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From: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
To: mpe@ellerman.id.au
Subject: [PATCH] powerpc/perf: Clear pending PMI in ppmu callbacks
Date: Tue,  6 Apr 2021 10:47:43 -0400
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Running perf fuzzer testsuite popped up below messages
in the dmesg logs:

"Can't find PMC that caused IRQ"

This means a PMU exception happened, but none of the PMC's (Performance
Monitor Counter) were found to be overflown. Perf interrupt handler checks
the PMC's to see which PMC has overflown and if none of the PMCs are
overflown ( counter value not >= 0x80000000 ), it throws warning:
"Can't find PMC that caused IRQ".

Powerpc has capability to mask and replay a performance monitoring
interrupt (PMI). In case of replayed PMI, there are some corner cases
that clears the PMCs after masking. In such cases, the perf interrupt
handler will not find the active PMC values that had caused the overflow
and thus leading to this message. This patchset attempts to fix those
corner cases.

However there is one more case in PowerNV where these messages are
emitted during system wide profiling or when a specific CPU is monitored
for an event. That is, when a counter overflow just before entering idle
and a PMI gets triggered after wakeup from idle. Since PMCs
are not saved in the idle path, perf interrupt handler will not
find overflown counter value and emits the "Can't find PMC" messages.
This patch documents this race condition in powerpc core-book3s.

Patch fixes the ppmu callbacks to disable pending interrupt before clearing
the overflown PMC and documents the race condition in idle path.

Athira Rajeev (1):
  powerpc/perf: Fix PMU callbacks to clear pending PMI before resetting
    an overflown PMC

 arch/powerpc/include/asm/pmc.h  | 11 +++++++++
 arch/powerpc/perf/core-book3s.c | 55 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+)

-- 
1.8.3.1