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Subject[PATCH 0/3 V2] time: use __builtin_constant_p() in msecs_to_jiffies
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In the overall kernel source there currently are
2544 msecs_to_jiffies
126 usecs_to_jiffies
and a few places that are using var * HZ / 1000 constructs
which are not always safe (no check of corner cases) and should
be switched to msecs_to_jiffies (roughly 25 left).
Allowing gcc to fold constants for these calls that in most
cases are passing in constants (roughly 95%) has some potential
to improve performance (and should save a few bytes).

size impact is marginal and for Powerpc it is
actually a slight increase.

As the changes to the top level Kbuild will impact every architecture
this is probably not enough - but I think suitable for a first review

Once this is clean a patch for usecs_to_jiffies will be provided as well

The patch set:
0001 moves timeconst.h from kernel/time/ to include/generated/ and makes
it available early enough so that the build can use the constants
for msecs_to_jiffies
0002 rename msecs_to_jiffies() to __msecs_to_jiffies()
move the common HZ range specific #ifdef'ed code into a inline helper
and #ifdef the variant to use. This allows to re-use the helpers in
both the const and non-const variant of msecs_to_jiffies()
modified msecs_to_jiffies() to checks for constant via
call to __builtin_constant_p() and call __msecs_to_jiffies() if it
can't determine that the argument is constant.
0003 documentation update and reformatting to kernel-doc format
for msecs_to_jiffies() and __msecs_to_jiffies() - for the helpers
its left as comments.

Verification:

kernel configs tested are defconfigs with e.g.
make x86_64_defconfig + CONFIG_TEST_LKM=m, GCONFIG_HZ_300=y. CONFIG_HZ=300

check kernel/softirq.c
#define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
used in __do_softirq() is folded into a single addq $1, %rax
conversely kernel/sched/core.c:sched_rr_handler() is not constant and
is a full call __msecs_to_jiffies

added the test-case proposed by Joe Perches <joe@perches.com>
into lib/test_module.c:test_module_init()
<snip>
unsigned int m;

for (m = 10; m < 200; m += 10)
pr_info("msecs_to_jiffies(%u) is %lu\n",
m, msecs_to_jiffies(m));

pr_info("msecs_to_jiffies(%u) is %lu\n",
10, msecs_to_jiffies(10));
pr_info("msecs_to_jiffies(%u) is %lu\n",
100, msecs_to_jiffies(100));
pr_info("msecs_to_jiffies(%u) is %lu\n",
1000, msecs_to_jiffies(1000));
<snip>

without the patch applied:
test_module_init:
pushq %rbp #
movq %rsp, %rbp #,
pushq %rbx #
movl $10, %ebx #, m
pushq %rcx #
.L2:
movl %ebx, %edi # m,
call msecs_to_jiffies #
movl %ebx, %esi # m,
movq %rax, %rdx # D.14503,
movq $.LC0, %rdi #,
xorl %eax, %eax #
addl $10, %ebx #, m
call printk #
cmpl $200, %ebx #, m
jne .L2 #,
movl $10, %edi #, <--- msecs_to_jiffies(10)
call msecs_to_jiffies # <--- runtime conversion
movl $10, %esi #,
movq %rax, %rdx # D.14504,
movq $.LC0, %rdi #,
xorl %eax, %eax #
call printk #
movl $100, %edi #,
call msecs_to_jiffies #
movl $100, %esi #,
movq %rax, %rdx # D.14505,
movq $.LC0, %rdi #,
xorl %eax, %eax #
call printk #
movl $1000, %edi #,
call msecs_to_jiffies #
movl $1000, %esi #,
movq %rax, %rdx # D.14506,
movq $.LC0, %rdi #,
xorl %eax, %eax #
call printk #
movq $.LC1, %rdi #,
xorl %eax, %eax #
call printk #
popq %rdx #
popq %rbx #
xorl %eax, %eax #
popq %rbp #
ret

with the patch applied:
test_module_init:
pushq %rbp #
movq %rsp, %rbp #,
pushq %rbx #
movl $10, %ebx #, m
pushq %rcx #
.L2:
movl %ebx, %edi # m,
call __msecs_to_jiffies #
movl %ebx, %esi # m,
movq %rax, %rdx # D.14545,
movq $.LC0, %rdi #,
xorl %eax, %eax #
addl $10, %ebx #, m
call printk #
cmpl $200, %ebx #, m
jne .L2 #,
movl $3, %edx #, <--- msecs_to_jiffies(10) == 3 jiffies
movl $10, %esi #, <--- const 10 passed to printk
movq $.LC0, %rdi #,
xorl %eax, %eax #
call printk #
movl $30, %edx #,
movl $100, %esi #,
movq $.LC0, %rdi #,
xorl %eax, %eax #
call printk #
movl $300, %edx #,
movl $1000, %esi #,
movq $.LC0, %rdi #,
xorl %eax, %eax #
call printk #
movq $.LC1, %rdi #,
xorl %eax, %eax #
call printk #
popq %rdx #
popq %rbx #
xorl %eax, %eax #
popq %rbp #
ret


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