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From:   Hsin-Yi Wang <hsinyi@chromium.org>
Date:   Mon, 13 Jun 2022 16:35:35 +0800
Message-ID: <CAJMQK-jpszOyA36gTSC=EMm+8+=0j63wErMwZ07cMUb2YBwEEQ@mail.gmail.com>
Subject: Re: [PATCH v5 3/3] squashfs: implement readahead
To:     Xiongwei Song <sxwjean@gmail.com>
Cc:     Phillip Lougher <phillip@squashfs.org.uk>,
        Matthew Wilcox <willy@infradead.org>,
        Xiongwei Song <Xiongwei.Song@windriver.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Zheng Liang <zhengliang6@huawei.com>,
        Zhang Yi <yi.zhang@huawei.com>, Hou Tao <houtao1@huawei.com>,
        Miao Xie <miaoxie@huawei.com>,
        "linux-mm @ kvack . org" <linux-mm@kvack.org>,
        "squashfs-devel @ lists . sourceforge . net" 
        <squashfs-devel@lists.sourceforge.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Jun 13, 2022 at 9:36 AM Xiongwei Song <sxwjean@gmail.com> wrote:
>
> Hi,
>
> On Fri, Jun 10, 2022 at 3:42 PM Phillip Lougher <phillip@squashfs.org.uk> wrote:
> >
> > On 09/06/2022 15:46, Xiongwei Song wrote:
> > > This version is bad for my test. I ran the test below
> > > "for cnt in $(seq 0 9); do echo 3 > /proc/sys/vm/drop_caches; echo
> > > "Loop ${cnt}:"; time -v find /software/test[0-9][0-9] | xargs -P 24 -i
> > > cat {} > /dev/null 2>/dev/null; echo ""; done"
> > > in 90 partitions.
> > >
> > > With 9eec1d897139 reverted:
> > > 1:06.18 (1m + 6.18s)
> > > 1:05.65
> > > 1:06.34
> > > 1:06.88
> > > 1:06.52
> > > 1:06.78
> > > 1:06.61
> > > 1:06.99
> > > 1:06.60
> > > 1:06.79
> > >
> > > With this version:
> > > 2:36.85 (2m + 36.85s)
> > > 2:28.89
> > > 1:43.46
> > > 1:41.50
> > > 1:42.75
> > > 1:43.46
> > > 1:43.67
> > > 1:44.41
> > > 1:44.91
> > > 1:45.44
> > >
> > > Any thoughts?
> >
> > Thank-you for your latest test results, and they tend to
> > imply that the latest version of the patch hasn't improved
> > performance in your use-case.
> >
> > One thing which is becoming clear here is that the devil is in
> > the detail, and your results being summaries are not capturing
> > enough detail to understand what is happening.  They show
> > something is wrong, but, don't give any guidance as to what
> > is happening.
> >
> > I think it will be difficult to capture more details from
> > your test case.  But, detail can be captured from summaries, by
> > varying the input and extrapolating from the results.
> >
> > By that I mean have you tried changing anything, and observed any
> > changed results?
> >
> > For instance have you tried any of the following
> >
> > 1.  Changing the parallelism of your test from 24 read threads.
> >      Does 1, 2, 4 etc parallel read threads change the observed
> >      behaviour?  In other words is the slow-down observed across
> >      all degrees of parallelism, or is there a critical point.
> >
> > 2. Does the Squashfs parallelism options in the kernel configuration
> >     change the behaviour?  Knowing if the number of "decompressors"
> >     available changes the difference in performance could be important.
> >
> > 3. Are your Squashfs filesystems built using fragments, or without
> >     fragments?  Rebuilding the filesystems without fragments, and
> >     observing any different performance, would help to pinpoint
> >     where the issue lies.
> >
> > 4. What is the block size used in your Squashfs filesystems.  Have
> >     you tried changing the block size, and seen what effect
> >     it has on the difference in performance between the patches?
> >
> > 5. You don't mention where your Squashfs filesystems are stored.
> >     Is this slow media or fast media?  Have you tried moving
> >     the Squashfs filesystems onto different media and observed
> >     any difference in performance between the patches?
> >
>
> Thanks for your response and inputs.  I really appreciated your help.
>  I can try these things  but can't provide the detailed results for
> now because I'm busy with a few things, hence It's hard to focus
> on this one thing for me.
>
> > The fact of the matter is there are many over-lapping factors
> > which affect the performance of Squashfs filesystems (like any
> > reasonably complex code), which may be elsewhere.  It can only
> > take a small change somewhere to have a dramatic affect on
> > performance.
> >
> > This is particularly the case with embedded systems, which
> > may be short on CPU performance, short on RAM, and have low
> > performance media, and be effectively operating on the "edge".
> > It can only take a small change, an update for instance, to
> > change from performing well to badly.
>
> Totally agree.
>
> >
> > I speak from experience, having spent over ten years in embedded
> > Linux as a senior engineer and then as a consultant.  I have
> > my own horror tales as a consultant, dealing with systems pushed
> > beyond the edge (with hacks), and the customer insisting they
> > didn't do anything to cause the system to finally break.
> >
> > Maybe it is off topic here.  But, I remember one instance where
> > a customer had a system out in the field, which "inexplicably"
> > started to lock up every 6 months or so.  This system had regular
> > updates "over the air", and I discovered the "lock up" only
> > started happening after the latest update.  It turns out the new version
> > of the application had grown a new feature which needed more
> > RAM than normal.  This feature wasn't used very often, but,
> > if it coincided with an infrequent "house-keeping" background task,
> > the system ran out of memory and locked up (they had disabled the OOM
> > killer).  This was so rare it might only coincide after six months.  No
> > bug, but a slow growth in working set RAM over a number of versions.
> >
> > In other words we may be looking at a knock-on side effect of
> > readahead, which is either caused by issues elsewhere or is
> > causing issues elsewhere.
> >
> > Dealing with it in isolation, as bug in the readahead code is going
> > to get us nowhere, looking for something that isn't there.
> >
> > I'm not saying that this is the case here.  But, the more detail
> > you can provide, and the more test variants you can provide will
> > help to determine what is the problem.
>
> Thanks for your sharing. I will provide detail later.
>
Hi,

Thanks for testing on v5. I've sent v6 which is based on the series:
https://patchwork.kernel.org/project/linux-mm/cover/20220611032133.5743-1-phillip@squashfs.org.uk/.

v6: https://patchwork.kernel.org/project/linux-mm/cover/20220613082802.1301238-1-hsinyi@chromium.org/

To apply the patch on linux-next, one might have to revert
ca1505bf4805 ("squashfs: implement readahead") and 9d58b94aa73a
("squashfs: always build "file direct" version of page actor") first.


> Regards,
> Xiongwei
>
> >
> > Thanks
> >
> > Phillip
> >
> >
> > >
> > > Regards,
> > > Xiongwei
> > >
> > > On Mon, Jun 6, 2022 at 11:03 PM Hsin-Yi Wang <hsinyi@chromium.org> wrote:
> > >>
> > >> Implement readahead callback for squashfs. It will read datablocks
> > >> which cover pages in readahead request. For a few cases it will
> > >> not mark page as uptodate, including:
> > >> - file end is 0.
> > >> - zero filled blocks.
> > >> - current batch of pages isn't in the same datablock.
> > >> - decompressor error.
> > >> Otherwise pages will be marked as uptodate. The unhandled pages will be
> > >> updated by readpage later.
> > >>
> > >> Suggested-by: Matthew Wilcox <willy@infradead.org>
> > >> Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
> > >> Reported-by: Matthew Wilcox <willy@infradead.org>
> > >> Reported-by: Phillip Lougher <phillip@squashfs.org.uk>
> > >> Reported-by: Xiongwei Song <Xiongwei.Song@windriver.com>
> > >> Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > >> Reported-by: Andrew Morton <akpm@linux-foundation.org>
> > >> ---
> > >> v4->v5:
> > >> - Handle short file cases reported by Marek and Matthew.
> > >> - Fix checkpatch error reported by Andrew.
> > >>
> > >> v4: https://lore.kernel.org/lkml/20220601103922.1338320-4-hsinyi@chromium.org/
> > >> v3: https://lore.kernel.org/lkml/20220523065909.883444-4-hsinyi@chromium.org/
> > >> v2: https://lore.kernel.org/lkml/20220517082650.2005840-4-hsinyi@chromium.org/
> > >> v1: https://lore.kernel.org/lkml/20220516105100.1412740-3-hsinyi@chromium.org/
> > >> ---
> > >>   fs/squashfs/file.c | 124 ++++++++++++++++++++++++++++++++++++++++++++-
> > >>   1 file changed, 123 insertions(+), 1 deletion(-)
> > >>
> > >> diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
> > >> index a8e495d8eb86..fbd096cd15f4 100644
> > >> --- a/fs/squashfs/file.c
> > >> +++ b/fs/squashfs/file.c
> > >> @@ -39,6 +39,7 @@
> > >>   #include "squashfs_fs_sb.h"
> > >>   #include "squashfs_fs_i.h"
> > >>   #include "squashfs.h"
> > >> +#include "page_actor.h"
> > >>
> > >>   /*
> > >>    * Locate cache slot in range [offset, index] for specified inode.  If
> > >> @@ -495,7 +496,128 @@ static int squashfs_read_folio(struct file *file, struct folio *folio)
> > >>          return 0;
> > >>   }
> > >>
> > >> +static void squashfs_readahead(struct readahead_control *ractl)
> > >> +{
> > >> +       struct inode *inode = ractl->mapping->host;
> > >> +       struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
> > >> +       size_t mask = (1UL << msblk->block_log) - 1;
> > >> +       unsigned short shift = msblk->block_log - PAGE_SHIFT;
> > >> +       loff_t start = readahead_pos(ractl) & ~mask;
> > >> +       size_t len = readahead_length(ractl) + readahead_pos(ractl) - start;
> > >> +       struct squashfs_page_actor *actor;
> > >> +       unsigned int nr_pages = 0;
> > >> +       struct page **pages;
> > >> +       int i, file_end = i_size_read(inode) >> msblk->block_log;
> > >> +       unsigned int max_pages = 1UL << shift;
> > >> +
> > >> +       readahead_expand(ractl, start, (len | mask) + 1);
> > >> +
> > >> +       if (file_end == 0)
> > >> +               return;
> > >> +
> > >> +       pages = kmalloc_array(max_pages, sizeof(void *), GFP_KERNEL);
> > >> +       if (!pages)
> > >> +               return;
> > >> +
> > >> +       actor = squashfs_page_actor_init_special(pages, max_pages, 0);
> > >> +       if (!actor)
> > >> +               goto out;
> > >> +
> > >> +       for (;;) {
> > >> +               pgoff_t index;
> > >> +               int res, bsize;
> > >> +               u64 block = 0;
> > >> +               unsigned int expected;
> > >> +
> > >> +               nr_pages = __readahead_batch(ractl, pages, max_pages);
> > >> +               if (!nr_pages)
> > >> +                       break;
> > >> +
> > >> +               if (readahead_pos(ractl) >= i_size_read(inode))
> > >> +                       goto skip_pages;
> > >> +
> > >> +               index = pages[0]->index >> shift;
> > >> +               if ((pages[nr_pages - 1]->index >> shift) != index)
> > >> +                       goto skip_pages;
> > >> +
> > >> +               expected = index == file_end ?
> > >> +                          (i_size_read(inode) & (msblk->block_size - 1)) :
> > >> +                           msblk->block_size;
> > >> +
> > >> +               bsize = read_blocklist(inode, index, &block);
> > >> +               if (bsize == 0)
> > >> +                       goto skip_pages;
> > >> +
> > >> +               if (nr_pages < max_pages) {
> > >> +                       struct squashfs_cache_entry *buffer;
> > >> +                       unsigned int block_mask = max_pages - 1;
> > >> +                       int offset = pages[0]->index - (pages[0]->index & ~block_mask);
> > >> +
> > >> +                       buffer = squashfs_get_datablock(inode->i_sb, block,
> > >> +                                                       bsize);
> > >> +                       if (buffer->error) {
> > >> +                               squashfs_cache_put(buffer);
> > >> +                               goto skip_pages;
> > >> +                       }
> > >> +
> > >> +                       expected -= offset * PAGE_SIZE;
> > >> +                       for (i = 0; i < nr_pages && expected > 0; i++,
> > >> +                                               expected -= PAGE_SIZE, offset++) {
> > >> +                               int avail = min_t(int, expected, PAGE_SIZE);
> > >> +
> > >> +                               squashfs_fill_page(pages[i], buffer,
> > >> +                                               offset * PAGE_SIZE, avail);
> > >> +                               unlock_page(pages[i]);
> > >> +                       }
> > >> +
> > >> +                       squashfs_cache_put(buffer);
> > >> +                       continue;
> > >> +               }
> > >> +
> > >> +               res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
> > >> +                                        actor);
> > >> +
> > >> +               if (res == expected) {
> > >> +                       int bytes;
> > >> +
> > >> +                       /* Last page may have trailing bytes not filled */
> > >> +                       bytes = res % PAGE_SIZE;
> > >> +                       if (bytes) {
> > >> +                               void *pageaddr;
> > >> +
> > >> +                               pageaddr = kmap_atomic(pages[nr_pages - 1]);
> > >> +                               memset(pageaddr + bytes, 0, PAGE_SIZE - bytes);
> > >> +                               kunmap_atomic(pageaddr);
> > >> +                       }
> > >> +
> > >> +                       for (i = 0; i < nr_pages; i++) {
> > >> +                               flush_dcache_page(pages[i]);
> > >> +                               SetPageUptodate(pages[i]);
> > >> +                       }
> > >> +               }
> > >> +
> > >> +               for (i = 0; i < nr_pages; i++) {
> > >> +                       unlock_page(pages[i]);
> > >> +                       put_page(pages[i]);
> > >> +               }
> > >> +       }
> > >> +
> > >> +       kfree(actor);
> > >> +       kfree(pages);
> > >> +       return;
> > >> +
> > >> +skip_pages:
> > >> +       for (i = 0; i < nr_pages; i++) {
> > >> +               unlock_page(pages[i]);
> > >> +               put_page(pages[i]);
> > >> +       }
> > >> +
> > >> +       kfree(actor);
> > >> +out:
> > >> +       kfree(pages);
> > >> +}
> > >>
> > >>   const struct address_space_operations squashfs_aops = {
> > >> -       .read_folio = squashfs_read_folio
> > >> +       .read_folio = squashfs_read_folio,
> > >> +       .readahead = squashfs_readahead
> > >>   };
> > >> --
> > >> 2.36.1.255.ge46751e96f-goog
> > >>
> > >>
> >