mds: use regular dispatch for processing beacons #57469

batrick · 2024-05-15T01:45:52Z

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src/messages/MMDSBeacon.h

leonid-s-usov

Is there a flow where the dispatchers are added dynamically, which would require a priority queue? Until now I was under the impression that dispatchers are added in a predictable static initialization flow, meaning that simply ordering the add_dispatcher_* calls should provide enough control over the invocation order.

If we have to go for the new priority argument, I think we should define standard high/default/low priorities, evenly spaced in the priority range. The default should be in the middle of the priority range, not on the lowest side.

src/msg/Messenger.h

batrick · 2024-05-15T14:28:19Z

Is there a flow where the dispatchers are added dynamically, which would require a priority queue? Until now I was under the impression that dispatchers are added in a predictable static initialization flow, meaning that simply ordering the add_dispatcher_* calls should provide enough control over the invocation order.

Dispatchers can be added dynamically but it is uncommon.

Adding dispatchers to the head/tail of the list is just too clumsy; there's no reason not to just define the order. It's rather important to get right because some dispatchers acquire highly contentious locks (mds_lock being one example).

If we have to go for the new priority argument, I think we should define standard high/default/low priorities, evenly spaced in the priority range. The default should be in the middle of the priority range, not on the lowest side.

Sure.

leonid-s-usov

OK then let's go with the priorities.

However, we should use the priority constants instead of literal magic numbers and try our best to leave calls unchanged where the default priority should apply.

src/mds/MDSDaemon.cc

src/mds/MDSRank.cc

src/mon/MonClient.cc

batrick · 2024-05-15T15:13:13Z

I've switched Messenger::(fast_)dispatchers to a std::vector from std::deque:

std::vector ensures the elements are contiguous
optimizing for add_dispatcher_head is not worthwhile when the dispatchers list is rarely changed and usually only at startup

leonid-s-usov · 2024-05-15T17:51:20Z

src/mds/MDSRank.cc

@@ -2137,7 +2137,7 @@ void MDSRank::active_start()

  dout(10) << __func__ << ": initializing metrics handler" << dendl;
  metrics_handler.init();
-  messenger->add_dispatcher_tail(&metrics_handler);
+  messenger->add_dispatcher_head(&metrics_handler, 20); // very high but behind Beacon


Still magic numbers. Now, one needs to go find where beacon handler is added and check that number. And if that number changes, how do they know to update this place?

If this should be high but behind beacon, this should be

messenger->add_dispatcher_tail(&metrics_handler, ::priority_high);

and beacon should stay

messenger->add_dispatcher_head(&beacon, ::priority_high);

For order checking. Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

So we can ensure that e.g. MDSRank::ms_dispatch is lowest priority so that we do not acquire the mds_lock when looking at beacons. This change maintains the current behavior when the priority is unset: the use of std::stable_sort will ensure that the add_dispatcher_head and add_dispatcher_tail calls will preserve order when dispatcher priorities are equal. Fixes: 7fc04be Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

Similar to the issue with MClientMetrics, beacons should also not be handled via fast dispatch because it's necessary to acquire Beacon::mutex. This is a big no-no as it may block one of the Messenger threads leading to improbable deadlocks or DoS. Instead, use the normal dispatch where acquiring locks is okay to do. Fixes: 7fc04be See-also: linux.git/f7c2f4f6ce16fb58f7d024f3e1b40023c4b43ff9 Fixes: https://tracker.ceph.com/issues/65658 Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

This tries to preserve existing order but uses priorities to make it explicit and robust to future dispatchers being added. Except: - The beacon and metrics dispatcher have the highest priorities. This is to ensure we process these messages before trying to acquire any expensive locks (like mds_lock). - The monc dispatcher also has a relatively high priority for the same reasons. This change affects other daemons which may have ordered a dispatcher ahead of the monc but I cannot think of a legitimate reason to nor do I see an instance of it. Fixes: 7fc04be Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

leonid-s-usov

I love it!

batrick · 2024-05-17T01:25:13Z

This PR is under test in https://tracker.ceph.com/issues/66086.

batrick requested a review from a team as a code owner May 15, 2024 01:45

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batrick force-pushed the i66014 branch from 7e982e7 to 428dd7b Compare May 15, 2024 01:51

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mds: use regular dispatch for processing metrics #57081

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mds: note when dispatcher is called

1b93fd5

For order checking. Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

batrick force-pushed the i66014 branch from 8defa5d to 515dfe1 Compare May 15, 2024 20:33

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batrick merged commit eb10f3d into ceph:main May 23, 2024
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batrick deleted the i66014 branch May 23, 2024 01:16

This was referenced May 23, 2024

squid: mds: use regular dispatch for processing beacons #57682

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reef: mds: use regular dispatch for processing beacons #57683

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quincy: mds: use regular dispatch for processing beacons #57684

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mds: use regular dispatch for processing beacons #57469

mds: use regular dispatch for processing beacons #57469

batrick commented May 15, 2024 •

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mds: use regular dispatch for processing beacons #57469

mds: use regular dispatch for processing beacons #57469

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batrick commented May 15, 2024 • edited

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batrick commented May 15, 2024 •

edited