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Re: [PATCH v6] Work around vhost-user-blk-test hang
From: |
Raphael Norwitz |
Subject: |
Re: [PATCH v6] Work around vhost-user-blk-test hang |
Date: |
Mon, 18 Oct 2021 22:33:02 +0000 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
On Mon, Oct 18, 2021 at 05:50:41PM -0400, Michael S. Tsirkin wrote:
> On Thu, Oct 14, 2021 at 04:32:23AM +0000, Raphael Norwitz wrote:
> > The vhost-user-blk-test qtest has been hanging intermittently for a
> > while. The root cause is not yet fully understood, but the hang is
> > impacting enough users that it is important to merge a workaround for
> > it.
> >
> > The race which causes the hang occurs early on in vhost-user setup,
> > where a vhost-user message is never received by the backend. Forcing
> > QEMU to wait until the storage-daemon has had some time to initialize
> > prevents the hang. Thus the existing storage-daemon pidfile option can
> > be used to implement a workaround cleanly and effectively, since it
> > creates a file only once the storage-daemon initialization is complete.
> >
> > This change implements a workaround for the vhost-user-blk-test hang by
> > making QEMU wait until the storage-daemon has written out a pidfile
> > before attempting to connect and send messages over the vhost-user
> > socket.
> >
> > Some relevent mailing list discussions:
> >
> > [1]
> > https://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kernel.org_qemu-2Ddevel_CAFEAcA8kYpz9LiPNxnWJAPSjc-3Dnv532bEdyfynaBeMeohqBp3A-40mail.gmail.com_&d=DwIBAg&c=s883GpUCOChKOHiocYtGcg&r=In4gmR1pGzKB8G5p6LUrWqkSMec2L5EtXZow_FZNJZk&m=eDRDFhe3H61BSSpDvy3PKzwQIa2grX5hNMhigtjMCJ8&s=c6OKIl0NMsDqP0-ZNnVjHhDq2psXIVszz-uBKw_8pEo&e=
> >
> > [2]
> > https://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kernel.org_qemu-2Ddevel_YWaky-252FKVbS-252FKZjlV-40stefanha-2Dx1.localdomain_&d=DwIBAg&c=s883GpUCOChKOHiocYtGcg&r=In4gmR1pGzKB8G5p6LUrWqkSMec2L5EtXZow_FZNJZk&m=eDRDFhe3H61BSSpDvy3PKzwQIa2grX5hNMhigtjMCJ8&s=B4EM_0f7TXqsh18YEKOg-cFHabUjsVA5Ie1riDXaB7A&e=
> >
> >
> > Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
> > Reviewed-by: Eric Blake <eblake@redhat.com>
>
>
> Um. Does not seem to make things better for me:
>
> **
> ERROR:../tests/qtest/vhost-user-blk-test.c:950:start_vhost_user_blk:
> assertion failed (retries < PIDFILE_RETRIES): (5 < 5)
> ERROR qtest-x86_64/qos-test - Bail out!
> ERROR:../tests/qtest/vhost-user-blk-test.c:950:start_vhost_user_blk:
> assertion failed (retries < PIDFILE_RETRIES): (5 < 5)
>
> At this point I just disabled the test in meson. No need to make
> everyone suffer.
Makes sense. Do you still want to persue the workaround?
If so, can you share some details on how you're running the test?
I've gone through 1000+ iterations using the script I posted here:
https://lore.kernel.org/qemu-devel/20210827165253.GA14291@raphael-debian-dev/
without hitting a failure.
>
>
> > ---
> > tests/qtest/vhost-user-blk-test.c | 29 ++++++++++++++++++++++++++++-
> > 1 file changed, 28 insertions(+), 1 deletion(-)
> >
> > diff --git a/tests/qtest/vhost-user-blk-test.c
> > b/tests/qtest/vhost-user-blk-test.c
> > index 6f108a1b62..c6626a286b 100644
> > --- a/tests/qtest/vhost-user-blk-test.c
> > +++ b/tests/qtest/vhost-user-blk-test.c
> > @@ -24,6 +24,7 @@
> > #define TEST_IMAGE_SIZE (64 * 1024 * 1024)
> > #define QVIRTIO_BLK_TIMEOUT_US (30 * 1000 * 1000)
> > #define PCI_SLOT_HP 0x06
> > +#define PIDFILE_RETRIES 5
> >
> > typedef struct {
> > pid_t pid;
>
>
> Don't like the arbitrary retries counter.
>
> Let's warn maybe, but on a busy machine we might not complete this
> in time ...
So you would like it to warn and keep trying forever? Or would you
rather set a much more lenient deadline? (1 min? 5 min?)
>
>
> > @@ -885,7 +886,8 @@ static void start_vhost_user_blk(GString *cmd_line, int
> > vus_instances,
> > int num_queues)
> > {
> > const char *vhost_user_blk_bin = qtest_qemu_storage_daemon_binary();
> > - int i;
> > + int i, retries;
> > + char *daemon_pidfile_path;
> > gchar *img_path;
> > GString *storage_daemon_command = g_string_new(NULL);
> > QemuStorageDaemonState *qsd;
> > @@ -898,6 +900,8 @@ static void start_vhost_user_blk(GString *cmd_line, int
> > vus_instances,
> > " -object memory-backend-memfd,id=mem,size=256M,share=on "
> > " -M memory-backend=mem -m 256M ");
> >
> > + daemon_pidfile_path = g_strdup_printf("/tmp/daemon-%d", getpid());
> > +
>
> Ugh. Predictable paths directly in /tmp are problematic .. mktemp?
>
Ack
> > for (i = 0; i < vus_instances; i++) {
> > int fd;
> > char *sock_path = create_listen_socket(&fd);
> > @@ -914,6 +918,9 @@ static void start_vhost_user_blk(GString *cmd_line, int
> > vus_instances,
> > i + 1, sock_path);
> > }
> >
> > + g_string_append_printf(storage_daemon_command, "--pidfile %s ",
> > + daemon_pidfile_path);
> > +
> > g_test_message("starting vhost-user backend: %s",
> > storage_daemon_command->str);
> > pid_t pid = fork();
> > @@ -930,7 +937,27 @@ static void start_vhost_user_blk(GString *cmd_line,
> > int vus_instances,
> > execlp("/bin/sh", "sh", "-c", storage_daemon_command->str, NULL);
> > exit(1);
> > }
> > +
> > + /*
> > + * FIXME: The loop here ensures the storage-daemon has come up properly
> > + * before allowing the test to proceed. This is a workaround for
> > + * a race which used to cause the vhost-user-blk-test to hang.
> > It
> > + * should be deleted once the root cause is fully understood and
> > + * fixed.
> > + */
> > + retries = 0;
> > + while (access(daemon_pidfile_path, F_OK) != 0) {
> > + g_assert_cmpint(retries, <, PIDFILE_RETRIES);
> > +
> > + retries++;
> > + g_usleep(1000);
> > + }
> > +
> > g_string_free(storage_daemon_command, true);
> > + if (access(daemon_pidfile_path, F_OK) == 0) {
> > + unlink(daemon_pidfile_path);
> > + }
> > + g_free(daemon_pidfile_path);
> >
> > qsd = g_new(QemuStorageDaemonState, 1);
> > qsd->pid = pid;
> > --
> > 2.20.1
>