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migration: Yield bitmap_mutex properly when sending/sleeping

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Don't take the bitmap mutex when sending pages, or when being throttled by
migration_rate_limit() (which is a bit tricky to call it here in ram code,
but seems still helpful).

It prepares for the possibility of concurrently sending pages in >1 threads
using the function ram_save_host_page() because all threads may need the
bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of
qemu_sem_wait() blocking for one thread will not block the other from
progressing.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
This commit is contained in:
Peter Xu 2022-10-11 17:55:50 -04:00 committed by Juan Quintela
parent eaa238ab31
commit f3321554ef
1 changed files with 35 additions and 11 deletions
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46
migration/ram.c
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@ -2452,9 +2452,14 @@ static void postcopy_preempt_reset_channel(RAMState *rs)
* a host page in which case the remainder of the hostpage is sent.
* Only dirty target pages are sent. Note that the host page size may
* be a huge page for this block.
*
* The saving stops at the boundary of the used_length of the block
* if the RAMBlock isn't a multiple of the host page size.
*
* The caller must be with ram_state.bitmap_mutex held to call this
* function. Note that this function can temporarily release the lock, but
* when the function is returned it'll make sure the lock is still held.
*
* Returns the number of pages written or negative on error
*
* @rs: current RAM state
@ -2462,6 +2467,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs)
*/
static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
{
bool page_dirty, preempt_active = postcopy_preempt_active();
int tmppages, pages = 0;
size_t pagesize_bits =
qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
@ -2485,22 +2491,40 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
break;
}
/* Check the pages is dirty and if it is send it */
if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
tmppages = ram_save_target_page(rs, pss);
if (tmppages < 0) {
return tmppages;
}
page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page);
pages += tmppages;
/* Check the pages is dirty and if it is send it */
if (page_dirty) {
/*
* Allow rate limiting to happen in the middle of huge pages if
* something is sent in the current iteration.
* Properly yield the lock only in postcopy preempt mode
* because both migration thread and rp-return thread can
* operate on the bitmaps.
*/
if (pagesize_bits > 1 && tmppages > 0) {
migration_rate_limit();
if (preempt_active) {
qemu_mutex_unlock(&rs->bitmap_mutex);
}
tmppages = ram_save_target_page(rs, pss);
if (tmppages >= 0) {
pages += tmppages;
/*
* Allow rate limiting to happen in the middle of huge pages if
* something is sent in the current iteration.
*/
if (pagesize_bits > 1 && tmppages > 0) {
migration_rate_limit();
}
}
if (preempt_active) {
qemu_mutex_lock(&rs->bitmap_mutex);
}
} else {
tmppages = 0;
}
if (tmppages < 0) {
return tmppages;
}
pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
} while ((pss->page < hostpage_boundary) &&
offset_in_ramblock(pss->block,