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b37ac1894a
At boot time, it is not necessary to delay between polls of
cpu_callin_map when waiting for a kicked CPU to come up. Remove the
delay intervals, but preserve the overall deadline (five seconds).
At run time, the first poll result is usually negative and we incur a
sleeping wait. If we spin on the callin word for a short time first,
we can reduce __cpu_up() from dozens of milliseconds to under 1ms in
the common case on a P9 LPAR:
$ ppc64_cpu --smt=off
$ bpftrace -e 'kprobe:__cpu_up {
@start[tid] = nsecs;
}
kretprobe:__cpu_up /@start[tid]/ {
@us = hist((nsecs - @start[tid]) / 1000);
delete(@start[tid]);
}' -c 'ppc64_cpu --smt=on'
Before:
@us:
[16K, 32K) 85 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[32K, 64K) 13 |@@@@@@@ |
After:
@us:
[128, 256) 95 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[256, 512) 3 |@ |
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220926220250.157022-1-nathanl@linux.ibm.com
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.