On Fri, Nov 20, 2020 at 07:53:34PM +0300, Andrey Gruzdev wrote:
> On 20.11.2020 19:43, Peter Xu wrote:
> > On Fri, Nov 20, 2020 at 07:15:07PM +0300, Andrey Gruzdev wrote:
> > > Yeah, I think we can re-use the postcopy queue code for faulting pages.
> > > I'm
> > > worring a little about some additional overhead dealing with urgent
> > > request
> > > semaphore. Also, the code won't change a lot, something like:
> > >
> > > [...]
> > > /* In case of 'write-tracking' migration we first try
> > > * to poll UFFD and sse if we have write page fault event */
> > > poll_fault_page(rs);
> > >
> > > again = true;
> > > found = get_queued_page(rs, &pss);
> > >
> > > if (!found) {
> > > /* priority queue empty, so just search for something dirty
> > > */
> > > found = find_dirty_block(rs, &pss, &again);
> > > }
> > > [...]
> >
> > Could I ask what's the "urgent request semaphore"? Thanks,
> >
>
> These function use it (the correct name is 'rate_limit_sem'):
>
> void migration_make_urgent_request(void)
> {
> qemu_sem_post(&migrate_get_current()->rate_limit_sem);
> }
>
> void migration_consume_urgent_request(void)
> {
> qemu_sem_wait(&migrate_get_current()->rate_limit_sem);
> }
>
> They are called from ram_save_queue_pages and unqueue_page, accordingly, to
> control migration rate limiter.
>
> bool migration_rate_limit(void)
> {
> [...]
> /*
> * Wait for a delay to do rate limiting OR
> * something urgent to post the semaphore.
> */
> int ms = s->iteration_start_time + BUFFER_DELAY - now;
> trace_migration_rate_limit_pre(ms);
> if (qemu_sem_timedwait(&s->rate_limit_sem, ms) == 0) {
> /*
> * We were woken by one or more urgent things but
> * the timedwait will have consumed one of them.
> * The service routine for the urgent wake will dec
> * the semaphore itself for each item it consumes,
> * so add this one we just eat back.
> */
> qemu_sem_post(&s->rate_limit_sem);
> urgent = true;
> }
> [...]
> }
>
Hmm... Why its overhead could be a problem? If it's an overhead that can be
avoided, then postcopy might want that too.
The thing is I really feel like the snapshot logic can leverage the whole idea
of existing postcopy (like get_queued_page/unqueue_page; it's probably due to
the fact that both of them want to "migrate some more urgent pages than the
background migration, due to either missing/wrprotected pages"), but I might
have something missing.
Thanks,
--
Peter Xu
