From a955ff66e7035a09a1284c4aaaa194aa69a72ec7 Mon Sep 17 00:00:00 2001 Message-Id: In-Reply-To: <405603258af5154387bea676be1f904b6713f6ae.1368111913.git.minovotn@redhat.com> References: <405603258af5154387bea676be1f904b6713f6ae.1368111913.git.minovotn@redhat.com> From: Amit Shah Date: Wed, 24 Apr 2013 08:18:26 +0200 Subject: [PATCH 52/65] qemu-char: eliminate busy waiting on can_read returning zero RH-Author: Amit Shah Message-id: <58da31f64d0b0b3910924ccaf97ca488700a61c6.1366724981.git.amit.shah@redhat.com> Patchwork-id: 50830 O-Subject: [RHEL6.5 qemu-kvm PATCH 52/65] qemu-char: eliminate busy waiting on can_read returning zero Bugzilla: 909059 RH-Acked-by: Hans de Goede RH-Acked-by: Gerd Hoffmann RH-Acked-by: Paolo Bonzini From: Paolo Bonzini The character backend refactoring introduced an undesirable busy wait. The busy wait happens if can_read returns zero and there is data available on the character device's file descriptor. Then, the I/O watch will fire continuously and, with TCG, the CPU thread will never run. 1) Char backend asks front end if it can write 2) Front end says no 3) poll() finds the char backend's descriptor is available 4) Goto (1) What we really want is this (note that step 3 avoids the busy wait): 1) Char backend asks front end if it can write 2) Front end says no 3) poll() goes on without char backend's descriptor 4) Goto (1) until qemu_chr_accept_input() called 5) Char backend asks front end if it can write 6) Front end says yes 7) poll() finds the char backend's descriptor is available 8) Backend handler called After this patch, the IOWatchPoll source and the watch source are separated. The IOWatchPoll is simply a hook that runs during the prepare phase on each main loop iteration. The hook adds/removes the actual source depending on the return value from can_read. A simple reproducer is qemu-system-i386 -serial mon:stdio ... followed by banging on the terminal as much as you can. :) Without this patch, emulation will hang. Signed-off-by: Paolo Bonzini Message-id: 1365177573-11817-1-git-send-email-pbonzini@redhat.com Signed-off-by: Anthony Liguori (cherry picked from commit d185c094b404b4ff392b77d1244c0233da7d53bd) Signed-off-by: Amit Shah --- qemu-char.c | 62 +++++++++++++++++++------------------------------------------ 1 file changed, 19 insertions(+), 43 deletions(-) Signed-off-by: Michal Novotny --- qemu-char.c | 62 +++++++++++++++++++------------------------------------------ 1 file changed, 19 insertions(+), 43 deletions(-) diff --git a/qemu-char.c b/qemu-char.c index adc2bb3..f134a4d 100644 --- a/qemu-char.c +++ b/qemu-char.c @@ -541,65 +541,51 @@ int send_all(int fd, const void *_buf, int len1) typedef struct IOWatchPoll { + GSource parent; + GSource *src; - int max_size; IOCanReadHandler *fd_can_read; void *opaque; - - QTAILQ_ENTRY(IOWatchPoll) node; } IOWatchPoll; -static QTAILQ_HEAD(, IOWatchPoll) io_watch_poll_list = - QTAILQ_HEAD_INITIALIZER(io_watch_poll_list); - static IOWatchPoll *io_watch_poll_from_source(GSource *source) { - IOWatchPoll *i; - - QTAILQ_FOREACH(i, &io_watch_poll_list, node) { - if (i->src == source) { - return i; - } - } - - return NULL; + return container_of(source, IOWatchPoll, parent); } static gboolean io_watch_poll_prepare(GSource *source, gint *timeout_) { IOWatchPoll *iwp = io_watch_poll_from_source(source); - - iwp->max_size = iwp->fd_can_read(iwp->opaque); - if (iwp->max_size == 0) { + bool now_active = iwp->fd_can_read(iwp->opaque) > 0; + bool was_active = g_source_get_context(iwp->src) != NULL; + if (was_active == now_active) { return FALSE; } - return g_io_watch_funcs.prepare(source, timeout_); + if (now_active) { + g_source_attach(iwp->src, NULL); + } else { + g_source_remove(g_source_get_id(iwp->src)); + } + return FALSE; } static gboolean io_watch_poll_check(GSource *source) { - IOWatchPoll *iwp = io_watch_poll_from_source(source); - - if (iwp->max_size == 0) { - return FALSE; - } - - return g_io_watch_funcs.check(source); + return FALSE; } static gboolean io_watch_poll_dispatch(GSource *source, GSourceFunc callback, gpointer user_data) { - return g_io_watch_funcs.dispatch(source, callback, user_data); + abort(); } static void io_watch_poll_finalize(GSource *source) { IOWatchPoll *iwp = io_watch_poll_from_source(source); - QTAILQ_REMOVE(&io_watch_poll_list, iwp, node); - g_io_watch_funcs.finalize(source); + g_source_unref(iwp->src); } static GSourceFuncs io_watch_poll_funcs = { @@ -616,24 +602,14 @@ static guint io_add_watch_poll(GIOChannel *channel, gpointer user_data) { IOWatchPoll *iwp; - GSource *src; - guint tag; - - src = g_io_create_watch(channel, G_IO_IN | G_IO_ERR | G_IO_HUP); - g_source_set_funcs(src, &io_watch_poll_funcs); - g_source_set_callback(src, (GSourceFunc)fd_read, user_data, NULL); - tag = g_source_attach(src, NULL); - g_source_unref(src); - iwp = g_malloc0(sizeof(*iwp)); - iwp->src = src; - iwp->max_size = 0; + iwp = (IOWatchPoll *) g_source_new(&io_watch_poll_funcs, sizeof(IOWatchPoll)); iwp->fd_can_read = fd_can_read; iwp->opaque = user_data; + iwp->src = g_io_create_watch(channel, G_IO_IN | G_IO_ERR | G_IO_HUP); + g_source_set_callback(iwp->src, (GSourceFunc)fd_read, user_data, NULL); - QTAILQ_INSERT_HEAD(&io_watch_poll_list, iwp, node); - - return tag; + return g_source_attach(&iwp->parent, NULL); } static GIOChannel *io_channel_from_fd(int fd) -- 1.7.11.7