On 01/20, Oleg Nesterov wrote:
>
> But I'll recheck this logic once again tomorrow, perhaps I misread
> pipe_write() when I made this patch.
Meanwhile I wrote a stupid test-case below.
Without the patch
State: S (sleeping)
voluntary_ctxt_switches: 74
nonvoluntary_ctxt_switches: 5
State: S (sleeping)
voluntary_ctxt_switches: 4169
nonvoluntary_ctxt_switches: 5
finally release the buffer
wrote next char!
With the patch
State: S (sleeping)
voluntary_ctxt_switches: 74
nonvoluntary_ctxt_switches: 3
State: S (sleeping)
voluntary_ctxt_switches: 74
nonvoluntary_ctxt_switches: 3
finally release the buffer
wrote next char!
As you can see, without this patch pipe_read() wakes the writer up
4095 times for no reason, the writer burns a bit of CPU and blocks
again after wakeup until the last read(fd[0], &c, 1).
Oleg.
-------------------------------------------------------------------------------
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <errno.h>
int main(void)
{
int fd[2], nb, cnt;
char cmd[1024], c;
assert(pipe(fd) == 0);
nb = 1; assert(ioctl(fd[1], FIONBIO, &nb) == 0);
while (write(fd[1], &c, 1) == 1);
assert(errno = -EAGAIN);
nb = 0; assert(ioctl(fd[1], FIONBIO, &nb) == 0);
// The pipe is full, the next write() will block.
sprintf(cmd, "grep -e State -e ctxt_switches /proc/%d/status", getpid());
if (!fork()) {
// wait until the parent sleeps in pipe_write()
usleep(10000);
system(cmd);
// trigger 4095 unnecessary wakeups
for (cnt = 0; cnt < 4095; ++cnt) {
assert(read(fd[0], &c, 1) == 1);
usleep(1000);
}
system(cmd);
// this should actually wake the writer
printf("finally release the buffer\n");
assert(read(fd[0], &c, 1) == 1);
return 0;
}
assert(write(fd[1], &c, 1) == 1);
printf("wrote next char!\n");
return 0;
}
