[patch 3/6] x86, perf, cqm: Remove pointless spinlock from state cache

Thomas Gleixner
2015-05-19T00:00:50Z

[-- Attachment #1: x86-perf-cqm-remove-pointless-spinlock.patch --]
[-- Type: text/plain, Size: 2730 bytes --]

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 3 functions which modify the content: start, stop and 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@linutronix.de>
---
arch/x86/kernel/cpu/perf_event_intel_cqm.c | 17 ++++++-----------
1 file changed, 6 insertions(+), 11 deletions(-)

Index: linux/arch/x86/kernel/cpu/perf_event_intel_cqm.c
===================================================================
--- linux.orig/arch/x86/kernel/cpu/perf_event_intel_cqm.c
+++ linux/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
{
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
* 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
} 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(unsign
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;