Re: [RFC 0/4] mm: zswap: add support for zswapin of large folios

Barry Song
2024-10-23T10:27:00Z

On Wed, Oct 23, 2024 at 11:07 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Wed, Oct 23, 2024 at 10:17 AM Usama Arif <usamaarif642@gmail.com> wrote:
> >
> >
> >
> > On 22/10/2024 21:46, Barry Song wrote:
> > > On Wed, Oct 23, 2024 at 4:26 AM Usama Arif <usamaarif642@gmail.com> wrote:
> > >>
> > >>
> > >>
> > >> On 21/10/2024 11:40, Usama Arif wrote:
> > >>>
> > >>>
> > >>> On 21/10/2024 06:09, Barry Song wrote:
> > >>>> On Fri, Oct 18, 2024 at 11:50 PM Usama Arif <usamaarif642@gmail.com> wrote:
> > >>>>>
> > >>>>> After large folio zswapout support added in [1], this patch adds
> > >>>>> support for zswapin of large folios to bring it on par with zram.
> > >>>>> This series makes sure that the benefits of large folios (fewer
> > >>>>> page faults, batched PTE and rmap manipulation, reduced lru list,
> > >>>>> TLB coalescing (for arm64 and amd)) are not lost at swap out when
> > >>>>> using zswap.
> > >>>>>
> > >>>>> It builds on top of [2] which added large folio swapin support for
> > >>>>> zram and provides the same level of large folio swapin support as
> > >>>>> zram, i.e. only supporting swap count == 1.
> > >>>>>
> > >>>>> Patch 1 skips swapcache for swapping in zswap pages, this should improve
> > >>>>> no readahead swapin performance [3], and also allows us to build on large
> > >>>>> folio swapin support added in [2], hence is a prerequisite for patch 3.
> > >>>>>
> > >>>>> Patch 3 adds support for large folio zswapin. This patch does not add
> > >>>>> support for hybrid backends (i.e. folios partly present swap and zswap).
> > >>>>>
> > >>>>> The main performance benefit comes from maintaining large folios *after*
> > >>>>> swapin, large folio performance improvements have been mentioned in previous
> > >>>>> series posted on it [2],[4], so have not added those. Below is a simple
> > >>>>> microbenchmark to measure the time needed *for* zswpin of 1G memory (along
> > >>>>> with memory integrity check).
> > >>>>>
> > >>>>> | no mTHP (ms) | 1M mTHP enabled (ms)
> > >>>>> Base kernel | 1165 | 1163
> > >>>>> Kernel with mTHP zswpin series | 1203 | 738
> > >>>>
> > >>>> Hi Usama,
> > >>>> Do you know where this minor regression for non-mTHP comes from?
> > >>>> As you even have skipped swapcache for small folios in zswap in patch1,
> > >>>> that part should have some gain? is it because of zswap_present_test()?
> > >>>>
> > >>>
> > >>> Hi Barry,
> > >>>
> > >>> The microbenchmark does a sequential read of 1G of memory, so it probably
> > >>> isnt very representative of real world usecases. This also means that
> > >>> swap_vma_readahead is able to readahead accurately all pages in its window.
> > >>> With this patch series, if doing 4K swapin, you get 1G/4K calls of fast
> > >>> do_swap_page. Without this patch, you get 1G/(4K*readahead window) of slow
> > >>> do_swap_page calls. I had added some prints and I was seeing 8 pages being
> > >>> readahead in 1 do_swap_page. The larger number of calls causes the slight
> > >>> regression (eventhough they are quite fast). I think in a realistic scenario,
> > >>> where readahead window wont be as large, there wont be a regression.
> > >>> The cost of zswap_present_test in the whole call stack of swapping page is
> > >>> very low and I think can be ignored.
> > >>>
> > >>> I think the more interesting thing is what Kanchana pointed out in
> > >>> https://lore.kernel.org/all/f2f2053f-ec5f-46a4-800d-50a3d2e61bff@gmail.com/
> > >>> I am curious, did you see this when testing large folio swapin and compression
> > >>> at 4K granuality? Its looks like swap thrashing so I think it would be common
> > >>> between zswap and zram. I dont have larger granuality zswap compression done,
> > >>> which is why I think there is a regression in time taken. (It could be because
> > >>> its tested on intel as well).
> > >>>
> > >>> Thanks,
> > >>> Usama
> > >>>
> > >>
> > >> Hi,
> > >>
> > >> So I have been doing some benchmarking after Kanchana pointed out a performance
> > >> regression in [1] of swapping in large folio. I would love to get thoughts from
> > >> zram folks on this, as thats where large folio swapin was first added [2].
> > >> As far as I can see, the current support in zram is doing large folio swapin
> > >> at 4K granuality. The large granuality compression in [3] which was posted
> > >> in March is not merged, so I am currently comparing upstream zram with this series.
> > >>
> > >> With the microbenchmark below of timing 1G swapin, there was a very large improvement
> > >> in performance by using this series. I think similar numbers would be seen in zram.
> > >
> > > Imagine running several apps on a phone and switching
> > > between them: A → B → C → D → E … → A → B … The app
> > > currently on the screen retains its memory, while the ones
> > > sent to the background are swapped out. When we bring
> > > those apps back to the foreground, their memory is restored.
> > > This behavior is quite similar to what you're seeing with
> > > your microbenchmark.
> > >
> >
> > Hi Barry,
> >
> > Thanks for explaining this! Do you know if there is some open source benchmark
> > we could use to show an improvement in app switching with large folios?
> >
>
> I’m fairly certain the Android team has this benchmark, but it’s not
> open source.
>
> A straightforward way to simulate this is to use a script that
> cyclically launches multiple applications, such as Chrome, Firefox,
> Office, PDF, and others.
>
> for example:
>
> launch chrome;
> launch firefox;
> launch youtube;
> ....
> launch chrome;
> launch firefox;
> ....
>
> On Android, we have "Android activity manager 'am' command" to do that.
> https://gist.github.com/tsohr/5711945
>
> Not quite sure if other windows managers have similar tools.
>
> > Also I guess swap thrashing can happen when apps are brought back to foreground?
> >
>
> Typically, the foreground app doesn't experience much swapping,
> as it is the most recently or frequently used. However, this may
> not hold for very low-end phones, where memory is significantly
> less than the app's working set. For instance, we can't expect a
> good user experience when playing a large game that requires 8GB
> of memory on a 4GB phone! :-)
> And for low-end phones, we never even enable mTHP.
>
> > >>
> > >> But when doing kernel build test, Kanchana saw a regression in [1]. I believe
> > >> its because of swap thrashing (causing large zswap activity), due to larger page swapin.
> > >> The part of the code that decides large folio swapin is the same between zswap and zram,
> > >> so I believe this would be observed in zram as well.
> > >
> > > Is this an extreme case where the workload's working set far
> > > exceeds the available memory by memcg limitation? I doubt mTHP
> > > would provide any real benefit from the start if the workload is bound to
> > > experience swap thrashing. What if we disable mTHP entirely?
> > >
> >
> > I would agree, this is an extreme case. I wanted (z)swap activity to happen so limited
> > memory.max to 4G.
> >
> > mTHP is beneficial in kernel test benchmarking going from no mTHP to 16K:
> >
> > ARM make defconfig; time make -j$(nproc) Image, cgroup memory.max=4G
> > metric no mTHP 16K mTHP=always
> > real 1m0.613s 0m52.008s
> > user 25m23.028s 25m19.488s
> > sys 25m45.466s 18m11.640s
> > zswpin 1911194 3108438
> > zswpout 6880815 9374628
> > pgfault 120430166 48976658
> > pgmajfault 1580674 2327086
> >
> >
>
> Interesting! We never use a phone to build the Linux kernel, but
> let me see if I can find some other machines to reproduce your data.

Hi Usama,

I suspect the regression occurs because you're running an edge case
where the memory cgroup stays nearly full most of the time (this isn't
an inherent issue with large folio swap-in). As a result, swapping in
mTHP quickly triggers a memcg overflow, causing a swap-out. The
next swap-in then recreates the overflow, leading to a repeating
cycle.

We need a way to stop the cup from repeatedly filling to the brim and
overflowing. While not a definitive fix, the following change might help
improve the situation:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c

index 17af08367c68..f2fa0eeb2d9a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -4559,7 +4559,10 @@ int mem_cgroup_swapin_charge_folio(struct folio
*folio, struct mm_struct *mm,
memcg = get_mem_cgroup_from_mm(mm);
rcu_read_unlock();

- ret = charge_memcg(folio, memcg, gfp);
+ if (folio_test_large(folio) && mem_cgroup_margin(memcg) <
MEMCG_CHARGE_BATCH)
+ ret = -ENOMEM;
+ else
+ ret = charge_memcg(folio, memcg, gfp);

css_put(&memcg->css);
return ret;
}

Please confirm if it makes the kernel build with memcg limitation
faster. If so, let's
work together to figure out an official patch :-) The above code hasn't consider
the parent memcg's overflow, so not an ideal fix.

>
> >
> >
> > >>
> > >> My initial thought was this might be because its intel, where you dont have the advantage
> > >> of TLB coalescing, so tested on AMD and ARM, but the regression is there on AMD
> > >> and ARM as well, though a bit less (have added the numbers below).
> > >>
> > >> The numbers show that the zswap activity increases and page faults decrease.
> > >> Overall this does result in sys time increasing and real time slightly increases,
> > >> likely because the cost of increased zswap activity is more than the benefit of
> > >> lower page faults.
> > >> I can see in [3] that pagefaults reduced in zram as well.
> > >>
> > >> Large folio swapin shows good numbers in microbenchmarks that just target reduce page
> > >> faults and sequential swapin only, but not in kernel build test. Is a similar regression
> > >> observed with zram when enabling large folio swapin on kernel build test? Maybe large
> > >> folio swapin makes more sense on workloads where mappings are kept for a longer time?
> > >>
> > >
> > > I suspect this is because mTHP doesn't always benefit workloads
> > > when available memory is quite limited compared to the working set.
> > > In that case, mTHP swap-in might introduce more features that
> > > exacerbate the problem. We used to have an extra control "swapin_enabled"
> > > for swap-in, but it never gained much traction:
> > > https://lore.kernel.org/linux-mm/20240726094618.401593-5-21cnbao@gmail.com/
> > > We can reconsider whether to include the knob, but if it's better
> > > to disable mTHP entirely for these cases, we can still adhere to
> > > the policy of "enabled".
> > >
> > Yes I think this makes sense to have. The only thing is, its too many knobs!
> > I personally think its already difficult to decide upto which mTHP size we
> > should enable (and I think this changes per workload). But if we add swapin_enabled
> > on top of that it can make things more difficult.
> >
> > > Using large block compression and decompression in zRAM will
> > > significantly reduce CPU usage, likely making the issue unnoticeable.
> > > However, the default minimum size for large block support is currently
> > > set to 64KB(ZSMALLOC_MULTI_PAGES_ORDER = 4).
> > >
> >
> > I saw that the patch was sent in March, and there werent any updates after?
> > Maybe I can try and cherry-pick that and see if we can develop large
> > granularity compression for zswap.
>
> will provide an updated version next week.
>
> >
> > >>
> > >> Kernel build numbers in cgroup with memory.max=4G to trigger zswap
> > >> Command for AMD: make defconfig; time make -j$(nproc) bzImage
> > >> Command for ARM: make defconfig; time make -j$(nproc) Image
> > >>
> > >>
> > >> AMD 16K+32K THP=always
> > >> metric mm-unstable mm-unstable + large folio zswapin series
> > >> real 1m23.038s 1m23.050s
> > >> user 53m57.210s 53m53.437s
> > >> sys 7m24.592s 7m48.843s
> > >> zswpin 612070 999244
> > >> zswpout 2226403 2347979
> > >> pgfault 20667366 20481728
> > >> pgmajfault 385887 269117
> > >>
> > >> AMD 16K+32K+64K THP=always
> > >> metric mm-unstable mm-unstable + large folio zswapin series
> > >> real 1m22.975s 1m23.266s
> > >> user 53m51.302s 53m51.069s
> > >> sys 7m40.168s 7m57.104s
> > >> zswpin 676492 1258573
> > >> zswpout 2449839 2714767
> > >> pgfault 17540746 17296555
> > >> pgmajfault 429629 307495
> > >> --------------------------
> > >> ARM 16K+32K THP=always
> > >> metric mm-unstable mm-unstable + large folio zswapin series
> > >> real 0m51.168s 0m52.086s
> > >> user 25m14.715s 25m15.765s
> > >> sys 17m18.856s 18m8.031s
> > >> zswpin 3904129 7339245
> > >> zswpout 11171295 13473461
> > >> pgfault 37313345 36011338
> > >> pgmajfault 2726253 1932642
> > >>
> > >>
> > >> ARM 16K+32K+64K THP=always
> > >> metric mm-unstable mm-unstable + large folio zswapin series
> > >> real 0m52.017s 0m53.828s
> > >> user 25m2.742s 25m0.046s
> > >> sys 18m24.525s 20m26.207s
> > >> zswpin 4853571 8908664
> > >> zswpout 12297199 15768764
> > >> pgfault 32158152 30425519
> > >> pgmajfault 3320717 2237015
> > >>
> > >>
> > >> Thanks!
> > >> Usama
> > >>
> > >>
> > >> [1] https://lore.kernel.org/all/f2f2053f-ec5f-46a4-800d-50a3d2e61bff@gmail.com/
> > >> [2] https://lore.kernel.org/all/20240821074541.516249-3-hanchuanhua@oppo.com/
> > >> [3] https://lore.kernel.org/all/20240327214816.31191-1-21cnbao@gmail.com/
> > >>
> > >>>
> > >>>>>
> > >>>>> The time measured was pretty consistent between runs (~1-2% variation).
> > >>>>> There is 36% improvement in zswapin time with 1M folios. The percentage
> > >>>>> improvement is likely to be more if the memcmp is removed.
> > >>>>>
> > >>>>> diff --git a/tools/testing/selftests/cgroup/test_zswap.c b/tools/testing/selftests/cgroup/test_zswap.c
> > >>>>> index 40de679248b8..77068c577c86 100644
> > >>>>> --- a/tools/testing/selftests/cgroup/test_zswap.c
> > >>>>> +++ b/tools/testing/selftests/cgroup/test_zswap.c
> > >>>>> @@ -9,6 +9,8 @@
> > >>>>> #include <string.h>
> > >>>>> #include <sys/wait.h>
> > >>>>> #include <sys/mman.h>
> > >>>>> +#include <sys/time.h>
> > >>>>> +#include <malloc.h>
> > >>>>>
> > >>>>> #include "../kselftest.h"
> > >>>>> #include "cgroup_util.h"
> > >>>>> @@ -407,6 +409,74 @@ static int test_zswap_writeback_disabled(const char *root)
> > >>>>> return test_zswap_writeback(root, false);
> > >>>>> }
> > >>>>>
> > >>>>> +static int zswapin_perf(const char *cgroup, void *arg)
> > >>>>> +{
> > >>>>> + long pagesize = sysconf(_SC_PAGESIZE);
> > >>>>> + size_t memsize = MB(1*1024);
> > >>>>> + char buf[pagesize];
> > >>>>> + int ret = -1;
> > >>>>> + char *mem;
> > >>>>> + struct timeval start, end;
> > >>>>> +
> > >>>>> + mem = (char *)memalign(2*1024*1024, memsize);
> > >>>>> + if (!mem)
> > >>>>> + return ret;
> > >>>>> +
> > >>>>> + /*
> > >>>>> + * Fill half of each page with increasing data, and keep other
> > >>>>> + * half empty, this will result in data that is still compressible
> > >>>>> + * and ends up in zswap, with material zswap usage.
> > >>>>> + */
> > >>>>> + for (int i = 0; i < pagesize; i++)
> > >>>>> + buf[i] = i < pagesize/2 ? (char) i : 0;
> > >>>>> +
> > >>>>> + for (int i = 0; i < memsize; i += pagesize)
> > >>>>> + memcpy(&mem[i], buf, pagesize);
> > >>>>> +
> > >>>>> + /* Try and reclaim allocated memory */
> > >>>>> + if (cg_write_numeric(cgroup, "memory.reclaim", memsize)) {
> > >>>>> + ksft_print_msg("Failed to reclaim all of the requested memory\n");
> > >>>>> + goto out;
> > >>>>> + }
> > >>>>> +
> > >>>>> + gettimeofday(&start, NULL);
> > >>>>> + /* zswpin */
> > >>>>> + for (int i = 0; i < memsize; i += pagesize) {
> > >>>>> + if (memcmp(&mem[i], buf, pagesize)) {
> > >>>>> + ksft_print_msg("invalid memory\n");
> > >>>>> + goto out;
> > >>>>> + }
> > >>>>> + }
> > >>>>> + gettimeofday(&end, NULL);
> > >>>>> + printf ("zswapin took %fms to run.\n", (end.tv_sec - start.tv_sec)*1000 + (double)(end.tv_usec - start.tv_usec) / 1000);
> > >>>>> + ret = 0;
> > >>>>> +out:
> > >>>>> + free(mem);
> > >>>>> + return ret;
> > >>>>> +}
> > >>>>> +
> > >>>>> +static int test_zswapin_perf(const char *root)
> > >>>>> +{
> > >>>>> + int ret = KSFT_FAIL;
> > >>>>> + char *test_group;
> > >>>>> +
> > >>>>> + test_group = cg_name(root, "zswapin_perf_test");
> > >>>>> + if (!test_group)
> > >>>>> + goto out;
> > >>>>> + if (cg_create(test_group))
> > >>>>> + goto out;
> > >>>>> +
> > >>>>> + if (cg_run(test_group, zswapin_perf, NULL))
> > >>>>> + goto out;
> > >>>>> +
> > >>>>> + ret = KSFT_PASS;
> > >>>>> +out:
> > >>>>> + cg_destroy(test_group);
> > >>>>> + free(test_group);
> > >>>>> + return ret;
> > >>>>> +}
> > >>>>> +
> > >>>>> /*
> > >>>>> * When trying to store a memcg page in zswap, if the memcg hits its memory
> > >>>>> * limit in zswap, writeback should affect only the zswapped pages of that
> > >>>>> @@ -584,6 +654,7 @@ struct zswap_test {
> > >>>>> T(test_zswapin),
> > >>>>> T(test_zswap_writeback_enabled),
> > >>>>> T(test_zswap_writeback_disabled),
> > >>>>> + T(test_zswapin_perf),
> > >>>>> T(test_no_kmem_bypass),
> > >>>>> T(test_no_invasive_cgroup_shrink),
> > >>>>> };
> > >>>>>
> > >>>>> [1] https://lore.kernel.org/all/20241001053222.6944-1-kanchana.p.sridhar@intel.com/
> > >>>>> [2] https://lore.kernel.org/all/20240821074541.516249-1-hanchuanhua@oppo.com/
> > >>>>> [3] https://lore.kernel.org/all/1505886205-9671-5-git-send-email-minchan@kernel.org/T/#u
> > >>>>> [4] https://lwn.net/Articles/955575/
> > >>>>>
> > >>>>> Usama Arif (4):
> > >>>>> mm/zswap: skip swapcache for swapping in zswap pages
> > >>>>> mm/zswap: modify zswap_decompress to accept page instead of folio
> > >>>>> mm/zswap: add support for large folio zswapin
> > >>>>> mm/zswap: count successful large folio zswap loads
> > >>>>>
> > >>>>> Documentation/admin-guide/mm/transhuge.rst | 3 +
> > >>>>> include/linux/huge_mm.h | 1 +
> > >>>>> include/linux/zswap.h | 6 ++
> > >>>>> mm/huge_memory.c | 3 +
> > >>>>> mm/memory.c | 16 +--
> > >>>>> mm/page_io.c | 2 +-
> > >>>>> mm/zswap.c | 120 ++++++++++++++-------
> > >>>>> 7 files changed, 99 insertions(+), 52 deletions(-)
> > >>>>>
> > >>>>> --
> > >>>>> 2.43.5
> > >>>>>
> > >>>>
> > >
>

Thanks
Barry