From 7b63ce61a6e369f2dadc73573be6d07f110bd3d4 Mon Sep 17 00:00:00 2001 From: jhb Date: Tue, 19 May 2015 19:15:19 +0000 Subject: [PATCH] Fix two bugs that could result in PMC sampling effectively stopping. In both cases, the the effect of the bug was that a very small positive number was written to the counter. This means that a large number of events needed to occur before the next sampling interrupt would trigger. Even with very frequently occurring events like clock cycles wrapping all the way around could take a long time. Both bugs occurred when updating the saved reload count for an outgoing thread on a context switch. First, the counter-independent code compares the current reload count against the count set when the thread switched in and generates a delta to apply to the saved count. If this delta causes the reload counter to go negative, it would add a full reload interval to wrap it around to a positive value. The fix is to add the full reload interval if the resulting counter is zero. Second, occasionally the raw counter value read during a context switch has actually wrapped, but an interrupt has not yet triggered. In this case the existing logic would return a very large reload count (e.g. 2^48 - 2 if the counter had overflowed by a count of 2). This was seen both for fixed-function and programmable counters on an E5-2643. Workaround this case by returning a reload count of zero. PR: 198149 Differential Revision: https://reviews.freebsd.org/D2557 Reviewed by: emaste MFC after: 1 week Sponsored by: Norse Corp, Inc. --- sys/dev/hwpmc/hwpmc_core.c | 8 ++++++++ sys/dev/hwpmc/hwpmc_mod.c | 2 +- 2 files changed, 9 insertions(+), 1 deletion(-) diff --git a/sys/dev/hwpmc/hwpmc_core.c b/sys/dev/hwpmc/hwpmc_core.c index aae5b2e0075..f22d2b328a3 100644 --- a/sys/dev/hwpmc/hwpmc_core.c +++ b/sys/dev/hwpmc/hwpmc_core.c @@ -203,6 +203,10 @@ core_pcpu_fini(struct pmc_mdep *md, int cpu) static pmc_value_t iaf_perfctr_value_to_reload_count(pmc_value_t v) { + + /* If the PMC has overflowed, return a reload count of zero. */ + if ((v & (1ULL << (core_iaf_width - 1))) == 0) + return (0); v &= (1ULL << core_iaf_width) - 1; return (1ULL << core_iaf_width) - v; } @@ -1806,6 +1810,10 @@ static const int niap_events = sizeof(iap_events) / sizeof(iap_events[0]); static pmc_value_t iap_perfctr_value_to_reload_count(pmc_value_t v) { + + /* If the PMC has overflowed, return a reload count of zero. */ + if ((v & (1ULL << (core_iap_width - 1))) == 0) + return (0); v &= (1ULL << core_iap_width) - 1; return (1ULL << core_iap_width) - v; } diff --git a/sys/dev/hwpmc/hwpmc_mod.c b/sys/dev/hwpmc/hwpmc_mod.c index bc3fa3322c3..d1b56f10835 100644 --- a/sys/dev/hwpmc/hwpmc_mod.c +++ b/sys/dev/hwpmc/hwpmc_mod.c @@ -1435,7 +1435,7 @@ pmc_process_csw_out(struct thread *td) tmp += pm->pm_sc.pm_reloadcount; mtx_pool_lock_spin(pmc_mtxpool, pm); pp->pp_pmcs[ri].pp_pmcval -= tmp; - if ((int64_t) pp->pp_pmcs[ri].pp_pmcval < 0) + if ((int64_t) pp->pp_pmcs[ri].pp_pmcval <= 0) pp->pp_pmcs[ri].pp_pmcval += pm->pm_sc.pm_reloadcount; mtx_pool_unlock_spin(pmc_mtxpool, pm); -- 2.45.0