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Date: Tue, 10 Feb 2009 10:21:58 +0800
From: "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To: Ingo Molnar <mingo@...e.hu>
Cc: Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf_counter: Make software counters work as per-cpu
counters
On Mon, 2009-02-09 at 12:48 +0100, Ingo Molnar wrote:
> * Paul Mackerras <paulus@...ba.org> wrote:
>
> > Impact: kernel crash fix
> >
> > Yanmin Zhang reported that using a PERF_COUNT_TASK_CLOCK software
> > counter as a per-cpu counter would reliably crash the system, because
> > it calls __task_delta_exec with a null pointer. The page fault,
> > context switch and cpu migration counters also won't function
> > correctly as per-cpu counters since they reference the current task.
> >
> > This fixes the problem by redirecting the task_clock counter to the
> > cpu_clock counter when used as a per-cpu counter, and by implementing
> > per-cpu page fault, context switch and cpu migration counters.
> >
> > Along the way, this:
> >
> > - Initializes counter->ctx earlier, in perf_counter_alloc, so that
> > sw_perf_counter_init can use it
> > - Adds code to kernel/sched.c to count task migrations into each
> > cpu, in rq->nr_migrations_in
> > - Exports the per-cpu context switch and task migration counts
> > via new functions added to kernel/sched.c
> > - Makes sure that if sw_perf_counter_init fails, we don't try to
> > initialize the counter as a hardware counter. Since the user has
> > passed a negative, non-raw event type, they clearly don't intend
> > for it to be interpreted as a hardware event.
> >
> > Signed-off-by: Paul Mackerras <paulus@...ba.org>
I tested it. The patch does fix the crash.
>
> Very nice, thanks Paul!
>
> > I'm a little concerned about the use of u64 for the existing
> > rq->nr_switches and the new rq->nr_migrations_in. On 32-bit machines
> > this will get updated in two halves, so there is a very small but
> > non-zero probability that reading it will give a bogus value. It's
> > not clear to me what the best way to fix it is, since not all 32-bit
> > platforms have atomic64_t. Maybe make nr_switches and
> > nr_migrations_in unsigned long, or atomic_t?
>
> It used to be 'just stats' so we dont really mind. But now that we
> expose it in a bit more systematic way i agree that it should be
> fixed. Updating to 'unsigned long' sounds good to me.
>
> > I have some sort of git problem with my perfcounters.git repository.
> > I'll let you know when I have that sorted out.
>
> ok - i've applied your patch from email to allow Yanmin to test tip:master.
Ingo,
Here is the patch to add system-wide collection for perfstat.c.
With the new parameter '-s', perfstat introduces very little overhead to benchmark
testing. If tracking performance data without '-s', sometimes perfstat has too much
impact on benchmark results, especially on netperf UDP-U-4k.
Yanmin
---
--- perfstat.c 2009-01-23 03:18:31.000000000 +0800
+++ perfstat_ymz.c 2009-02-10 10:17:46.000000000 +0800
@@ -172,14 +172,18 @@ static char *sw_event_names [] = {
};
#define MAX_COUNTERS 64
+#define MAX_NR_CPUS 256
static int nr_counters = 0;
+static int nr_cpus = 0;
static int event_id[MAX_COUNTERS] =
{ -2, -5, -4, -3, 0, 1, 2, 3};
static int event_raw[MAX_COUNTERS];
+static int system_wide = 0;
+
static void display_help(void)
{
printf(
@@ -195,6 +199,7 @@ static void display_help(void)
" 4: branch instructions\n"
" 5: branch prediction misses\n\n"
" 6: bus cycles\n"
+ " -s # system-wide collection\n"
" -c <cmd..> --command=<cmd..> # command+arguments to be timed.\n"
"\n");
@@ -269,7 +274,7 @@ static void process_options(int argc, ch
{"command", no_argument, NULL, 'c'},
{NULL, 0, NULL, 0 }
};
- int c = getopt_long(argc, argv, "+:e:c",
+ int c = getopt_long(argc, argv, "+:e:c:s",
long_options, &option_index);
if (c == -1)
break;
@@ -277,6 +282,9 @@ static void process_options(int argc, ch
switch (c) {
case 'c':
break;
+ case 's':
+ system_wide = 1;
+ break;
case 'e':
parse_events(optarg);
break;
@@ -302,7 +310,7 @@ char fault_here[1000000];
#define PR_TASK_PERF_COUNTERS_DISABLE 31
#define PR_TASK_PERF_COUNTERS_ENABLE 32
-static int fd[MAX_COUNTERS];
+static int fd[MAX_NR_CPUS][MAX_COUNTERS];
static void create_counter(int counter)
{
@@ -313,16 +321,29 @@ static void create_counter(int counter)
hw_event.raw = event_raw[counter];
hw_event.record_type = PERF_RECORD_SIMPLE;
hw_event.nmi = 0;
- hw_event.inherit = 1;
- hw_event.disabled = 1;
- fd[counter] = sys_perf_counter_open(&hw_event, 0, -1, -1);
- if (fd[counter] < 0) {
- printf("perfstat error: syscall returned with %d (%s)\n",
- fd[counter], strerror(errno));
- exit(-1);
+ if (system_wide) {
+ int cpu;
+ for (cpu = 0; cpu < nr_cpus; cpu ++) {
+ fd[cpu][counter] = sys_perf_counter_open(&hw_event, -1, cpu, -1);
+ if (fd[cpu][counter] < 0) {
+ printf("perfstat error: syscall returned with %d (%s)\n",
+ fd[cpu][counter], strerror(errno));
+ exit(-1);
+ }
+
+ }
+ } else {
+ hw_event.inherit = 1;
+ hw_event.disabled = 1;
+
+ fd[0][counter] = sys_perf_counter_open(&hw_event, 0, -1, -1);
+ if (fd[0][counter] < 0) {
+ printf("perfstat error: syscall returned with %d (%s)\n",
+ fd[0][counter], strerror(errno));
+ exit(-1);
+ }
}
- assert(fd[counter] >= 0);
}
@@ -344,6 +365,13 @@ int main(int argc, char *argv[])
process_options(argc, argv);
+ if (system_wide) {
+ nr_cpus = sysconf(_SC_NPROCESSORS_ONLN);
+ assert(nr_cpus <= MAX_NR_CPUS);
+ assert(nr_cpus >= 0);
+ } else
+ nr_cpus = 1;
+
for (counter = 0; counter < nr_counters; counter++)
create_counter(counter);
@@ -375,12 +403,16 @@ int main(int argc, char *argv[])
fprintf(stderr, "\n");
for (counter = 0; counter < nr_counters; counter++) {
- u64 count;
-
- res = read(fd[counter],
- (char *) &count, sizeof(count));
- assert(res == sizeof(count));
+ int cpu;
+ u64 count, single_count;
+ count = 0;
+ for (cpu = 0; cpu < nr_cpus; cpu ++) {
+ res = read(fd[cpu][counter],
+ (char *) &single_count, sizeof(single_count));
+ assert(res == sizeof(single_count));
+ count += single_count;
+ }
if (!event_raw[counter] &&
(event_id[counter] == PERF_COUNT_CPU_CLOCK ||
--
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