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

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

On Mon, Oct 21, 2024 at 9:14 PM Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>
>
>
> On 2024/10/21 15:55, Barry Song wrote:
> > 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?
>
> Here is for SGP_FALLOC, vmf = NULL, we could use folio_zero_user(folio,
> 0), I think the performance is worse, will retest once I can access
> hardware.

Perhaps, since the current code uses clear_hugepage(). Does using
index << PAGE_SHIFT as the addr_hint offer any benefit?

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