On Fri, Apr 02, 2021 at 05:46:52PM +0200, Juergen Gross wrote:
On 30.03.21 19:33, Paul E. McKenney wrote:
On Wed, Mar 24, 2021 at 11:18:03AM +0100, Jürgen Groß wrote:
On 02.03.21 07:28, Juergen Gross wrote:
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 35feb60474bf4f7 ("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
This example (being an artificial one, produced with a previous version
of this patch without the "hdlend" event), shows that cpu#6 started to
handle an IPI (cnt 3e72-3e74), bit didn't start to handle another IPI
(sent by cpu#4, cnt 3e7f). The next request from cpu#1 for cpu#6 was
queued (3eb3), but no IPI was needed (cnt 3eb4, there was the event
from cpu#4 in the queue already).
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@xxxxxxxx>
Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Just an update regarding current status with debugging the underlying
issue:
On a customer's machine with a backport of this patch applied we've
seen another case of the hang. In the logs we've found:
smp: csd: Detected non-responsive CSD lock (#1) on CPU#18, waiting
5000000046 ns for CPU#06 do_flush_tlb_all+0x0/0x30( (null)).
smp: csd: CSD lock (#1) unresponsive.
smp: csd: cnt(0000000): 0000->0000 queue
smp: csd: cnt(0000001): ffff->0006 idle
smp: csd: cnt(0025dba): 0012->0006 queue
smp: csd: cnt(0025dbb): 0012->0006 noipi
smp: csd: cnt(01d1333): 001a->0006 pinged
smp: csd: cnt(01d1334): ffff->0006 gotipi
smp: csd: cnt(01d1335): ffff->0006 handle
smp: csd: cnt(01d1336): ffff->0006 dequeue (src cpu 0 == empty)
smp: csd: cnt(01d1337): ffff->0006 hdlend (src cpu 0 == early)
smp: csd: cnt(01d16cb): 0012->0005 ipi
smp: csd: cnt(01d16cc): 0012->0006 queue
smp: csd: cnt(01d16cd): 0012->0006 ipi
smp: csd: cnt(01d16f3): 0012->001a ipi
smp: csd: cnt(01d16f4): 0012->ffff ping
smp: csd: cnt(01d1750): ffff->0018 hdlend (src cpu 0 == early)
smp: csd: cnt(01d1751): 0012->ffff pinged
smp: csd: cnt now: 01d1769
So we see that cpu#18 (0012 hex) is waiting for cpu#06 (first line of the
data).
The next 4 lines of the csd actions are not really interesting, as they are
rather old.
Then we see that cpu 0006 did handle a request rather recently (cnt 01d1333
- 01d1337): cpu 001a pinged it via an IPI and it got the IPI, entered the
handler, dequeued a request, and handled it.
Nearly all of the rest shows the critical request: cpu 0012 did a loop over
probably all other cpus and queued the requests and marked them to be IPI-ed
(including cpu 0006, cnt 01d16cd). Then the cpus marked to receive an IPI
were pinged (cnt 01d16f4 and cnt 01d1751).
The entry cnt 01d1750 is not of interest here.
This data confirms that on sending side everything seems to be okay at the
level above the actual IPI sending. On receiver side there seems no IPI to
be seen, but there is no visible reason for a race either.
It seems as if we need more debugging in the deeper layers: is the IPI
really sent out, and is something being received on the destination cpu?
I'll have another try with even more debugging code, probably in private
on the customer machine first.
Apologies for the late reply, was out last week.
Excellent news, and thank you!
For my part, I have put together a rough prototype script that allows
me to run scftorture on larger groups of systems and started running it,
though I am hoping that 1,000 is far more than will be required.
Your diagnosis of a lost IPI matches what we have been able to glean
from the occasional occurrences in the wild on our systems, for whatever
that might be worth. The hope is to get something that reproduces more
quickly, which would allow deeper debugging at this end as well.
Sometimes one is lucky.
I've found a reproducer while hunting another bug. The test on that
machine will trigger the csd_lock timeout about once a day.
Nice!!! You are way ahead of me!
I've used my new debug kernel and found that the IPI is really sent
out (more precise: the hypervisor has been requested to do so, and
it didn't report an error). On the target cpu there was no interrupt
received after that, so the IPI has not been swallowed on the target
cpu by the Linux kernel.
Will now try to instrument the hypervisor to get more data.
I am increasing the number and types of systems and the test duration.
Ijust started running three different systems with IPI workloads in both
guests and within host over the weekend.
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