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SubjectRe: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()
On Wed, Oct 16, 2013 at 03:31:25PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 16, 2013 at 08:46:49AM -0400, Don Zickus wrote:
> > On Wed, Oct 16, 2013 at 12:57:55PM +0200, Peter Zijlstra wrote:
> > > A prettier patch below. The main difference is on-demand allocation of
> > > the scratch buffer.
> >
> > I'll see if I can sanity test this in the next couple hours.
> >
> > Further testing yesterday showed that intel_pmu_drain_pebs_nhm still
> > has long latencies somewhere. With 15 minute reboots, isolation goes
> > slooow.
>
> Pick a smaller box? I seem to be able to reproduce on my wsm-ep, which
> boots inside a minute :-)
>
> root@westmere:~# cd /debug/tracing/
> root@westmere:/debug/tracing# echo function > current_tracer
> root@westmere:/debug/tracing# cat available_filter_functions | grep ^inat > set_ftrace_notrace
> root@westmere:/debug/tracing# cat available_filter_functions | grep ^insn | grep -v get_length >> set_ftrace_notrace
>
> Run: perf top --stdio -e 'cycles:pp' in another window and when the
> console output shows:
>
> [ 610.319486] perf samples too long (19310 > 19230), lowering kernel.perf_event_max_sample_rate to 7000
>
> quickly press enter here:

BTW; you can also replace this bit of manual intervention with something
like:

There's 3 changes:
- changed atomic_t into regular int; there's nothing atomic about
atomic_set vs atomic_read, so atomic_t is pointless
- made perf_proc_update_handler() clear the running_sample_length
state.
- added if (avg_local_sample_len > 30000) tracing_off().

Of course you should tweak the 30000 to match whatever value you're
interested in. But tracing_off() does the same as that:

echo 0 > tracing_on

And avoids being too late and having lost the trace buffer content.

---
kernel/events/core.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index c716385f6483..ea787d0d0e78 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -175,8 +175,10 @@ int sysctl_perf_event_sample_rate __read_mostly = DEFAULT_MAX_SAMPLE_RATE;
static int max_samples_per_tick __read_mostly = DIV_ROUND_UP(DEFAULT_MAX_SAMPLE_RATE, HZ);
static int perf_sample_period_ns __read_mostly = DEFAULT_SAMPLE_PERIOD_NS;

-static atomic_t perf_sample_allowed_ns __read_mostly =
- ATOMIC_INIT( DEFAULT_SAMPLE_PERIOD_NS * DEFAULT_CPU_TIME_MAX_PERCENT / 100);
+static int perf_sample_allowed_ns __read_mostly =
+ DEFAULT_SAMPLE_PERIOD_NS * DEFAULT_CPU_TIME_MAX_PERCENT / 100;
+
+static DEFINE_PER_CPU(u64, running_sample_length);

void update_perf_cpu_limits(void)
{
@@ -184,7 +186,7 @@ void update_perf_cpu_limits(void)

tmp *= sysctl_perf_cpu_time_max_percent;
do_div(tmp, 100);
- atomic_set(&perf_sample_allowed_ns, tmp);
+ ACCESS_ONCE(perf_sample_allowed_ns) = tmp;
}

static int perf_rotate_context(struct perf_cpu_context *cpuctx);
@@ -194,6 +196,7 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
loff_t *ppos)
{
int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+ int cpu;

if (ret || !write)
return ret;
@@ -202,6 +205,9 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate;
update_perf_cpu_limits();

+ for_each_possible_cpu(cpu)
+ per_cpu(running_sample_length, cpu) = 0;
+
return 0;
}

@@ -228,14 +234,13 @@ int perf_cpu_time_max_percent_handler(struct ctl_table *table, int write,
* we detect that events are taking too long.
*/
#define NR_ACCUMULATED_SAMPLES 128
-DEFINE_PER_CPU(u64, running_sample_length);

void perf_sample_event_took(u64 sample_len_ns)
{
u64 avg_local_sample_len;
u64 local_samples_len;

- if (atomic_read(&perf_sample_allowed_ns) == 0)
+ if (ACCESS_ONCE(perf_sample_allowed_ns) == 0)
return;

/* decay the counter by 1 average sample */
@@ -251,12 +256,15 @@ void perf_sample_event_took(u64 sample_len_ns)
*/
avg_local_sample_len = local_samples_len/NR_ACCUMULATED_SAMPLES;

- if (avg_local_sample_len <= atomic_read(&perf_sample_allowed_ns))
+ if (avg_local_sample_len <= ACCESS_ONCE(perf_sample_allowed_ns))
return;

if (max_samples_per_tick <= 1)
return;

+ if (avg_local_sample_len > 30000)
+ tracing_off();
+
max_samples_per_tick = DIV_ROUND_UP(max_samples_per_tick, 2);
sysctl_perf_event_sample_rate = max_samples_per_tick * HZ;
perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate;
@@ -265,7 +273,7 @@ void perf_sample_event_took(u64 sample_len_ns)
"perf samples too long (%lld > %d), lowering "
"kernel.perf_event_max_sample_rate to %d\n",
avg_local_sample_len,
- atomic_read(&perf_sample_allowed_ns),
+ ACCESS_ONCE(perf_sample_allowed_ns),
sysctl_perf_event_sample_rate);

update_perf_cpu_limits();

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