linux-kernel - [tip:perf/core] perf/x86/intel/cqm: Remove pointless spinlock from state cache

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Message-ID: <tip-9e7eaac95af6c1aecaf558b8c7a1757d5f2d2ad7@git.kernel.org>
Date:	Wed, 27 May 2015 03:03:31 -0700
From:	tip-bot for Thomas Gleixner <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	matt.fleming@...el.com, kanaka.d.juvva@...el.com,
	tglx@...utronix.de, peterz@...radead.org,
	linux-kernel@...r.kernel.org, hpa@...or.com, will.auld@...el.com,
	vikas.shivappa@...ux.intel.com, mingo@...nel.org,
	torvalds@...ux-foundation.org
Subject: [tip:perf/core] perf/x86/intel/cqm:
  Remove pointless spinlock from state cache

Commit-ID:  9e7eaac95af6c1aecaf558b8c7a1757d5f2d2ad7
Gitweb:     http://git.kernel.org/tip/9e7eaac95af6c1aecaf558b8c7a1757d5f2d2ad7
Author:     Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Tue, 19 May 2015 00:00:53 +0000
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 27 May 2015 09:17:39 +0200

perf/x86/intel/cqm: Remove pointless spinlock from state cache

'struct intel_cqm_state' is a strict per CPU cache of the rmid and the
usage counter. It can never be modified from a remote CPU.

The three functions which modify the content: intel_cqm_event[start|stop|del]
(del maps to stop) are called from the perf core with interrupts disabled
which is enough protection for the per CPU state values.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Matt Fleming <matt.fleming@...el.com>
Cc: Kanaka Juvva <kanaka.d.juvva@...el.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Cc: Will Auld <will.auld@...el.com>
Link: http://lkml.kernel.org/r/20150518235150.001006529@linutronix.de
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/cpu/perf_event_intel_cqm.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_cqm.c b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
index 3e9a7fb..63391f8 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
@@ -17,11 +17,16 @@ static unsigned int cqm_max_rmid = -1;
 static unsigned int cqm_l3_scale; /* supposedly cacheline size */
 
 struct intel_cqm_state {
-	raw_spinlock_t		lock;
 	u32			rmid;
 	int			cnt;
 };
 
+/*
+ * The cached intel_cqm_state is strictly per CPU and can never be
+ * updated from a remote CPU. Both functions which modify the state
+ * (intel_cqm_event_start and intel_cqm_event_stop) are called with
+ * interrupts disabled, which is sufficient for the protection.
+ */
 static DEFINE_PER_CPU(struct intel_cqm_state, cqm_state);
 
 /*
@@ -963,15 +968,12 @@ static void intel_cqm_event_start(struct perf_event *event, int mode)
 {
 	struct intel_cqm_state *state = this_cpu_ptr(&cqm_state);
 	u32 rmid = event->hw.cqm_rmid;
-	unsigned long flags;
 
 	if (!(event->hw.cqm_state & PERF_HES_STOPPED))
 		return;
 
 	event->hw.cqm_state &= ~PERF_HES_STOPPED;
 
-	raw_spin_lock_irqsave(&state->lock, flags);
-
 	if (state->cnt++)
 		WARN_ON_ONCE(state->rmid != rmid);
 	else
@@ -984,21 +986,17 @@ static void intel_cqm_event_start(struct perf_event *event, int mode)
 	 * Technology component.
 	 */
 	wrmsr(MSR_IA32_PQR_ASSOC, rmid, 0);
-
-	raw_spin_unlock_irqrestore(&state->lock, flags);
 }
 
 static void intel_cqm_event_stop(struct perf_event *event, int mode)
 {
 	struct intel_cqm_state *state = this_cpu_ptr(&cqm_state);
-	unsigned long flags;
 
 	if (event->hw.cqm_state & PERF_HES_STOPPED)
 		return;
 
 	event->hw.cqm_state |= PERF_HES_STOPPED;
 
-	raw_spin_lock_irqsave(&state->lock, flags);
 	intel_cqm_event_read(event);
 
 	if (!--state->cnt) {
@@ -1013,8 +1011,6 @@ static void intel_cqm_event_stop(struct perf_event *event, int mode)
 	} else {
 		WARN_ON_ONCE(!state->rmid);
 	}
-
-	raw_spin_unlock_irqrestore(&state->lock, flags);
 }
 
 static int intel_cqm_event_add(struct perf_event *event, int mode)
@@ -1257,7 +1253,6 @@ static void intel_cqm_cpu_prepare(unsigned int cpu)
 	struct intel_cqm_state *state = &per_cpu(cqm_state, cpu);
 	struct cpuinfo_x86 *c = &cpu_data(cpu);
 
-	raw_spin_lock_init(&state->lock);
 	state->rmid = 0;
 	state->cnt  = 0;
 
--
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