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Subject[PATCH 3/3] perf/x86/intel: force resched when TFA sysctl is modified
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This patch provides guarantee to the sysadmin that when TFA is disabled, no PMU
event is using PMC3 when the echo command returns. Vice-Versa, when TFA
is enabled, PMU can use PMC3 immediately (to eliminate possible multiplexing).

$ perf stat -a -I 1000 --no-merge -e branches,branches,branches,branches
1.000123979 125,768,725,208 branches
1.000562520 125,631,000,456 branches
1.000942898 125,487,114,291 branches
1.001333316 125,323,363,620 branches
2.004721306 125,514,968,546 branches
2.005114560 125,511,110,861 branches
2.005482722 125,510,132,724 branches
2.005851245 125,508,967,086 branches
3.006323475 125,166,570,648 branches
3.006709247 125,165,650,056 branches
3.007086605 125,164,639,142 branches
3.007459298 125,164,402,912 branches
4.007922698 125,045,577,140 branches
4.008310775 125,046,804,324 branches
4.008670814 125,048,265,111 branches
4.009039251 125,048,677,611 branches
5.009503373 125,122,240,217 branches
5.009897067 125,122,450,517 branches

Then on another connection, sysadmin does:
$ echo 1 >/sys/devices/cpu/allow_tsx_force_abort

Then perf stat adjusts the events immediately:

5.010286029 125,121,393,483 branches
5.010646308 125,120,556,786 branches
6.011113588 124,963,351,832 branches
6.011510331 124,964,267,566 branches
6.011889913 124,964,829,130 branches
6.012262996 124,965,841,156 branches
7.012708299 124,419,832,234 branches [79.69%]
7.012847908 124,416,363,853 branches [79.73%]
7.013225462 124,400,723,712 branches [79.73%]
7.013598191 124,376,154,434 branches [79.70%]
8.014089834 124,250,862,693 branches [74.98%]
8.014481363 124,267,539,139 branches [74.94%]
8.014856006 124,259,519,786 branches [74.98%]
8.014980848 124,225,457,969 branches [75.04%]
9.015464576 124,204,235,423 branches [75.03%]
9.015858587 124,204,988,490 branches [75.04%]
9.016243680 124,220,092,486 branches [74.99%]
9.016620104 124,231,260,146 branches [74.94%]

And vice-versa if the syadmin does:
$ echo 0 >/sys/devices/cpu/allow_tsx_force_abort

Events are again spread over the 4 counters:

10.017096277 124,276,230,565 branches [74.96%]
10.017237209 124,228,062,171 branches [75.03%]
10.017478637 124,178,780,626 branches [75.03%]
10.017853402 124,198,316,177 branches [75.03%]
11.018334423 124,602,418,933 branches [85.40%]
11.018722584 124,602,921,320 branches [85.42%]
11.019095621 124,603,956,093 branches [85.42%]
11.019467742 124,595,273,783 branches [85.42%]
12.019945736 125,110,114,864 branches
12.020330764 125,109,334,472 branches
12.020688740 125,109,818,865 branches
12.021054020 125,108,594,014 branches
13.021516774 125,109,164,018 branches
13.021903640 125,108,794,510 branches
13.022270770 125,107,756,978 branches
13.022630819 125,109,380,471 branches
14.023114989 125,133,140,817 branches
14.023501880 125,133,785,858 branches
14.023868339 125,133,852,700 branches

Signed-off-by: Stephane Eranian <eranian@google.com>
---
arch/x86/events/core.c | 4 +++
arch/x86/events/intel/core.c | 60 ++++++++++++++++++++++++++++++++++--
arch/x86/events/perf_event.h | 1 +
3 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 12d7d591843e..314173f89cc8 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -677,6 +677,10 @@ static inline int is_x86_event(struct perf_event *event)
return event->pmu == &pmu;
}

+struct pmu *x86_get_pmu(void)
+{
+ return &pmu;
+}
/*
* Event scheduler state:
*
diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index a4b7711ef0ee..8d356c2096bc 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -4483,6 +4483,60 @@ static ssize_t freeze_on_smi_store(struct device *cdev,
return count;
}

+static void update_tfa_sched(void *ignored)
+{
+ struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+ struct pmu *pmu = x86_get_pmu();
+ struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);
+ struct perf_event_context *task_ctx = cpuctx->task_ctx;
+
+ /* prevent any changes to the two contexts */
+ perf_ctx_lock(cpuctx, task_ctx);
+
+ /*
+ * check if PMC3 is used
+ * and if so force schedule out for all event types all contexts
+ */
+ if (test_bit(3, cpuc->active_mask))
+ perf_ctx_resched(cpuctx, task_ctx, EVENT_ALL|EVENT_CPU);
+
+ perf_ctx_unlock(cpuctx, task_ctx);
+}
+
+static ssize_t show_sysctl_tfa(struct device *cdev,
+ struct device_attribute *attr,
+ char *buf)
+{
+ return snprintf(buf, 40, "%d\n", allow_tsx_force_abort);
+}
+
+static ssize_t set_sysctl_tfa(struct device *cdev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+{
+ unsigned long val;
+ ssize_t ret;
+
+ ret = kstrtoul(buf, 0, &val);
+ if (ret)
+ return ret;
+
+ /* looking for boolean value */
+ if (val > 2)
+ return -EINVAL;
+
+ /* no change */
+ if (val == allow_tsx_force_abort)
+ return count;
+
+ allow_tsx_force_abort ^= 1;
+
+ on_each_cpu(update_tfa_sched, NULL, 1);
+
+ return count;
+}
+
+
static DEVICE_ATTR_RW(freeze_on_smi);

static ssize_t branches_show(struct device *cdev,
@@ -4515,7 +4569,9 @@ static struct attribute *intel_pmu_caps_attrs[] = {
NULL
};

-static DEVICE_BOOL_ATTR(allow_tsx_force_abort, 0644, allow_tsx_force_abort);
+static DEVICE_ATTR(allow_tsx_force_abort, 0644,
+ show_sysctl_tfa,
+ set_sysctl_tfa);

static struct attribute *intel_pmu_attrs[] = {
&dev_attr_freeze_on_smi.attr,
@@ -5026,7 +5082,7 @@ __init int intel_pmu_init(void)
x86_pmu.get_event_constraints = tfa_get_event_constraints;
x86_pmu.enable_all = intel_tfa_pmu_enable_all;
x86_pmu.commit_scheduling = intel_tfa_commit_scheduling;
- intel_pmu_attrs[1] = &dev_attr_allow_tsx_force_abort.attr.attr;
+ intel_pmu_attrs[1] = &dev_attr_allow_tsx_force_abort.attr;
}

pr_cont("Skylake events, ");
diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index fff8868f92a8..437255603810 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -785,6 +785,7 @@ static struct perf_pmu_events_ht_attr event_attr_##v = { \
.event_str_ht = ht, \
}

+struct pmu *x86_get_pmu(void);
extern struct x86_pmu x86_pmu __read_mostly;

static inline bool x86_pmu_has_lbr_callstack(void)
--
2.21.0.392.gf8f6787159e-goog
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