Re: [PATCH] mm: shmem: convert to use folio_zero_range()

[Barry Song <21cnbao@gmail.com>]

On Mon, Oct 21, 2024 at 8:47 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Mon, Oct 21, 2024 at 7:09 PM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> >
> >
> >
> > On 2024/10/21 13:38, Barry Song wrote:
> > > On Mon, Oct 21, 2024 at 6:16 PM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> > >>
> > >>
> > >>
> > >> On 2024/10/21 12:15, Barry Song wrote:
> > >>> On Fri, Oct 18, 2024 at 8:48 PM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> > >>>>
> > >>>>
> > >>>>
> > >>>> On 2024/10/18 15:32, Kefeng Wang wrote:
> > >>>>>
> > >>>>>
> > >>>>> On 2024/10/18 13:23, Barry Song wrote:
> > >>>>>> On Fri, Oct 18, 2024 at 6:20 PM Kefeng Wang
> > >>>>>> <wangkefeng.wang@huawei.com> wrote:
> > >>>>>>>
> > >>>>>>>
> > >>>>>>>
> > >>>>>>> On 2024/10/17 23:09, Matthew Wilcox wrote:
> > >>>>>>>> On Thu, Oct 17, 2024 at 10:25:04PM +0800, Kefeng Wang wrote:
> > >>>>>>>>> Directly use folio_zero_range() to cleanup code.
> > >>>>>>>>
> > >>>>>>>> Are you sure there's no performance regression introduced by this?
> > >>>>>>>> clear_highpage() is often optimised in ways that we can't optimise for
> > >>>>>>>> a plain memset().  On the other hand, if the folio is large, maybe a
> > >>>>>>>> modern CPU will be able to do better than clear-one-page-at-a-time.
> > >>>>>>>>
> > >>>>>>>
> > >>>>>>> Right, I missing this, clear_page might be better than memset, I change
> > >>>>>>> this one when look at the shmem_writepage(), which already convert to
> > >>>>>>> use folio_zero_range() from clear_highpage(), also I grep
> > >>>>>>> folio_zero_range(), there are some other to use folio_zero_range().
> > >>>>>>>
> > >>>>>>> fs/bcachefs/fs-io-buffered.c:           folio_zero_range(folio, 0,
> > >>>>>>> folio_size(folio));
> > >>>>>>> fs/bcachefs/fs-io-buffered.c:                   folio_zero_range(f,
> > >>>>>>> 0, folio_size(f));
> > >>>>>>> fs/bcachefs/fs-io-buffered.c:                   folio_zero_range(f,
> > >>>>>>> 0, folio_size(f));
> > >>>>>>> fs/libfs.c:     folio_zero_range(folio, 0, folio_size(folio));
> > >>>>>>> fs/ntfs3/frecord.c:             folio_zero_range(folio, 0,
> > >>>>>>> folio_size(folio));
> > >>>>>>> mm/page_io.c:   folio_zero_range(folio, 0, folio_size(folio));
> > >>>>>>> mm/shmem.c:             folio_zero_range(folio, 0, folio_size(folio));
> > >>>>>>>
> > >>>>>>>
> > >>>>>>>> IOW, what performance testing have you done with this patch?
> > >>>>>>>
> > >>>>>>> No performance test before, but I write a testcase,
> > >>>>>>>
> > >>>>>>> 1) allocate N large folios (folio_alloc(PMD_ORDER))
> > >>>>>>> 2) then calculate the diff(us) when clear all N folios
> > >>>>>>>        clear_highpage/folio_zero_range/folio_zero_user
> > >>>>>>> 3) release N folios
> > >>>>>>>
> > >>>>>>> the result(run 5 times) shown below on my machine,
> > >>>>>>>
> > >>>>>>> N=1,
> > >>>>>>>            clear_highpage  folio_zero_range    folio_zero_user
> > >>>>>>>       1      69                   74                 177
> > >>>>>>>       2      57                   62                 168
> > >>>>>>>       3      54                   58                 234
> > >>>>>>>       4      54                   58                 157
> > >>>>>>>       5      56                   62                 148
> > >>>>>>> avg       58                   62.8               176.8
> > >>>>>>>
> > >>>>>>>
> > >>>>>>> N=100
> > >>>>>>>            clear_highpage  folio_zero_range    folio_zero_user
> > >>>>>>>       1    11015                 11309               32833
> > >>>>>>>       2    10385                 11110               49751
> > >>>>>>>       3    10369                 11056               33095
> > >>>>>>>       4    10332                 11017               33106
> > >>>>>>>       5    10483                 11000               49032
> > >>>>>>> avg     10516.8               11098.4             39563.4
> > >>>>>>>
> > >>>>>>> N=512
> > >>>>>>>            clear_highpage  folio_zero_range   folio_zero_user
> > >>>>>>>       1    55560                 60055              156876
> > >>>>>>>       2    55485                 60024              157132
> > >>>>>>>       3    55474                 60129              156658
> > >>>>>>>       4    55555                 59867              157259
> > >>>>>>>       5    55528                 59932              157108
> > >>>>>>> avg     55520.4               60001.4            157006.6
> > >>>>>>>
> > >>>>>>>
> > >>>>>>>
> > >>>>>>> folio_zero_user with many cond_resched(), so time fluctuates a lot,
> > >>>>>>> clear_highpage is better folio_zero_range as you said.
> > >>>>>>>
> > >>>>>>> Maybe add a new helper to convert all folio_zero_range(folio, 0,
> > >>>>>>> folio_size(folio))
> > >>>>>>> to use clear_highpage + flush_dcache_folio?
> > >>>>>>
> > >>>>>> If this also improves performance for other existing callers of
> > >>>>>> folio_zero_range(), then that's a positive outcome.
> > >>>>>
> > ...
> >
> > >>> hi Kefeng,
> > >>> what's your point? providing a helper like clear_highfolio() or similar?
> > >>
> > >> Yes, from above test, using clear_highpage/flush_dcache_folio is better
> > >> than using folio_zero_range() for folio zero(especially for large
> > >> folio), so I'd like to add a new helper, maybe name it folio_zero()
> > >> since it zero the whole folio.
> > >
> > > we already have a helper like folio_zero_user()?
> > > it is not good enough?
> >
> > Since it is with many cond_resched(), the performance is worst...
>
> Not exactly? It should have zero cost for a preemptible kernel.
> For a non-preemptible kernel, it helps avoid clearing the folio
> from occupying the CPU and starving other processes, right?

--- a/mm/shmem.c
+++ b/mm/shmem.c

@@ -2393,10 +2393,7 @@ static int shmem_get_folio_gfp(struct inode
*inode, pgoff_t index,
         * it now, lest undo on failure cancel our earlier guarantee.
         */

        if (sgp != SGP_WRITE && !folio_test_uptodate(folio)) {
-               long i, n = folio_nr_pages(folio);
-
-               for (i = 0; i < n; i++)
-                       clear_highpage(folio_page(folio, i));
+               folio_zero_user(folio, vmf->address);
                flush_dcache_folio(folio);
                folio_mark_uptodate(folio);
        }

Do we perform better or worse with the following?