Peter Xu <pet...@redhat.com> writes:
> On Wed, Jan 31, 2024 at 12:05:08PM -0300, Fabiano Rosas wrote:
>> pet...@redhat.com writes:
>>
>> > From: Peter Xu <pet...@redhat.com>
>> >
>> > Multifd send side has two fields to indicate error quits:
>> >
>> > - MultiFDSendParams.quit
>> > - &multifd_send_state->exiting
>> >
>> > Merge them into the global one. The replacement is done by changing all
>> > p->quit checks into the global var check. The global check doesn't need
>> > any lock.
>> >
>> > A few more things done on top of this altogether:
>> >
>> > - multifd_send_terminate_threads()
>> >
>> > Moving the xchg() of &multifd_send_state->exiting upper, so as to cover
>> > the tracepoint, migrate_set_error() and migrate_set_state().
>>
>> Good.
>>
>> >
>> > - multifd_send_sync_main()
>> >
>> > In the 2nd loop, add one more check over the global var to make sure we
>> > don't keep the looping if QEMU already decided to quit.
>>
>> Yes, also because we don't necessarily enter at multifd_send_page()
>> every time.
>>
>> >
>> > - multifd_tls_outgoing_handshake()
>> >
>> > Use multifd_send_terminate_threads() to set the error state. That has
>> > a benefit of updating MigrationState.error to that error too, so we can
>> > persist that 1st error we hit in that specific channel.
>>
>> Makes sense.
>>
>> >
>> > - multifd_new_send_channel_async()
>> >
>> > Take similar approach like above, drop the migrate_set_error() because
>> > multifd_send_terminate_threads() already covers that. Unwrap the
>> > helper
>> > multifd_new_send_channel_cleanup() along the way; not really needed.
>> >
>> > Signed-off-by: Peter Xu <pet...@redhat.com>
>> > ---
>> > migration/multifd.h | 2 --
>> > migration/multifd.c | 85 ++++++++++++++++++---------------------------
>> > 2 files changed, 33 insertions(+), 54 deletions(-)
>> >
>> > diff --git a/migration/multifd.h b/migration/multifd.h
>> > index 35d11f103c..7c040cb85a 100644
>> > --- a/migration/multifd.h
>> > +++ b/migration/multifd.h
>> > @@ -95,8 +95,6 @@ typedef struct {
>> > QemuMutex mutex;
>> > /* is this channel thread running */
>> > bool running;
>> > - /* should this thread finish */
>> > - bool quit;
>> > /* multifd flags for each packet */
>> > uint32_t flags;
>> > /* global number of generated multifd packets */
>> > diff --git a/migration/multifd.c b/migration/multifd.c
>> > index b8d2c96533..2c98023d67 100644
>> > --- a/migration/multifd.c
>> > +++ b/migration/multifd.c
>> > @@ -372,6 +372,11 @@ struct {
>> > MultiFDMethods *ops;
>> > } *multifd_send_state;
>> >
>> > +static bool multifd_send_should_exit(void)
>> > +{
>> > + return qatomic_read(&multifd_send_state->exiting);
>> > +}
>> > +
>> > /*
>> > * The migration thread can wait on either of the two semaphores. This
>> > * function can be used to kick the main thread out of waiting on either
>> > of
>> > @@ -409,7 +414,7 @@ static int multifd_send_pages(void)
>> > MultiFDSendParams *p = NULL; /* make happy gcc */
>> > MultiFDPages_t *pages = multifd_send_state->pages;
>> >
>> > - if (qatomic_read(&multifd_send_state->exiting)) {
>> > + if (multifd_send_should_exit()) {
>> > return -1;
>> > }
v>> >
>> > @@ -421,14 +426,11 @@ static int multifd_send_pages(void)
>> > */
>> > next_channel %= migrate_multifd_channels();
>> > for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) {
>> > - p = &multifd_send_state->params[i];
>> > -
>> > - qemu_mutex_lock(&p->mutex);
>> > - if (p->quit) {
>> > - error_report("%s: channel %d has already quit!", __func__, i);
>> > - qemu_mutex_unlock(&p->mutex);
>> > + if (multifd_send_should_exit()) {
>> > return -1;
>> > }
>> > + p = &multifd_send_state->params[i];
>> > + qemu_mutex_lock(&p->mutex);
>> > if (!p->pending_job) {
>> > p->pending_job++;
>> > next_channel = (i + 1) % migrate_multifd_channels();
>>
>> Hm, I'm not sure it's correct to check 'exiting' outside of the
>> lock. While it is an atomic operation, it is not atomic in relation to
>> pending_job...
>>
>> ... looking closer, it seems that we can do what you suggest because
>> p->pending_job is not touched by the multifd_send_thread in case of
>> error, which means this function will indeed miss the 'exiting' flag,
>> but pending_job > 0 means it will loop to the next channel and _then_ it
>> will see the 'exiting' flag.
>
> It could still be the last channel we iterate, then IIUC we can still try
> to assign a job to a thread even if a concurrent error is set there.
>
> However IMHO it's okay; the error in the sender thread should ultimately
> set migrate_set_error() and the main thread should detect that in the
> migration loop, then we'll still quit. The extra queued job shouldn't
> matter, IMHO.
>
>>
>> > @@ -483,6 +485,16 @@ static void multifd_send_terminate_threads(Error *err)
>> > {
>> > int i;
>> >
>> > + /*
>> > + * We don't want to exit each threads twice. Depending on where
>> > + * we get the error, or if there are two independent errors in two
>> > + * threads at the same time, we can end calling this function
>> > + * twice.
>> > + */
>> > + if (qatomic_xchg(&multifd_send_state->exiting, 1)) {
>> > + return;
>> > + }
>> > +
>> > trace_multifd_send_terminate_threads(err != NULL);
>> >
>> > if (err) {
>> > @@ -497,26 +509,13 @@ static void multifd_send_terminate_threads(Error
>> > *err)
>> > }
>> > }
>> >
>> > - /*
>> > - * We don't want to exit each threads twice. Depending on where
>> > - * we get the error, or if there are two independent errors in two
>> > - * threads at the same time, we can end calling this function
>> > - * twice.
>> > - */
>> > - if (qatomic_xchg(&multifd_send_state->exiting, 1)) {
>> > - return;
>> > - }
>> > -
>> > for (i = 0; i < migrate_multifd_channels(); i++) {
>> > MultiFDSendParams *p = &multifd_send_state->params[i];
>> >
>> > - qemu_mutex_lock(&p->mutex);
>> > - p->quit = true;
>>
>> Now that you removed this, we decoupled kicking the threads from setting
>> the exit/error, so this function could be split in two.
>>
>> We could set the exiting flag at the places the error occurred (multifd
>> threads, thread creation, etc) and "terminate the threads" at
>> multifd_save_cleanup(). That second part we already do actually:
>>
>> void multifd_save_cleanup(void) {
>> ...
>> multifd_send_terminate_threads(NULL);
>> ^see?
>> for (i = 0; i < migrate_multifd_channels(); i++) {
>> MultiFDSendParams *p = &multifd_send_state->params[i];
>>
>> if (p->running) {
>> qemu_thread_join(&p->thread);
>> }
>> }
>> ...
>> }
>>
>> I think there's no reason anymore for the channels to kick each
>> other. They would all be waiting at p->sem and multifd_send_cleanup()
>> would kick + join them.
>
> Sounds good here.
>
> I'll attach one patch like this, feel free to have an early look:
>
> =====
>
> From f9a3d63d5cca0068daaea4c72392803f4b29dcb5 Mon Sep 17 00:00:00 2001
> From: Peter Xu <pet...@redhat.com>
> Date: Thu, 1 Feb 2024 17:01:54 +0800
> Subject: [PATCH] migration/multifd: Split multifd_send_terminate_threads()
>
> Split multifd_send_terminate_threads() into two functions:
>
> - multifd_send_set_error(): used when an error happened on the sender
> side, set error and quit state only
>
> - multifd_send_terminate_threads(): used only by the main thread to kick
> all multifd send threads out of sleep, for the last recycling.
>
> Use multifd_send_set_error() in the three old call sites where only the
> error will be set.
>
> Use multifd_send_terminate_threads() in the last one where the main thread
> will kick the multifd threads at last in multifd_save_cleanup().
>
> Both helpers will need to set quitting=1.
>
> Suggested-by: Fabiano Rosas <faro...@suse.de>
> Signed-off-by: Peter Xu <pet...@redhat.com>
New patch looks good.
> ---
> migration/multifd.c | 27 ++++++++++++++++++---------
> migration/trace-events | 2 +-
> 2 files changed, 19 insertions(+), 10 deletions(-)
>
> diff --git a/migration/multifd.c b/migration/multifd.c
> index c71e74b101..95dc29c8c7 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -536,10 +536,9 @@ int multifd_queue_page(RAMBlock *block, ram_addr_t
> offset)
> return 1;
> }
>
> -static void multifd_send_terminate_threads(Error *err)
> +/* Multifd send side hit an error; remember it and prepare to quit */
> +static void multifd_send_set_error(Error *err)
> {
> - int i;
> -
> /*
> * We don't want to exit each threads twice. Depending on where
> * we get the error, or if there are two independent errors in two
> @@ -550,8 +549,6 @@ static void multifd_send_terminate_threads(Error *err)
> return;
> }
>
> - trace_multifd_send_terminate_threads(err != NULL);
> -
> if (err) {
> MigrationState *s = migrate_get_current();
> migrate_set_error(s, err);
> @@ -563,7 +560,19 @@ static void multifd_send_terminate_threads(Error *err)
> MIGRATION_STATUS_FAILED);
> }
> }
> +}
> +
> +static void multifd_send_terminate_threads(void)
> +{
> + int i;
> +
> + trace_multifd_send_terminate_threads();
>
> + /*
> + * Tell everyone we're quitting. No xchg() needed here; we simply
> + * always set it.
> + */
> + qatomic_set(&multifd_send_state->exiting, 1);
> for (i = 0; i < migrate_multifd_channels(); i++) {
> MultiFDSendParams *p = &multifd_send_state->params[i];
>
> @@ -586,7 +595,7 @@ void multifd_save_cleanup(void)
> if (!migrate_multifd()) {
> return;
> }
> - multifd_send_terminate_threads(NULL);
> + multifd_send_terminate_threads();
> for (i = 0; i < migrate_multifd_channels(); i++) {
> MultiFDSendParams *p = &multifd_send_state->params[i];
We could then move the qemu_thread_join loop into
multifd_send_terminate_threads().
(and fix all the bugs we have so that we only progress past
multifd_send_terminate_threads() once all threads have exited and no
more thread is going to spawn)
>
> @@ -778,7 +787,7 @@ out:
> if (ret) {
> assert(local_err);
> trace_multifd_send_error(p->id);
> - multifd_send_terminate_threads(local_err);
> + multifd_send_set_error(local_err);
> multifd_send_kick_main(p);
> error_free(local_err);
> }
> @@ -814,7 +823,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
>
> trace_multifd_tls_outgoing_handshake_error(ioc, error_get_pretty(err));
>
> - multifd_send_terminate_threads(err);
> + multifd_send_set_error(err);
> multifd_send_kick_main(p);
> error_free(err);
> }
> @@ -896,7 +905,7 @@ static void multifd_new_send_channel_async(QIOTask *task,
> gpointer opaque)
> }
>
> trace_multifd_new_send_channel_async_error(p->id, local_err);
> - multifd_send_terminate_threads(local_err);
> + multifd_send_set_error(local_err);
> multifd_send_kick_main(p);
> object_unref(OBJECT(ioc));
> error_free(local_err);
> diff --git a/migration/trace-events b/migration/trace-events
> index de4a743c8a..298ad2b0dd 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -141,7 +141,7 @@ multifd_send_error(uint8_t id) "channel %u"
> multifd_send_sync_main(long packet_num) "packet num %ld"
> multifd_send_sync_main_signal(uint8_t id) "channel %u"
> multifd_send_sync_main_wait(uint8_t id) "channel %u"
> -multifd_send_terminate_threads(bool error) "error %d"
> +multifd_send_terminate_threads(void) ""
> multifd_send_thread_end(uint8_t id, uint64_t packets, uint64_t normal_pages)
> "channel %u packets %" PRIu64 " normal pages %" PRIu64
> multifd_send_thread_start(uint8_t id) "%u"
> multifd_tls_outgoing_handshake_start(void *ioc, void *tioc, const char
> *hostname) "ioc=%p tioc=%p hostname=%s"
> --
> 2.43.0
