Re: [PATCH v3 3/7] support UFFD write fault processing in ram_save_iterate()

[Andrey Gruzdev]

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;
        }
[...]
}

--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                virtuzzo.com