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

[Martin Lau <kafai@fb.com>]

Hi Steve,

Do you have a chance to give it another try?

Thanks,
Martin

On Thu, Jun 26, 2014 at 11:52:44PM -0700, Martin Lau wrote:
> 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
> > 
> > 

> #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;
> }