Re: [PATCH v2 3/3] squashfs: implement readahead

From: Phillip Lougher
Date: Thu May 19 2022 - 23:02:17 EST

On 19/05/2022 09:09, Hsin-Yi Wang wrote:
On Tue, May 17, 2022 at 4:28 PM Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> 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 or not enough in a
datablock.
Otherwise pages will be marked as uptodate. The unhandled pages will be
updated by readpage later.

Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Reported-by: Phillip Lougher <phillip@xxxxxxxxxxxxxxx>
Reported-by: Xiongwei Song <sxwjean@xxxxxxxxx>
---
v1->v2: remove unused check on readahead_expand().
v1: https://lore.kernel.org/lkml/20220516105100.1412740-3-hsinyi@xxxxxxxxxxxx/
---

Hi Phillip and Matthew,

Regarding the performance issue of this patch, I saw a possible
performance gain if we only read the first block instead of reading
until nr_pages == 0.

To be more clear, apply the following diff (Please ignore the skipping
of nr_pages check first. This is a demonstration of "only read and
update the first block per readahead call"):

diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
index aad6823f0615..c52f7c4a7cfe 100644
--- a/fs/squashfs/file.c
+++ b/fs/squashfs/file.c
@@ -524,10 +524,8 @@ static void squashfs_readahead(struct
readahead_control *ractl)
if (!actor)
goto out;

- for (;;) {
+ {
nr_pages = __readahead_batch(ractl, pages, max_pages);
- if (!nr_pages)
- break;

if (readahead_pos(ractl) >= i_size_read(inode) ||
nr_pages < max_pages)


All the performance numbers:
1. original: 39s
2. revert "mm: put readahead pages in cache earlier": 2.8s
3. v2 of this patch: 2.7s
4. v2 of this patch and apply the diff: 1.8s

In my testing data, normally it reads and updates 1~2 blocks per
readahead call. The change might not make sense since the performance
improvement may only happen in certain cases.
What do you think? Or is the performance of the current patch
considered reasonable?

It entirely depends on where the speed improvement comes from.

From experience, the speed improvement is probably worthwhile,
and probably isn't gained at the expense of worse performance
on other work-loads.

But this is a guestimate, based on the fact timings 2 and 3
(2.8s v 2.7s) are almost identical. Which implies the v2
patch isn't now doing any more work than the previous
baseline before the "mm: put readahead pages in cache earlier"
patch (*).

As such the speed improvement must be coming from increased
parallelism. Such as moving from serially reading the
readahead blocks to parallel reading.

But, without looking at any trace output, that is just a
guestimate.

Phillip

(*) multiply decompressing the same blocks, which
is the cause of the performance regression.

Thanks.

testing env:
- arm64 on kernel 5.10
- data: ~ 300K pack file contains some android files

fs/squashfs/file.c | 77 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 76 insertions(+), 1 deletion(-)

diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
index a8e495d8eb86..e10a55c5b1eb 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 +496,81 @@ 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;
+ size_t 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;
+ u64 block = 0;
+ int bsize, res, i, index;
+ int 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 (;;) {
+ nr_pages = __readahead_batch(ractl, pages, max_pages);
+ if (!nr_pages)
+ break;
+
+ if (readahead_pos(ractl) >= i_size_read(inode) ||
+ nr_pages < max_pages)
+ goto skip_pages;
+
+ index = pages[0]->index >> shift;
+ if ((pages[nr_pages - 1]->index >> shift) != index)
+ goto skip_pages;
+
+ bsize = read_blocklist(inode, index, &block);
+ if (bsize == 0)
+ goto skip_pages;
+
+ res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
+ actor);
+
+ if (res >= 0)
+ for (i = 0; i < nr_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
};
--
2.36.0.550.gb090851708-goog