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031e3bd898
On a 128 cores AMD machine, there are 8 cores in nohz_full mode, and
the others are used for housekeeping. When many housekeeping cpus are
in idle state, we can observe huge time burn in the loop for searching
nearest busy housekeeper cpu by ftrace.
9) | get_nohz_timer_target() {
9) | housekeeping_test_cpu() {
9) 0.390 us | housekeeping_get_mask.part.1();
9) 0.561 us | }
9) 0.090 us | __rcu_read_lock();
9) 0.090 us | housekeeping_cpumask();
9) 0.521 us | housekeeping_cpumask();
9) 0.140 us | housekeeping_cpumask();
...
9) 0.500 us | housekeeping_cpumask();
9) | housekeeping_any_cpu() {
9) 0.090 us | housekeeping_get_mask.part.1();
9) 0.100 us | sched_numa_find_closest();
9) 0.491 us | }
9) 0.100 us | __rcu_read_unlock();
9) + 76.163 us | }
for_each_cpu_and() is a micro function, so in get_nohz_timer_target()
function the
for_each_cpu_and(i, sched_domain_span(sd),
housekeeping_cpumask(HK_FLAG_TIMER))
equals to below:
for (i = -1; i = cpumask_next_and(i, sched_domain_span(sd),
housekeeping_cpumask(HK_FLAG_TIMER)), i < nr_cpu_ids;)
That will cause that housekeeping_cpumask() will be invoked many times.
The housekeeping_cpumask() function returns a const value, so it is
unnecessary to invoke it every time. This patch can minimize the worst
searching time from ~76us to ~16us in my testing.
Similarly, the find_new_ilb() function has the same problem.
Co-developed-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Yuan ZhaoXiong <yuanzhaoxiong@baidu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/1622985115-51007-1-git-send-email-yuanzhaoxiong@baidu.com
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| init | ||
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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.