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.
> 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
