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

Martin Lau
2014-07-10T22:20:36Z

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;
> }