migration/postcopy: Fix high frequency sync
With current code base I can observe extremely high sync count during precopy, as long as one enables postcopy-ram=on before switchover to postcopy. To provide some context of when QEMU decides to do a full sync: it checks must_precopy (which implies "data must be sent during precopy phase"), and as long as it is lower than the threshold size we calculated (out of bandwidth and expected downtime) QEMU will kick off the slow/exact sync. However, when postcopy is enabled (even if still during precopy phase), RAM only reports all pages as can_postcopy, and report must_precopy==0. Then "must_precopy <= threshold_size" mostly always triggers and enforces a slow sync for every call to migration_iteration_run() when postcopy is enabled even if not used. That is insane. It turns out it was a regress bug introduced in the previous refactoring in 8.0 as reported by Nina [1]: (a)c8df4a7aef
("migration: Split save_live_pending() into state_pending_*") Then a workaround patch is applied at the end of release (8.0-rc4) to fix it: (b)28ef5339c3
("migration: fix ram_state_pending_exact()") However that "workaround" was overlooked when during the cleanup in this 9.0 release in this commit.. (c)b0504edd40
("migration: Drop unnecessary check in ram's pending_exact()") Then the issue was re-exposed as reported by Nina [1]. The problem with (b) is that it only fixed the case for RAM, rather than all the rest of iterators. Here a slow sync should only be required if all dirty data (precopy+postcopy) is less than the threshold_size that QEMU calculated. It is even debatable whether a sync is needed when switched to postcopy. Currently ram_state_pending_exact() will be mostly noop if switched to postcopy, and that logic seems to apply too for all the rest of iterators, as sync dirty bitmap during a postcopy doesn't make much sense. However let's leave such change for later, as we're in rc phase. So rather than reusing commit (b), this patch provides the complete fix for all iterators. When at it, cleanup a little bit on the lines around. [1] https://gitlab.com/qemu-project/qemu/-/issues/1565 Reported-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com> Fixes:b0504edd40
("migration: Drop unnecessary check in ram's pending_exact()") Reviewed-by: Fabiano Rosas <farosas@suse.de> Link: https://lore.kernel.org/r/20240320214453.584374-1-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com>
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@ -3199,17 +3199,16 @@ typedef enum {
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*/
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static MigIterateState migration_iteration_run(MigrationState *s)
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{
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uint64_t must_precopy, can_postcopy;
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uint64_t must_precopy, can_postcopy, pending_size;
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Error *local_err = NULL;
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bool in_postcopy = s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE;
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bool can_switchover = migration_can_switchover(s);
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qemu_savevm_state_pending_estimate(&must_precopy, &can_postcopy);
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uint64_t pending_size = must_precopy + can_postcopy;
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pending_size = must_precopy + can_postcopy;
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trace_migrate_pending_estimate(pending_size, must_precopy, can_postcopy);
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if (must_precopy <= s->threshold_size) {
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if (pending_size < s->threshold_size) {
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qemu_savevm_state_pending_exact(&must_precopy, &can_postcopy);
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pending_size = must_precopy + can_postcopy;
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trace_migrate_pending_exact(pending_size, must_precopy, can_postcopy);
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