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

Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> · Mon, 21 Oct 2024 16:14:40 +0800

On 2024/10/21 15:55, Barry Song wrote:
On Mon, Oct 21, 2024 at 8:47 PM Barry Song <21cnbao@xxxxxxxxx> wrote:

On Mon, Oct 21, 2024 at 7:09 PM Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:



On 2024/10/21 13:38, Barry Song wrote:
On Mon, Oct 21, 2024 at 6:16 PM Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:



On 2024/10/21 12:15, Barry Song wrote:
On Fri, Oct 18, 2024 at 8:48 PM Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> 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@xxxxxxxxxx> 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.









