linux-kernel - [PATCH] perf, pt, bts: Get rid of itrace

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Date:   Thu, 30 Mar 2017 18:39:56 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: [PATCH] perf, pt, bts: Get rid of itrace_started

I just noticed that hw.itrace_started and hw.config are aliased to the
same location. Now, the PT driver happens to use both, which works out
fine by sheer luck:

 - STORE(hw.itrace_start) is ordered before STORE(hw.config), in the
 program order, although there are no compiler barriers to ensure that,
 - to the perf_log_itrace_start() hw.itrace_start looks set at the same
 time as when it is intended to be set because both stores happen in the
 same path,
 - hw.config is never reset to zero in the PT driver.

Now, the use of hw.config by the PT driver makes more sense (it being a
HW PMU) than messing around with itrace_started, which is an awkward API
to begin with.

This patch replaces hw.itrace_started with an attach_state bit and an
API call for the PMU drivers to use to communicate the condition.

Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
---
 arch/x86/events/intel/bts.c | 2 +-
 arch/x86/events/intel/pt.c  | 5 +++--
 include/linux/perf_event.h  | 5 ++---
 kernel/events/core.c        | 7 ++++++-
 4 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
index 8ae8c5ce3a..c036ed56db 100644
--- a/arch/x86/events/intel/bts.c
+++ b/arch/x86/events/intel/bts.c
@@ -268,7 +268,7 @@ static void bts_event_start(struct perf_event *event, int flags)
 	bts->ds_back.bts_absolute_maximum = cpuc->ds->bts_absolute_maximum;
 	bts->ds_back.bts_interrupt_threshold = cpuc->ds->bts_interrupt_threshold;
 
-	event->hw.itrace_started = 1;
+	perf_event_itrace_started(event);
 	event->hw.state = 0;
 
 	__bts_event_start(event);
diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
index ae8324d65e..81fd41d5a0 100644
--- a/arch/x86/events/intel/pt.c
+++ b/arch/x86/events/intel/pt.c
@@ -471,8 +471,9 @@ static void pt_config(struct perf_event *event)
 	struct pt *pt = this_cpu_ptr(&pt_ctx);
 	u64 reg;
 
-	if (!event->hw.itrace_started) {
-		event->hw.itrace_started = 1;
+	/* First round: clear STATUS, in particular the PSB byte counter. */
+	if (!event->hw.config) {
+		perf_event_itrace_started(event);
 		wrmsrl(MSR_IA32_RTIT_STATUS, 0);
 	}
 
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 24a635887f..30eacfa4fa 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -147,9 +147,6 @@ struct hw_perf_event {
 			struct list_head	cqm_groups_entry;
 			struct list_head	cqm_group_entry;
 		};
-		struct { /* itrace */
-			int			itrace_started;
-		};
 		struct { /* amd_power */
 			u64	pwr_acc;
 			u64	ptsc;
@@ -541,6 +538,7 @@ struct swevent_hlist {
 #define PERF_ATTACH_GROUP	0x02
 #define PERF_ATTACH_TASK	0x04
 #define PERF_ATTACH_TASK_DATA	0x08
+#define PERF_ATTACH_ITRACE	0x10
 
 struct perf_cgroup;
 struct ring_buffer;
@@ -862,6 +860,7 @@ extern int perf_aux_output_skip(struct perf_output_handle *handle,
 				unsigned long size);
 extern void *perf_get_aux(struct perf_output_handle *handle);
 extern void perf_aux_output_flag(struct perf_output_handle *handle, u64 flags);
+extern void perf_event_itrace_started(struct perf_event *event);
 
 extern int perf_pmu_register(struct pmu *pmu, const char *name, int type);
 extern void perf_pmu_unregister(struct pmu *pmu);
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6e75a5c941..2e23f5373f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7239,6 +7239,11 @@ static void perf_log_throttle(struct perf_event *event, int enable)
 	perf_output_end(&handle);
 }
 
+void perf_event_itrace_started(struct perf_event *event)
+{
+	event->attach_state |= PERF_ATTACH_ITRACE;
+}
+
 static void perf_log_itrace_start(struct perf_event *event)
 {
 	struct perf_output_handle handle;
@@ -7254,7 +7259,7 @@ static void perf_log_itrace_start(struct perf_event *event)
 		event = event->parent;
 
 	if (!(event->pmu->capabilities & PERF_PMU_CAP_ITRACE) ||
-	    event->hw.itrace_started)
+	    event->attach_state & PERF_ATTACH_ITRACE)
 		return;
 
 	rec.header.type	= PERF_RECORD_ITRACE_START;
-- 
2.11.0