Date: Thu, 3 Mar 2016 13:43:12 -0800
From: Andi Kleen <ak@linux.intel.com>
To: Stephane Eranian <eranian@google.com>
Cc: linux-kernel@vger.kernel.org, acme@redhat.com, peterz@infradead.org,
        mingo@elte.hu, kan.liang@intel.com, jolsa@redhat.com,
        namhyung@kernel.org, adrian.hunter@intel.com
Subject: Re: [PATCH 2/3] perf/x86/pebs: add workaround for broken OVFL status
 on HSW
Message-ID: <20160303214312.GI23621@tassilo.jf.intel.com>
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> +		/*
> +		 * There are cases where, even though, the PEBS ovfl bit is set in
> +		 * GLOBAL_OVF_STATUS, the PEBS events may also have their overflow bits
> +		 * set for their counters. We must clear them here because they have
> +		 * been processed as exact samples in the drain_pebs() routine. They
> +		 * must not be processed again in the for_each_bit_set() loop for
> +		 * regular samples below.
> +		 */
> +		status &= ~cpuc->pebs_enabled;
> +		status &= x86_pmu.intel_ctrl | GLOBAL_STATUS_TRACE_TOPAPMI;

If you just clear the bits here they will not be acked and stay around
forever in GLOBAL_STATUS, which causes other problems.

You need a separate ack_status variable that contains all bits and is always
acked.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only