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Subject[PATCH] x86: mce: New MCE logging design

* Huang Ying <> wrote:

> Hi, Ingo,
> I think it is a interesting idea to replace the original MCE logging
> part implementation (MCE log ring buffer + /dev/mcelog misc device)
> with tracing infrastructure. The tracing infrastructure has definitely
> more features than original method, but it is more complex than
> original method too.

TRACE_EVENT() can be used to define the tracepoint itself - but most of
the benefit comes from using the performance events subsystem for event
reporting. It's a perfect match for MCE events - see below.

> The tracing infrastructure is good at tracing. But I think it may be
> too complex to be used as a (hardware) error processing mechanism. I
> think the error processing should be simple and relatively independent
> with other part of the kernel to avoid triggering more (hardware)
> error during error processing. What do you think about this?

The fact that event reporting is used for other things as well might
even make it more fault-resilient than open-coded MCE code: just due to
it possibly being hot in the cache already (and thus not needing extra
DRAM cycles to a possibly faulty piece of hardware) while the MCE code,
not used by anything else, has to be brought into the cache for error
processing - triggering more faults.

But ... that argument is even largely irrelevant: the overriding core
kernel principle was always that of reuse of facilities. We want to keep
performance event reporting simple and robust too, so re-using
facilities is a good thing and the needs of MCE logging will improve
perf events - and vice versa.

Furthermore, to me as x86 maintainer it matters a lot that a piece of
code i maintain is clean, well-designed and maintainable in the long
run. The current /dev/mcelog code is not clean and not maintainable,
and that needs to be fixed before it can be extended.

That MCE logging also becomes more robust and more feature-rich as a
result is a happy side-effect.

> Even if we will reach consensus at some point to re-write MCE logging
> part totally. We still need to fix the bugs in original implementation
> before the new implementation being ready. Do you agree?

Well, this is all 2.6.33 stuff and that's plenty of time to do the
generic events + perf events approach.

Anyway, i think we can work on these MCE enhancements much better if we
move it to the level of actual patches. Let me give you a (very small)
demonstration of my requests.

The patch attached below adds two new events to the MCE code, to the
thermal throttling code:


If you apply the patch below to latest -tip and boot that kernel and do:

cd tools/perf/
make -j install

And type 'perf list', those two new MCE events will show up in the list
of available events:

# perf list
mce:mce_thermal_throttle_hot [Tracepoint event]
mce:mce_thermal_throttle_cold [Tracepoint event]

Initially you can check whether those events are occuring:

# perf stat -a -e mce:mce_thermal_throttle_hot sleep 1

Performance counter stats for 'sleep 1':

0 mce:mce_thermal_throttle_hot # 0.000 M/sec

1.000672214 seconds time elapsed

None on a normal system. Now, i tried it on a system that gets frequent
thermal events, and there it gives:

# perf stat -a -e mce:mce_thermal_throttle_hot sleep 1

Performance counter stats for 'sleep 1':

73 mce:mce_thermal_throttle_hot # 0.000 M/sec

1.000765260 seconds time elapsed

Now i can observe the frequency of those thermal throttling events:

# perf stat --repeat 10 -a -e mce:mce_thermal_throttle_hot sleep 1

Performance counter stats for 'sleep 1' (10 runs):

184 mce:mce_thermal_throttle_hot # 0.000 M/sec ( +- 1.971% )

1.006488821 seconds time elapsed ( +- 0.203% )

184/sec is pretty hefty, with a relatively stable rate.

Now i try to see exactly which code is affected by those thermal
throttling events. For that i use 'perf record', and profile the whole
system for 10 seconds:

# perf record -f -e mce:mce_thermal_throttle_hot -c 1 -a sleep 10
[ perf record: Woken up 4 times to write data ]
[ perf record: Captured and wrote 0.802 MB (~35026 samples) ]

And i used 'perf report' to analyze where the MCE events occured:

# Samples: 2495
# Overhead Command Shared Object Symbol
# ........ .............. ........................ ......
55.39% hackbench [vdso] [.] 0x000000f77e1430
35.27% hackbench f77c2430 [.] 0x000000f77c2430
3.05% hackbench [kernel] [k] ia32_setup_sigcontext
0.88% bash [vdso] [.] 0x000000f776f430
0.64% bash f7731430 [.] 0x000000f7731430
0.52% hackbench ./hackbench [.] sender
0.48% nice f7753430 [.] 0x000000f7753430
0.36% hackbench ./hackbench [.] receiver
0.24% perf /lib64/ [.] __GI_read
0.24% hackbench /lib/ [.] __libc_read

Most of the events concentrate themselves to around syscall entry
points. That's not because the system is doing that most of the time -
it's probably because the fast syscall entry point is a critical path of
the CPU and overheating events likely concentrate themselves to around
that place.

Now lets how we can do very detailed MCE logs, with per CPU data,
timestamps and other events mixed. For that i use perf record again:

# perf record -R -f -e mce:mce_thermal_throttle_hot:r -e mce:mce_thermal_throttle_cold:r -e irq:irq_handler_entry:r -M -c 1 -a sleep 10
[ perf record: Woken up 12 times to write data ]
[ perf record: Captured and wrote 1.167 MB (~50990 samples) ]

And i use 'perf trace' to look at the result:

hackbench-5070 [000] 1542.044184301: mce_thermal_throttle_hot: throttle_count=150852
hackbench-5070 [000] 1542.044672817: mce_thermal_throttle_cold: throttle_count=150852
hackbench-5056 [000] 1542.047029212: mce_thermal_throttle_hot: throttle_count=150853
hackbench-5057 [000] 1542.047518733: mce_thermal_throttle_cold: throttle_count=150853
hackbench-5058 [000] 1542.049822672: mce_thermal_throttle_hot: throttle_count=150854
hackbench-5046 [000] 1542.050312612: mce_thermal_throttle_cold: throttle_count=150854
hackbench-5072 [000] 1542.052720357: mce_thermal_throttle_hot: throttle_count=150855
hackbench-5072 [000] 1542.053210612: mce_thermal_throttle_cold: throttle_count=150855
:5074-5074 [000] 1542.055689468: mce_thermal_throttle_hot: throttle_count=150856
hackbench-5011 [000] 1542.056179849: mce_thermal_throttle_cold: throttle_count=150856
:5061-5061 [000] 1542.056937152: mce_thermal_throttle_hot: throttle_count=150857
hackbench-5048 [000] 1542.057428970: mce_thermal_throttle_cold: throttle_count=150857
hackbench-5053 [000] 1542.060372255: mce_thermal_throttle_hot: throttle_count=150858
hackbench-5054 [000] 1542.060863006: mce_thermal_throttle_cold: throttle_count=150858

The first thing we can see it from the trace, only the first core
reports thermal events (the box is a dual-core box). The other thing is
the timing: the CPU is in 'hot' state for about ~490 microseconds, then
in 'cold' state for ~250 microseconds.

So the CPU spends about 65% of its time in hot-throttled state, to cool
down enough to switch back to cold mode again.

Also, most of the overheating events hit the 'hackbench' app. (which is
no surprise on this box as it is only running that workload - but might
be interesting on other systems with more mixed workloads.)

So even in its completely MCE-unaware form, and with this small patch
that took 5 minutes to write, the performance events subsystem and
tooling can already give far greater insight into characteristics of
these events than the MCE subsystem did before.

Obviously the reported events can be used for different, MCE specific
things as well:

- Report an alert on the desktop (without having to scan the syslog all
the time)

- Mail the sysadmin

- Do some specific policy action such as soft-throttle the workload
until the situation and the log data can be examined.

As i outlined it in my previous mail, the perf events subsystem provides
a wide range of facilities that can be used for that purpose.

If there are aspects of MCE reporting usecases that necessiate the
extension of perf events, then we can do that - both sides will benefit
from it. What we dont want to do is to prolongue the pain caused by the
inferior, buggy and poorly designed /dev/mcelog ABI and code. It's not
serving our users well and people are spending time on fixing something
that has been misdesigned from the get go instead of concentrating on
improving the right facilities.

So this is really how i see the future of the MCE logging subsystem: use
meaningful generic events that empowers users, admins and tools to do
something intelligent with the available stream of MCE data. The
hardware tells us a _lot_ of information - lets use and expose that in a
structured form.


arch/x86/kernel/cpu/mcheck/therm_throt.c | 9 +++++
include/trace/events/mce.h | 47 +++++++++++++++++++++++++++++++
2 files changed, 55 insertions(+), 1 deletion(-)

Index: linux/arch/x86/kernel/cpu/mcheck/therm_throt.c
--- linux.orig/arch/x86/kernel/cpu/mcheck/therm_throt.c
+++ linux/arch/x86/kernel/cpu/mcheck/therm_throt.c
@@ -31,6 +31,9 @@
#include <asm/mce.h>
#include <asm/msr.h>

+#include <trace/events/mce.h>
/* How long to wait between reporting thermal events */
#define CHECK_INTERVAL (300 * HZ)

@@ -118,8 +121,12 @@ static int therm_throt_process(bool is_t
was_throttled = state->is_throttled;
state->is_throttled = is_throttled;

- if (is_throttled)
+ if (is_throttled) {
+ trace_mce_thermal_throttle_hot(state->throttle_count);
+ } else {
+ trace_mce_thermal_throttle_cold(state->throttle_count);
+ }

if (time_before64(now, state->next_check) &&
state->throttle_count != state->last_throttle_count)
Index: linux/include/trace/events/mce.h
--- /dev/null
+++ linux/include/trace/events/mce.h
@@ -0,0 +1,47 @@
+#define TRACE_SYSTEM mce
+#if !defined(_TRACE_MCE_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_MCE_H
+#include <linux/ktime.h>
+#include <linux/tracepoint.h>
+ TP_PROTO(unsigned int throttle_count),
+ TP_ARGS(throttle_count),
+ TP_STRUCT__entry(
+ __field( u64, throttle_count )
+ ),
+ TP_fast_assign(
+ __entry->throttle_count = throttle_count;
+ ),
+ TP_printk("throttle_count=%Lu", __entry->throttle_count)
+ TP_PROTO(unsigned int throttle_count),
+ TP_ARGS(throttle_count),
+ TP_STRUCT__entry(
+ __field( u64, throttle_count )
+ ),
+ TP_fast_assign(
+ __entry->throttle_count = throttle_count;
+ ),
+ TP_printk("throttle_count=%Lu", __entry->throttle_count)
+#endif /* _TRACE_MCE_H */
+/* This part must be outside protection */
+#include <trace/define_trace.h>

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