lib/zstd: use div_u64() to let it build on 32-bit
diff mbox

Message ID 20170627041823.1300-1-kilobyte@angband.pl
State New
Headers show

Commit Message

Adam Borowski June 27, 2017, 4:18 a.m. UTC
David Sterba wrote:
> > Thus, you want do_div() instead of /; do check widths and signedness of
> > arguments.
>
> No do_div please, div_u64 or div64_u64.

Good to know, the interface of do_div() is indeed weird.

I guess Nick has found and fixed the offending divisions in his tree
already, but this patch I'm sending is what I'm testing.

One thing to note is that it divides u64 by size_t, so the actual operation
differs on 32 vs 64-bit.  Yet the code fails to handle compressing pieces
bigger than 4GB in other places -- so use of size_t is misleading.  Perhaps
u32 would better convey this limitation?

Anyway, that this code didn't even compile on 32-bit also means it hasn't
been tested.  I just happen to have such an ARM machine doing Debian archive
rebuilds; I've rewritten the chroots with compress=zstd; this should be a
nice non-artificial test.  The load consists of snapshot+dpkg+gcc/etc+
assorted testsuites, two sbuild instances.  Seems to work fine for a whole
hour (yay!) already, let's see if there'll be any explosions.


-- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 --
Note that "total" is limited to 2³²-1 elsewhere despite being declared
as size_t, so it's ok to use 64/32 -- it's much faster on eg. x86-32
than 64/64.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
---
 lib/zstd/fse_compress.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Comments

Nick Terrell June 27, 2017, 5:27 a.m. UTC | #1
Adam, I’ve applied the same patch in my tree. I’ll send out the update [1]
once it's reviewed, since I also reduced the stack usage of functions
using over 1 KB of stack space.

You’re right that div_u64() will work, since the FSE functions are only
called on blocks of at most 128 KB at a time. Perhaps a u32 would be
clearer, but I would prefer to leave the signatures as is, to stay closer
to upstream. Upstream FSE should work with sizes larger than 4 GB, but
since it can't happen in zstd, it isn't a priority.

I have userland tests set up mocking the linux kernel headers, and tested
32-bit mode there, but neglected to test the kernel on a 32-bit VM, which
I’ve now corrected. Thanks for testing the patch on your ARM machine!

[1] https://github.com/facebook/zstd/pull/738/files

On 6/26/17, 9:18 PM, "Adam Borowski" <kilobyte@angband.pl> wrote:

    David Sterba wrote:
    > > Thus, you want do_div() instead of /; do check widths and signedness of

    > > arguments.

    >

    > No do_div please, div_u64 or div64_u64.

    
    Good to know, the interface of do_div() is indeed weird.
    
    I guess Nick has found and fixed the offending divisions in his tree
    already, but this patch I'm sending is what I'm testing.
    
    One thing to note is that it divides u64 by size_t, so the actual operation
    differs on 32 vs 64-bit.  Yet the code fails to handle compressing pieces
    bigger than 4GB in other places -- so use of size_t is misleading.  Perhaps
    u32 would better convey this limitation?
    
    Anyway, that this code didn't even compile on 32-bit also means it hasn't
    been tested.  I just happen to have such an ARM machine doing Debian archive
    rebuilds; I've rewritten the chroots with compress=zstd; this should be a
    nice non-artificial test.  The load consists of snapshot+dpkg+gcc/etc+
    assorted testsuites, two sbuild instances.  Seems to work fine for a whole
    hour (yay!) already, let's see if there'll be any explosions.
    
    
    -- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 ---- >8 --
    Note that "total" is limited to 2³²-1 elsewhere despite being declared
    as size_t, so it's ok to use 64/32 -- it's much faster on eg. x86-32
    than 64/64.
    
    Signed-off-by: Adam Borowski <kilobyte@angband.pl>

    ---
     lib/zstd/fse_compress.c | 5 +++--
     1 file changed, 3 insertions(+), 2 deletions(-)
    
    diff --git a/lib/zstd/fse_compress.c b/lib/zstd/fse_compress.c
    index e016bb177833..f59f9ebfe9c0 100644
    --- a/lib/zstd/fse_compress.c
    +++ b/lib/zstd/fse_compress.c
    @@ -49,6 +49,7 @@
     #include "fse.h"
     #include <linux/compiler.h>
     #include <linux/string.h> /* memcpy, memset */
    +#include <linux/math64.h>
     
     /* **************************************************************
     *  Error Management
    @@ -575,7 +576,7 @@ static size_t FSE_normalizeM2(short *norm, U32 tableLog, const unsigned *count,
     	{
     		U64 const vStepLog = 62 - tableLog;
     		U64 const mid = (1ULL << (vStepLog - 1)) - 1;
    -		U64 const rStep = ((((U64)1 << vStepLog) * ToDistribute) + mid) / total; /* scale on remaining */
    +		U64 const rStep = div_u64((((U64)1 << vStepLog) * ToDistribute) + mid, total); /* scale on remaining */
     		U64 tmpTotal = mid;
     		for (s = 0; s <= maxSymbolValue; s++) {
     			if (norm[s] == NOT_YET_ASSIGNED) {
    @@ -609,7 +610,7 @@ size_t FSE_normalizeCount(short *normalizedCounter, unsigned tableLog, const uns
     	{
     		U32 const rtbTable[] = {0, 473195, 504333, 520860, 550000, 700000, 750000, 830000};
     		U64 const scale = 62 - tableLog;
    -		U64 const step = ((U64)1 << 62) / total; /* <== here, one division ! */
    +		U64 const step = div_u64((U64)1 << 62, total); /* <== here, one division ! */
     		U64 const vStep = 1ULL << (scale - 20);
     		int stillToDistribute = 1 << tableLog;
     		unsigned s;
    -- 
    2.13.1
David Sterba June 27, 2017, 12:57 p.m. UTC | #2
Please don't top post.

On Tue, Jun 27, 2017 at 05:27:51AM +0000, Nick Terrell wrote:
> Adam, I’ve applied the same patch in my tree. I’ll send out the update [1]
> once it's reviewed, since I also reduced the stack usage of functions
> using over 1 KB of stack space.

Which function needs 1KB of stack space? That's quite a lot.

I can see in [1] that there are some on-stack buffers replaced by
pointers to the workspace. That's good, but I would like to know if
there's any hidden gem that grags the precious stack space.

> You’re right that div_u64() will work, since the FSE functions are only
> called on blocks of at most 128 KB at a time. Perhaps a u32 would be
> clearer, but I would prefer to leave the signatures as is, to stay closer
> to upstream. Upstream FSE should work with sizes larger than 4 GB, but
> since it can't happen in zstd, it isn't a priority.

Hm, I'd suggest to create a version optimized for kernel, eg. expecting
that 4+ GB buffer will never be used and you can use the most fittin in
type. This should affect only the function signatures, not the
algorithm implementation, so porting future zstd changes should be
straightforward.
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Adam Borowski June 29, 2017, 1:02 a.m. UTC | #3
On Tue, Jun 27, 2017 at 05:27:51AM +0000, Nick Terrell wrote:
> Adam, I’ve applied the same patch in my tree. I’ll send out the update [1]
> once it's reviewed, since I also reduced the stack usage of functions
> using over 1 KB of stack space.
> 
> I have userland tests set up mocking the linux kernel headers, and tested
> 32-bit mode there, but neglected to test the kernel on a 32-bit VM, which
> I’ve now corrected. Thanks for testing the patch on your ARM machine!

Is there a version I should be testing?

I got a bunch of those:
[10170.448783] kworker/u8:6: page allocation stalls for 60720ms, order:0, mode:0x14000c2(GFP_KERNEL|__GFP_HIGHMEM), nodemask=(null)
[10170.448819] kworker/u8:6 cpuset=/ mems_allowed=0
[10170.448842] CPU: 3 PID: 13430 Comm: kworker/u8:6 Not tainted 4.12.0-rc7-00034-gdff47ed160bb #1
[10170.448846] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
[10170.448872] Workqueue: btrfs-endio btrfs_endio_helper
[10170.448910] [<c010de1c>] (unwind_backtrace) from [<c010adb8>] (show_stack+0x10/0x14)
[10170.448925] [<c010adb8>] (show_stack) from [<c0442b00>] (dump_stack+0x78/0x8c)
[10170.448942] [<c0442b00>] (dump_stack) from [<c01b0178>] (warn_alloc+0xc0/0x170)
[10170.448952] [<c01b0178>] (warn_alloc) from [<c01b0c3c>] (__alloc_pages_nodemask+0x97c/0xe30)
[10170.448964] [<c01b0c3c>] (__alloc_pages_nodemask) from [<c01e217c>] (__vmalloc_node_range+0x144/0x27c)
[10170.448976] [<c01e217c>] (__vmalloc_node_range) from [<c01e2550>] (__vmalloc_node.constprop.10+0x48/0x50)
[10170.448982] [<c01e2550>] (__vmalloc_node.constprop.10) from [<c01e25ec>] (vmalloc+0x2c/0x34)
[10170.448990] [<c01e25ec>] (vmalloc) from [<c038f7cc>] (zstd_alloc_workspace+0x6c/0xb8)
[10170.448997] [<c038f7cc>] (zstd_alloc_workspace) from [<c038fcb8>] (find_workspace+0x120/0x1f4)
[10170.449002] [<c038fcb8>] (find_workspace) from [<c038ff60>] (end_compressed_bio_read+0x1d4/0x3b0)
[10170.449016] [<c038ff60>] (end_compressed_bio_read) from [<c0130e14>] (process_one_work+0x1d8/0x3f0)
[10170.449026] [<c0130e14>] (process_one_work) from [<c0131a18>] (worker_thread+0x38/0x558)
[10170.449035] [<c0131a18>] (worker_thread) from [<c0136854>] (kthread+0x124/0x154)
[10170.449042] [<c0136854>] (kthread) from [<c01076f8>] (ret_from_fork+0x14/0x3c)

which never happened with compress=lzo, and a 2GB RAM machine that runs 4
threads of various builds runs into memory pressure quite often.  On the
other hand, I used 4.11 for lzo so this needs more testing before I can
blame the zstd code.

Also, I had network problems all day today so the machine was mostly idle
instead of doing further tests -- not quite going to pull sources to build
over a phone connection.

I'm on linus:4.12-rc7 with only a handful of btrfs patches (v3 of Qu's chunk
check, some misc crap) -- I guess I should use at least btrfs-for-4.13.  Or
would you prefer full-blown next?


Meow!

Patch
diff mbox

diff --git a/lib/zstd/fse_compress.c b/lib/zstd/fse_compress.c
index e016bb177833..f59f9ebfe9c0 100644
--- a/lib/zstd/fse_compress.c
+++ b/lib/zstd/fse_compress.c
@@ -49,6 +49,7 @@ 
 #include "fse.h"
 #include <linux/compiler.h>
 #include <linux/string.h> /* memcpy, memset */
+#include <linux/math64.h>
 
 /* **************************************************************
 *  Error Management
@@ -575,7 +576,7 @@  static size_t FSE_normalizeM2(short *norm, U32 tableLog, const unsigned *count,
 	{
 		U64 const vStepLog = 62 - tableLog;
 		U64 const mid = (1ULL << (vStepLog - 1)) - 1;
-		U64 const rStep = ((((U64)1 << vStepLog) * ToDistribute) + mid) / total; /* scale on remaining */
+		U64 const rStep = div_u64((((U64)1 << vStepLog) * ToDistribute) + mid, total); /* scale on remaining */
 		U64 tmpTotal = mid;
 		for (s = 0; s <= maxSymbolValue; s++) {
 			if (norm[s] == NOT_YET_ASSIGNED) {
@@ -609,7 +610,7 @@  size_t FSE_normalizeCount(short *normalizedCounter, unsigned tableLog, const uns
 	{
 		U32 const rtbTable[] = {0, 473195, 504333, 520860, 550000, 700000, 750000, 830000};
 		U64 const scale = 62 - tableLog;
-		U64 const step = ((U64)1 << 62) / total; /* <== here, one division ! */
+		U64 const step = div_u64((U64)1 << 62, total); /* <== here, one division ! */
 		U64 const vStep = 1ULL << (scale - 20);
 		int stillToDistribute = 1 << tableLog;
 		unsigned s;