Messages in this thread Patch in this message | | | Subject | Re: [PATCH] scripts: coccinelle: check for !(un)?likely usage | From | Denis Efremov <> | Date | Sun, 25 Aug 2019 21:59:10 +0300 |
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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/@) { > >
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