>> Longer term I can try and have a look at if we can do something with
>>>> workingset_restore_anon to improve things.
>>>>
>>>> Thanks,
>>>> Usama
>
> Thanks
> Barry
