UbiquitousOS
/
OpenCloudOS-Kernel
mirror of https://gitee.com/OpenCloudOS/OpenCloudOS-Kernel.git
11
0
Fork 0
Code Issues Projects Releases Wiki Activity

clocksource: Simplify the clocks_calc_max_nsecs() logic 

The previous clocks_calc_max_nsecs() code had some unecessarily
complex bit logic to find the max interval that could cause
multiplication overflows. Since this is not in the hot
path, just do the divide to make it easier to read.

The previous implementation also had a subtle issue
that it avoided overflows with signed 64-bit values, where
as the intervals are always unsigned. This resulted in
overly conservative intervals, which other safety margins
were then added to, reducing the intended interval length.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1426133800-29329-2-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
John Stultz 2015-03-11 21:16:29 -07:00 committed by Ingo Molnar
parent 0bbdb4258b
commit 6086e346fd
1 changed files with 3 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
kernel/time/clocksource.c
View File

@ -476,19 +476,10 @@ u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask)
/*
* Calculate the maximum number of cycles that we can pass to the
* cyc2ns function without overflowing a 64-bit signed result. The
* maximum number of cycles is equal to ULLONG_MAX/(mult+maxadj)
* which is equivalent to the below.
* max_cycles < (2^63)/(mult + maxadj)
* max_cycles < 2^(log2((2^63)/(mult + maxadj)))
* max_cycles < 2^(log2(2^63) - log2(mult + maxadj))
* max_cycles < 2^(63 - log2(mult + maxadj))
* max_cycles < 1 << (63 - log2(mult + maxadj))
* Please note that we add 1 to the result of the log2 to account for
* any rounding errors, ensure the above inequality is satisfied and
* no overflow will occur.
* cyc2ns() function without overflowing a 64-bit result.
*/
max_cycles = 1ULL << (63 - (ilog2(mult + maxadj) + 1));
max_cycles = ULLONG_MAX;
do_div(max_cycles, mult+maxadj);
/*
* The actual maximum number of cycles we can defer the clocksource is