From 234fdd2815ce8fe4da6782109580f3b166aeb97b Mon Sep 17 00:00:00 2001 From: David Sterba Date: Mon, 27 Jul 2020 20:59:20 +0200 Subject: btrfs: remove redundant check in up check_setget_bounds 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 Signed-off-by: David Sterba --- fs/btrfs/struct-funcs.c | 11 +++-------- 1 file changed, 3 insertions(+), 8 deletions(-) diff --git a/fs/btrfs/struct-funcs.c b/fs/btrfs/struct-funcs.c index f429256f56db..12455b2b41de 100644 --- a/fs/btrfs/struct-funcs.c +++ b/fs/btrfs/struct-funcs.c @@ -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; } -- cgit v1.2.3-59-g8ed1b