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btrfs: remove redundant check in up check_setget_bounds

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There are two separate checks in the bounds checker, the first one being
a special case of the second. As this function is performance critical
due to checking access to any eb member, reducing the size can slightly
improve performance.

On a release build on x86_64 the helper is completely inlined so the
function call overhead is also gone.

There was a report of 5% performance drop on metadata heavy workload,
that disappeared after disabling asserts. The most significant part of
that is the bounds checker.

https://lore.kernel.org/linux-btrfs/20200724164147.39925-1-josef@toxicpanda.com/

After the analysis, the optimized code removes the worst overhead which
is the function call and the performance was restored.

https://lore.kernel.org/linux-btrfs/20200730110943.GE3703@twin.jikos.cz/

1. baseline, asserts on, setget check on

run time:		46s
run time with perf:	48s

2. asserts on, comment out setget check

run time:		44s
run time with perf:	47s

So this is confirms the 5% difference

3. asserts on, optimized seget check

run time:		44s
run time with perf:	47s

The optimizations are reducing the number of ifs to 1 and inlining the
hot path. Low-level stuff, gets the performance back. Patch below.

4. asserts off, no setget check

run time:		44s
run time with perf:	45s

This verifies that asserts other than the setget check have negligible
impact on performance and it's not harmful to keep them on.

Analysis where the performance is lost:

* check_setget_bounds is short function, but it's still a function call,
  changing the flow of instructions and given how many times it's
  called the overhead adds up

* there are two conditions, one to check if the range is
  completely outside (member_offset > eb->len) or partially inside
  (member_offset + size > eb->len)

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
David Sterba 2020-07-27 20:59:20 +02:00
parent 51c0674a56
commit 234fdd2815
1 changed files with 3 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
fs/btrfs/struct-funcs.c
View File

@ -12,15 +12,10 @@ static bool check_setget_bounds(const struct extent_buffer *eb,
{
const unsigned long member_offset = (unsigned long)ptr + off;
if (member_offset > eb->len) {
if (unlikely(member_offset + size > eb->len)) {
btrfs_warn(eb->fs_info,
"bad eb member start: ptr 0x%lx start %llu member offset %lu size %d",
(unsigned long)ptr, eb->start, member_offset, size);
return false;
}
if (member_offset + size > eb->len) {
btrfs_warn(eb->fs_info,
"bad eb member end: ptr 0x%lx start %llu member offset %lu size %d",
"bad eb member %s: ptr 0x%lx start %llu member offset %lu size %d",
(member_offset > eb->len ? "start" : "end"),
(unsigned long)ptr, eb->start, member_offset, size);
return false;
}