[tip: perf/urgent] perf/x86/intel: Add a quirk for the calculation of the number of counters on Alder Lake

From: tip-bot2 for Kan Liang
Date: Tue Jan 18 2022 - 06:18:10 EST


The following commit has been merged into the perf/urgent branch of tip:

Commit-ID: 7fa981cad216e9f64f49e22112f610c0bfed91bc
Gitweb: https://git.kernel.org/tip/7fa981cad216e9f64f49e22112f610c0bfed91bc
Author: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
AuthorDate: Tue, 11 Jan 2022 10:20:38 -08:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 18 Jan 2022 12:09:47 +01:00

perf/x86/intel: Add a quirk for the calculation of the number of counters on Alder Lake

For some Alder Lake machine with all E-cores disabled in a BIOS, the
below warning may be triggered.

[ 2.010766] hw perf events fixed 5 > max(4), clipping!

Current perf code relies on the CPUID leaf 0xA and leaf 7.EDX[15] to
calculate the number of the counters and follow the below assumption.

For a hybrid configuration, the leaf 7.EDX[15] (X86_FEATURE_HYBRID_CPU)
is set. The leaf 0xA only enumerate the common counters. Linux perf has
to manually add the extra GP counters and fixed counters for P-cores.
For a non-hybrid configuration, the X86_FEATURE_HYBRID_CPU should not
be set. The leaf 0xA enumerates all counters.

However, that's not the case when all E-cores are disabled in a BIOS.
Although there are only P-cores in the system, the leaf 7.EDX[15]
(X86_FEATURE_HYBRID_CPU) is still set. But the leaf 0xA is updated
to enumerate all counters of P-cores. The inconsistency triggers the
warning.

Several software ways were considered to handle the inconsistency.
- Drop the leaf 0xA and leaf 7.EDX[15] CPUID enumeration support.
Hardcode the number of counters. This solution may be a problem for
virtualization. A hypervisor cannot control the number of counters
in a Linux guest via changing the guest CPUID enumeration anymore.
- Find another CPUID bit that is also updated with E-cores disabled.
There may be a problem in the virtualization environment too. Because
a hypervisor may disable the feature/CPUID bit.
- The P-cores have a maximum of 8 GP counters and 4 fixed counters on
ADL. The maximum number can be used to detect the case.
This solution is implemented in this patch.

Fixes: ee72a94ea4a6 ("perf/x86/intel: Fix fixed counter check warning for some Alder Lake")
Reported-by: Damjan Marion (damarion) <damarion@xxxxxxxxx>
Reported-by: Chan Edison <edison_chan_gz@xxxxxxxxxxx>
Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Damjan Marion (damarion) <damarion@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/1641925238-149288-1-git-send-email-kan.liang@xxxxxxxxxxxxxxx
---
arch/x86/events/intel/core.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index fd9f908..d5f940c 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -6236,6 +6236,19 @@ __init int intel_pmu_init(void)
pmu->num_counters = x86_pmu.num_counters;
pmu->num_counters_fixed = x86_pmu.num_counters_fixed;
}
+
+ /*
+ * Quirk: For some Alder Lake machine, when all E-cores are disabled in
+ * a BIOS, the leaf 0xA will enumerate all counters of P-cores. However,
+ * the X86_FEATURE_HYBRID_CPU is still set. The above codes will
+ * mistakenly add extra counters for P-cores. Correct the number of
+ * counters here.
+ */
+ if ((pmu->num_counters > 8) || (pmu->num_counters_fixed > 4)) {
+ pmu->num_counters = x86_pmu.num_counters;
+ pmu->num_counters_fixed = x86_pmu.num_counters_fixed;
+ }
+
pmu->max_pebs_events = min_t(unsigned, MAX_PEBS_EVENTS, pmu->num_counters);
pmu->unconstrained = (struct event_constraint)
__EVENT_CONSTRAINT(0, (1ULL << pmu->num_counters) - 1,