In order to help identifying problems with IPI handling and remote
function execution add some more data to IPI debugging code.

There have been multiple reports of CPUs looping long times (many
seconds) in smp_call_function_many() waiting for another CPU executing
a function like tlb flushing. Most of these reports have been for
cases where the kernel was running as a guest on top of KVM or Xen
(there are rumours of that happening under VMWare, too, and even on
bare metal).

Finding the root cause hasn't been successful yet, even after more than
2 years of chasing this bug by different developers.

Commit:

  35feb604 ("kernel/smp: Provide CSD lock timeout diagnostics")

tried to address this by adding some debug code and by issuing another
IPI when a hang was detected. This helped mitigating the problem
(the repeated IPI unlocks the hang), but the root cause is still unknown.

Current available data suggests that either an IPI wasn't sent when it
should have been, or that the IPI didn't result in the target CPU
executing the queued function (due to the IPI not reaching the CPU,
the IPI handler not being called, or the handler not seeing the queued
request).

Try to add more diagnostic data by introducing a global atomic counter
which is being incremented when doing critical operations (before and
after queueing a new request, when sending an IPI, and when dequeueing
a request). The counter value is stored in percpu variables which can
be printed out when a hang is detected.

The data of the last event (consisting of sequence counter, source
CPU, target CPU, and event type) is stored in a global variable. When
a new event is to be traced, the data of the last event is stored in
the event related percpu location and the global data is updated with
the new event's data. This allows to track two events in one data
location: one by the value of the event data (the event before the
current one), and one by the location itself (the current event).

A typical printout with a detected hang will look like this:

csd: Detected non-responsive CSD lock (#1) on CPU#1, waiting 5000000003 ns for CPU#06 scf_handler_1+0x0/0x50(0xffffa2a881bb1410).
	csd: CSD lock (#1) handling prior scf_handler_1+0x0/0x50(0xffffa2a8813823c0) request.
        csd: cnt(00008cc): ffff->0000 dequeue (src cpu 0 == empty)
        csd: cnt(00008cd): ffff->0006 idle
        csd: cnt(0003668): 0001->0006 queue
        csd: cnt(0003669): 0001->0006 ipi
        csd: cnt(0003e0f): 0007->000a queue
        csd: cnt(0003e10): 0001->ffff ping
        csd: cnt(0003e71): 0003->0000 ping
        csd: cnt(0003e72): ffff->0006 gotipi
        csd: cnt(0003e73): ffff->0006 handle
        csd: cnt(0003e74): ffff->0006 dequeue (src cpu 0 == empty)
        csd: cnt(0003e7f): 0004->0006 ping
        csd: cnt(0003e80): 0001->ffff pinged
        csd: cnt(0003eb2): 0005->0001 noipi
        csd: cnt(0003eb3): 0001->0006 queue
        csd: cnt(0003eb4): 0001->0006 noipi
        csd: cnt now: 0003f00

The idea is to print only relevant entries. Those are all events which
are associated with the hang (so sender side events for the source CPU
of the hanging request, and receiver side events for the target CPU),
and the related events just before those (for adding data needed to
identify a possible race). Printing all available data would be
possible, but this would add large amounts of data printed on larger
configurations.
Signed-off-by: Juergen Gross <jgross@suse.com>
[ Minor readability edits. Breaks col80 but is far more readable. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20210301101336.7797-4-jgross@suse.com