Cc: Ingo Molnar <mingo@...>, Christoph Lameter <clameter@...>, Kyle Moffett <mrmacman_g4@...>, Cyrus Massoumi <cyrusm@...>, LKML Kernel <linux-kernel@...>, Andrew Morton <akpm@...>, Mike Galbraith <efault@...>, Paul Menage <menage@...>, Peter Williams <pwil3058@...>
On Sat, Nov 17, 2007 at 12:26:41AM +0100, Dmitry Adamushko wrote:
For consistency, I tested this using /dev/rtc. I set the rtc frequency
to 16 Hz, and replaced the main loop of my high (-19) priority thread
with:
while (1) {
unsigned long data;
for (i = 0; i < 3; i++) {
if (read(rtc, &data, sizeof data) != sizeof data) {
perror("read");
return 1;
}
}
fcntl(rtc, F_SETFL, O_NONBLOCK);
while (read(rtc, &data, sizeof data) < 0);
fcntl(rtc, F_SETFL, 0);
}
Now it's busy-looping for 62ms, and sleeping for three consecutive
62.5ms chunks totalling 187.5ms.
The results aren't quite what I was expecting. I have only observed
this so far in test cases where I have a very high wakeup frequency,
so I wasn't expecting this to work. I did, however, still observe the
problem where occasionally I get into a state where one CPU is mostly
idle.
Qualitatively, this feels a bit different. With the higher clock
frequency it seemed like the CPU would easily get "stuck" in this
state where it's mostly idle, and it would stay there for a long
time. With the low wakeup frequency, I'm seeing it toggle between the
busy and mostly-idle states more quickly.
I tried a similar test using usleep() and gettimeofday() rather than
/dev/rtc:
while (1) {
usleep(300000);
gettimeofday(&t1, NULL);
do {
gettimeofday(&t2, NULL);
} while (t2.tv_usec - t1.tv_usec +
(t2.tv_sec - t1.tv_sec) * 1000000 < 100000);
}
With this test program, I haven't yet seen a CPU imbalance that lasts
longer than a fraction of a second.
--Micah
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