f105dfec0a
When tick_nohz_full_cpu() is called with smp_processor_id(), the latter
is unconditionally evaluated whether the static key is on or off. It is
not necessary in the off-case though, so make sure the cpu expression
is executed at the last moment.
Illustrate with the following test function:
int tick_nohz_test(void)
{
return tick_nohz_full_cpu(smp_processor_id());
}
The resulting code before was:
mov %gs:0x7eea92d1(%rip),%eax # smp_processor_id() fetch
nopl 0x0(%rax,%rax,1)
xor %eax,%eax
retq
cmpb $0x0,0x29d393a(%rip) # <tick_nohz_full_running>
je tick_nohz_test+0x29 # jump to below eax clear
mov %eax,%eax
bt %rax,0x29d3936(%rip) # <tick_nohz_full_mask>
setb %al
movzbl %al,%eax
retq
xor %eax,%eax
retq
Now it becomes:
nopl 0x0(%rax,%rax,1)
xor %eax,%eax
retq
cmpb $0x0,0x29d3871(%rip) # <tick_nohz_full_running>
je tick_nohz_test+0x29 # jump to below eax clear
mov %gs:0x7eea91f0(%rip),%eax # smp_processor_id() fetch, after static key
mov %eax,%eax
bt %rax,0x29d3866(%rip) # <tick_nohz_full_mask>
setb %al
movzbl %al,%eax
retq
xor %eax,%eax
retq
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210512232924.150322-2-frederic@kernel.org
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.