Re: [PATCH] ring-buffer: Fix polling on trace_pipe

Martin Lau
2014-06-27T06:52:56Z

[-- Attachment #1: Type: text/plain, Size: 1219 bytes --]

Hi Steve,

I retried the test program (with the kernel patch). It does block from
time to time. I spotted the ee.events is not set to EPOLLIN before
calling epll_ctl(eft, EPOLL_CTL_ADD,...). I fixed it and ran
the test in a bash-loop.

I have the kafai-2 version attached (with some more logging in case
if it still blocks).

Can you retry?

Thanks,
Martin

On Thu, Jun 26, 2014 at 08:53:27PM -0400, Steven Rostedt wrote:
> On Thu, 26 Jun 2014 11:34:46 -0700
> Martin Lau <kafai@fb.com> wrote:
>
> > Hi Steve,
> >
> > Can the modified test program reproduce the problem in your test setup?
>
> Ah sorry, got distracted by other work.
>
> OK, I just tried it out, and here's my results:
>
> I ran you code with my current kernel and this is what I got:
>
> # ./ftrace-test-epoll-kafai
> <...>-3183 [002] ...1 81.777891: tracing_mark_write:
> some data 3183: waitting for more data......
> 3184: written more data
>
> And it just hung there.
>
>
> Then I applied your patch, compiled and booted it, and ran the test
> again, and I got this:
>
> # ./ftrace-test-epoll-kafai
>
> It just hung there. No forward progress.
>
> I don't think that was the result you intended.
>
> -- Steve
>
>

[-- Attachment #2: ftrace-test-epoll-kafai-2.c --]
[-- Type: text/plain, Size: 2392 bytes --]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/epoll.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>

static const char * debugfs_list[] = {
"/debug/tracing",
"/sys/kernel/debug/tracing",
"/d/tracing",
NULL,
};

static const char *debugfs;
static int markfd;
static int trace_pipe_fd;

static void alog(const char *name, int ret)
{
printf("%d: %s: %d\n", getpid(), name, ret);
}

static const char *find_debugfs(void)
{
struct stat st;
int i;
int r;

for (i = 0; debugfs_list[i]; i++) {
r = stat(debugfs_list[i], &st);
if (r < 0)
continue;
if (S_ISDIR(st.st_mode))
return debugfs_list[i];
}
return NULL;
}

static char * make_path(const char *file)
{
char *path;
int size;

size = strlen(debugfs) + strlen(file) + 2;
path = malloc(size);
if (!path) {
perror("malloc");
exit(-1);
}
sprintf(path, "%s/%s", debugfs, file);
return path;
}

static void mark_write(const char *str)
{
int ret;
ret = write(markfd, str, strlen(str));
alog("write(markfd)", ret);
}

static void read_trace_pipe(void)
{
char buf[1024];
int r;

while ((r = read(trace_pipe_fd, buf, 1024)) > 0)
printf("%.*s", r, buf);
}

int main (int argc, char **argv)
{
struct epoll_event ee;
char *marker;
char *pipe;
int efd;
int ret;
pid_t dwrt_pid;

debugfs = find_debugfs();
if (!debugfs) {
fprintf(stderr, "Could not find debugfs\n");
exit(-1);
}

marker = make_path("trace_marker");
pipe = make_path("trace_pipe");

markfd = open(marker, O_WRONLY);
if (markfd < 0) {
perror("marker");
exit(-1);
}
trace_pipe_fd = open(pipe, O_RDONLY|O_NONBLOCK);
if (trace_pipe_fd < 0) {
perror("trace_pipe");
exit(-1);
}

efd = epoll_create(1);
if (efd < 0) {
perror("epoll_create");
exit(-1);
}

mark_write("some data");
memset(&ee, 0, sizeof(ee));
ee.events = EPOLLIN;
ret = epoll_ctl(efd, EPOLL_CTL_ADD, trace_pipe_fd, &ee);
if (ret < 0) {
perror("epoll_ctl");
exit(-1);
}
alog("waiting data......", 0);
ret = epoll_wait(efd, &ee, 1, -1);
alog("epoll_wait()", ret);
read_trace_pipe();
dwrt_pid = fork();
assert(dwrt_pid != -1);
if (dwrt_pid == 0) {
sleep(10);
mark_write("more data");
} else {
alog("waiting form more data......", 0);
ret = epoll_wait(efd, &ee, 1, -1);
alog("epoll_wait()", ret);
read_trace_pipe();
wait(NULL);
}
return 0;
}