Excerpts from Athira Rajeev's message of November 20, 2021 12:36 am: > > >> On 21-Jul-2021, at 11:18 AM, Athira Rajeev <atraj...@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
It seems good to me. It already has my R-B. Would be good if we can get this one merged. Thanks, Nick
