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

[Athira Rajeev]

> On 21-Jul-2021, at 11:18 AM, Athira Rajeev <atrajeev at linux.vnet.ibm.com> wrote:
> 
> 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

Hi,

Please let me know if there are any review comments for this patch.

Thanks
Athira
> 
> 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
>