On 25.08.2019 19:37, Joe Perches wrote:
> On Sun, 2019-08-25 at 16:05 +0300, Denis Efremov wrote:
>> This patch adds coccinelle script for detecting !likely and !unlikely
>> usage. It's better to use unlikely instead of !likely and vice versa.
>
> Please explain _why_ is it better in the changelog.
>
In my naive understanding the negation (!) before the likely/unlikely
could confuse the compiler and the initial branch-prediction intent
could be "falsified". I would say that either you need to move the
negation under the bracket "!unlikely(cond) -> unlikely(!cond)" or
you need to use likely instead "!unlikely(cond) -> likely(cond)".
However, I'm not a compiler expert to state that this is a general
rule. But, we've got 2 special macro for branch predicting, not one.
There is also ftrace in-between. I will try to do some simple
benchmarking.
> btw: there are relatively few uses like this in the kernel.
>
> $ git grep -P '!\s*(?:un)?likely\s*\(' | wc -l
> 40
>
> afaict: It may save 2 bytes of x86/64 object code.
>
> For instance:
>
> $ diff -urN kernel/tsacct.lst.old kernel/tsacct.lst.new|less
> --- kernel/tsacct.lst.old 2019-08-25 09:21:39.936570183 -0700
> +++ kernel/tsacct.lst.new 2019-08-25 09:22:20.774324886 -0700
> @@ -24,158 +24,153 @@
> 15: 48 89 fb mov %rdi,%rbx
> u64 time, delta;
>
> - if (!likely(tsk->mm))
> + if (unlikely(tsk->mm))
> 18: 4c 8d ab 28 02 00 00 lea 0x228(%rbx),%r13
> 1f: e8 00 00 00 00 callq 24 <__acct_update_integrals+0x24>
> 20: R_X86_64_PLT32 __sanitizer_cov_trace_pc-0x4
> 24: 4c 89 ef mov %r13,%rdi
> 27: e8 00 00 00 00 callq 2c <__acct_update_integrals+0x2c>
> 28: R_X86_64_PLT32 __asan_load8_noabort-0x4
> - 2c: 4c 8b bb 28 02 00 00 mov 0x228(%rbx),%r15
> - 33: 4d 85 ff test %r15,%r15
> - 36: 74 34 je 6c <__acct_update_integrals+0x6c>
> + 2c: 48 83 bb 28 02 00 00 cmpq $0x0,0x228(%rbx)
> + 33: 00
> + 34: 75 34 jne 6a <__acct_update_integrals+0x6a>
> return;
I think it's incorrect to say so in general. For example, on x86/64:
$ make mrproper
$ make allyesconfig
$ make && mv vmlinux vmlinux-000
$ make coccicheck MODE=patch COCCI=scripts/coccinelle/misc/unlikely.cocci |
patch -p1
$ make
$ ./scripts/bloat-o-meter ./vmlinux-000 ./vmlinux
add/remove: 0/0 grow/shrink: 3/4 up/down: 41/-35 (6)
Function old new delta
dpaa2_io_service_rearm 357 382 +25
intel_pmu_hw_config 1277 1285 +8
get_sigframe.isra.constprop 1657 1665 +8
csum_partial_copy_from_user 605 603 -2
wait_consider_task 3807 3797 -10
__acct_update_integrals 384 373 -11
pipe_to_sendpage 459 447 -12
Total: Before=312759461, After=312759467, chg +0.00%
It definitely influence the way the compiler optimizes the code.
>
> And here's a possible equivalent checkpatch test.
> ---
> scripts/checkpatch.pl | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> index 287fe73688f0..364603ad1a47 100755
> --- a/scripts/checkpatch.pl
> +++ b/scripts/checkpatch.pl
> @@ -6529,6 +6529,24 @@ sub process {
> "Using $1 should generally have parentheses around
> the comparison\n" . $herecurr);
> }
>
> +# !(likely|unlikely)(condition) use should be (unlikely|likely)(condition)
> + if ($perl_version_ok &&
> + $line =~ /(\!\s*((?:un)?likely))\s*$balanced_parens/) {
> + my $match = $1;
> + my $type = $2;
> + my $reverse;
> + if ($type eq "likely") {
> + $reverse = "unlikely";
> + } else {
> + $reverse = "likely";
> + }
> + if (WARN("LIKELY_MISUSE",
> + "Prefer $reverse over $match\n" . $herecurr) &&
> + $fix) {
> + $fixed[$fixlinenr] =~
> s/\Q$match\E\s*\(/$reverse(/;
> + }
> + }
> +
> # whine mightly about in_atomic
> if ($line =~ /\bin_atomic\s*\(/) {
> if ($realfile =~ m@^drivers/@) {
>
>
