Subject: [PATCH V4 0/1] powerpc/perf: Clear pending PMI in ppmu callbacks

From: Athira Rajeev <[email-masked]>
Date: Wed, 21 Jul 2021 01:48:28 -0400

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.

Changelog:
changes from v3 -> v4
   Addressed review comments from Nicholas Piggin
   - Added comment explaining the need to clear MMCR0 PMXE bit in
     pmu disable callback.
   - Added a check to display warning if there is a PMI pending
     bit set in Paca without any overflown PMC.
   - Removed the condition check before clearing pending PMI
     in 'clear_pmi_irq_pending' function.
   - Added reviewed by from Nicholas Piggin.

Changes from v2 -> v3
   Addressed review comments from Nicholas Piggin
   - Moved the clearing of PMI bit to power_pmu_disable.
     In previous versions, this was done in power_pmu_del,
     power_pmu_stop/enable callbacks before clearing of PMC's.
   - power_pmu_disable is called before any event gets deleted
     or stopped. If more than one event is running in the PMU,
     we may clear the PMI bit for an event which is not going
     to be deleted/stopped. Hence introduced check in
     power_pmu_enable to set back PMI to avoid dropping of valid
     samples in such cases.
   - Disable MMCR0 PMXE bit in pmu disable callback which otherwise
     could trigger PMI when PMU is getting disabled.
Changes from v1 -> v2
   Addressed review comments from Nicholas Piggin
   - Moved the PMI pending check and clearing function
     to arch/powerpc/include/asm/hw_irq.h and renamed
     function to "get_clear_pmi_irq_pending"
   - Along with checking for pending PMI bit in Paca,
     look for PMAO bit in MMCR0 register to decide on
     pending PMI interrupt.

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

 arch/powerpc/include/asm/hw_irq.h | 38 +++++++++++++++++++++++++
 arch/powerpc/perf/core-book3s.c   | 59 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 96 insertions(+), 1 deletion(-)

-- 
1.8.3.1