We can disable preempt in stop_cpu(), It can reduce the cpu's waiting time
and make stop_machine path faster.
Signed-off-by: Xiao Guangrong <[email protected]>
---
kernel/stop_machine.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 912823e..feaa947 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -68,7 +68,7 @@ static void stop_cpu(struct work_struct *unused)
{
enum stopmachine_state curstate = STOPMACHINE_NONE;
struct stop_machine_data *smdata = &idle;
- int cpu = smp_processor_id();
+ int cpu = get_cpu();
int err;
if (!active_cpus) {
@@ -103,6 +103,7 @@ static void stop_cpu(struct work_struct *unused)
}
} while (curstate != STOPMACHINE_EXIT);
+ put_cpu();
local_irq_enable();
}
--
1.6.1.2
On Wed, 16 Sep 2009 15:26:54 +0800
Xiao Guangrong <[email protected]> wrote:
> We can disable preempt in stop_cpu(), It can reduce the cpu's waiting time
> and make stop_machine path faster.
>
"it can"? But does it? Are there any measurements or observations to
back this up?
The kstop threads are already created with create_rt_workqueue() (which
has super-secret undocumented semantics which I have cleverly
discovered mean "use SCHED_FIFO").
So we won't be getting preempted here anyway?
Andrew Morton wrote:
> On Wed, 16 Sep 2009 15:26:54 +0800
> Xiao Guangrong <[email protected]> wrote:
>
>> We can disable preempt in stop_cpu(), It can reduce the cpu's waiting time
>> and make stop_machine path faster.
>>
>
> "it can"? But does it? Are there any measurements or observations to
> back this up?
>
> The kstop threads are already created with create_rt_workqueue() (which
> has super-secret undocumented semantics which I have cleverly
> discovered mean "use SCHED_FIFO").
>
Ah, indeed, please ignore this mail, sorry for bother you all.
Thanks,
Xiao
> So we won't be getting preempted here anyway?
>
>
On Wed, 16 Sep 2009 04:56:54 pm Xiao Guangrong wrote:
> We can disable preempt in stop_cpu(), It can reduce the cpu's waiting time
> and make stop_machine path faster.
>
> Signed-off-by: Xiao Guangrong <[email protected]>
Do you have a benchmark? I had a lot of trouble showing any latency
problem with stop_machine...
Thanks!
Rusty.
Here's the rough code I used (modprobe does a stop_machine, so makes a
convenient test), Kathy helped write it:
#define _GNU_SOURCE
#include <sched.h>
#include <sys/time.h>
#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <err.h>
#include <string.h>
#include "time_diff.h"
/* "Copyright 2007 <[email protected]> IBM Corp" */
#define streq(a,b) (strcmp((a),(b)) == 0)
static uint64_t timeval_to_usecs(struct timeval convert)
{ /* this function works out the number of microseconds */
return (convert.tv_sec * (uint64_t)1000000 + convert.tv_usec);
}
static void *grab_file(const char *filename, unsigned long *size)
{
unsigned int max = 16384;
int ret, fd;
void *buffer = malloc(max);
if (!buffer)
return NULL;
if (streq(filename, "-"))
fd = dup(STDIN_FILENO);
else
fd = open(filename, O_RDONLY, 0);
if (fd < 0)
return NULL;
*size = 0;
while ((ret = read(fd, buffer + *size, max - *size)) > 0) {
*size += ret;
if (*size == max)
buffer = realloc(buffer, max *= 2);
}
if (ret < 0) {
free(buffer);
buffer = NULL;
}
close(fd);
return buffer;
}
extern long init_module(void *, unsigned long, const char *);
/* If module is NULL, merely go through the motions. */
static void do_modprobe(int cpu, int pollfd, int secs, const char *module)
{
struct sched_param sparam = { .sched_priority = 99 };
cpu_set_t this_cpu;
fd_set readfds;
int error;
struct timeval timeout = { .tv_sec = 5 };
void *file;
unsigned long len;
if (module) {
file = grab_file(module, &len);
if (!file)
err(1, "Loading file %s", module);
}
CPU_ZERO(&this_cpu);
CPU_SET(cpu, &this_cpu);
if (sched_setaffinity(getpid(), sizeof(cpu_set_t), &this_cpu) != 0)
err(1, "Could not move modprobe to cpu %i", cpu);
if (sched_setscheduler(getpid(), SCHED_FIFO, &sparam) != 0)
err(1, "Could not set FIFO scheduler for modprobe");
/* Wait for go signal. */
FD_ZERO(&readfds);
FD_SET(pollfd, &readfds);
/* We can timeout. */
if (select(pollfd + 1, &readfds, NULL, NULL, &timeout) != 1)
exit(1);
/* Sleep until halfway through. */
usleep(secs * 500000);
if (module) {
error = init_module(file, len, "");
if (error)
err(1, "init_module '%s'", module);
}
printf("Modprobe done on cpu %i\n", cpu);
exit(0);
}
static void measure_latency(int cpu, int secs, int writefd, int pollfd)
{
struct timeval start_time, now, elapsed_time, previous_time, diff;
uint64_t least, max_diff, no_of_diffs;
cpu_set_t this_cpu;
fd_set readfds;
struct timeval timeout = { .tv_sec = 5 };
char buf[1024];
struct sched_param sparam = { .sched_priority = 50 };
least = UINT64_MAX;
max_diff = 0;
no_of_diffs = 0;
CPU_ZERO(&this_cpu);
CPU_SET(cpu, &this_cpu);
if (sched_setaffinity(getpid(), sizeof(cpu_set_t), &this_cpu) != 0)
err(1, "Could not move to cpu %i", cpu);
if (sched_setscheduler(getpid(), SCHED_FIFO, &sparam) != 0)
err(1, "Could not set FIFO scheduler");
/* Note that we're ready. */
write(writefd, "", 1);
/* Wait for go signal. */
FD_ZERO(&readfds);
FD_SET(pollfd, &readfds);
/* We can timeout. */
if (select(pollfd + 1, &readfds, NULL, NULL, &timeout) != 1)
exit(1);
gettimeofday(&start_time, NULL);
previous_time = start_time;
do {
gettimeofday(&now, NULL);
/* call conv_timeval func; apply to now and previous time; calc diff */
time_diff(&previous_time, &now, &diff);
if (timeval_to_usecs(diff) > max_diff)
max_diff = timeval_to_usecs(diff);
if (timeval_to_usecs(diff) < least) /* This should always return 0 */
least = timeval_to_usecs(diff);
/* Work out time to elapse since the starting time */
time_diff(&start_time, &now, &elapsed_time);
/* reset previous_time to now */
previous_time = now;
no_of_diffs++;
} while (elapsed_time.tv_sec < secs);
sprintf(buf, "CPU %u: %llu diffs, min/avg/max = %llu/%llu/%llu\n",
cpu, no_of_diffs,
least,
timeval_to_usecs(elapsed_time) / no_of_diffs,
max_diff);
write(STDOUT_FILENO, buf, strlen(buf));
exit(0);
}
int main(int argc, char *argv[])
{
int i, secs, status, tochildren[2], fromchild[2], arg;
const char *module;
if (argc < 3) {
printf("Usage: %s [--modprobe=<module>] <seconds> <cpunum>...\n", argv[0]);
exit(1);
}
arg = 1;
if (strncmp(argv[arg], "--modprobe=", strlen("--modprobe=")) == 0) {
module = argv[arg] + 11;
arg++;
} else
module = NULL;
if (pipe(tochildren) != 0 || pipe(fromchild) != 0)
err(1, "Creating pipes");
secs = atoi(argv[arg++]);
switch (fork()) {
case -1:
err(1, "fork failed");
case 0:
do_modprobe(atoi(argv[arg]), tochildren[0], secs, module);
}
for (i = arg+1; i < argc; i++) {
char c;
switch (fork()) {
case -1:
err(1, "fork failed");
case 0:
measure_latency(atoi(argv[i]), secs,
fromchild[1], tochildren[0]);
}
if (read(fromchild[0], &c, 1) != 1)
err(1, "Read from child failed");
}
/* Tell them to go. */
write(tochildren[1], "", 1);
/* Wait for the children. */
status = 0;
for (i = arg; i < argc; i++) {
if (status == 0) {
wait(&status);
if (!WIFEXITED(status) || WEXITSTATUS(status) != 0)
status = 1;
} else
wait(NULL);
}
return status;
}