Re: [PATCH 10/10] Use down_read_critical() for /sys/<pid>/exe and /sys/<pid>/maps files

From: David Howells
Date: Wed May 19 2010 - 11:22:37 EST

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Michel Lespinasse <walken@xxxxxxxxxx> wrote:

> This helps in the following situation:
> - Thread A takes a page fault while reading or writing memory.
> do_page_fault() acquires the mmap_sem for read and blocks on disk
> (either reading the page from file, or hitting swap) for a long time.
> - Thread B does an mmap call and blocks trying to acquire the mmap_sem
> for write
> - Thread C is a monitoring process trying to read every /proc/pid/maps
> in the system. This requires acquiring the mmap_sem for read. Thread C
> blocks behind B, waiting for A to release the rwsem. If thread C
> could be allowed to run in parallel with A, it would probably get done
> long before thread A's disk access completes, thus not actually slowing
> down thread B.
>
> Test results with down_read_critical_test (10 seconds):

That seems to work. I have some rwsem benchmarks for you using my
synchronisation primitive test (see attached). I run it like so:

insmod /tmp/synchro-test.ko rd=10 wr=2 dg=1 do_sched=1

with 10 readers, 2 writers and a downgrader on the same rwsem, and permitting
scheduling between attempts to grab and release the semaphore. The test runs
for 5 seconds on a box that isn't doing anything else, and has had almost
everything that might normally be running (including syslog) disabled or
removed.

Without your patches:

THREADS S INTVL LOCKS TAKEN AND RELEASED
------------------- /LOAD -------------------------------------------------
MTX SEM RD WR DG MTX TAKEN SEM TAKEN RD TAKEN WR TAKEN DG TAKEN
=== === === === === = ===== ========= ========= ========= ========= =========
0 0 10 2 1 s 2/2 0 0 2087911 590464 194394
0 0 10 2 1 s 2/2 0 0 2030497 591511 196851
0 0 10 2 1 s 2/2 0 0 2078523 605317 199645

With your patches

0 0 10 2 1 s 2/2 0 0 2054720 605691 198472
0 0 10 2 1 s 2/2 0 0 2051376 597976 196962
0 0 10 2 1 s 2/2 0 0 2011909 595706 197482

So there doesn't seem to be much in the way of degradation. In fact, there
seems to have been more degradation somewhere in the general kernel since I
played with version 1 of your patches a week ago. At that time, the vanilla
kernel gave this without your patches applied:

0 0 10 2 1 s 2/2 0 0 2217200 636227 210255
0 0 10 2 1 s 2/2 0 0 2217903 629175 212926
0 0 10 2 1 s 2/2 0 0 2224136 647912 212506

Feel free to have a play.

BTW, the code also works on FRV in NOMMU mode (which uses the spinlock-based
rwsem version).

However... I'm not certain that giving a process that's accessing
/proc/pid/maps priority over a second process that may be trying to do mmap or
page fault or something internally is a good idea.

It would be useful if you made clearer what you're actually measuring from
/proc/pid/maps...

David
---
/* synchro-test.c: run some threads to test the synchronisation primitives
*
* Copyright (C) 2005, 2006 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@xxxxxxxxxx)
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version
* 2 of the License, or (at your option) any later version.
*
* The module should be run as something like:
*
* insmod synchro-test.ko rd=2 wr=2
* insmod synchro-test.ko mx=1
* insmod synchro-test.ko sm=2 ism=1
* insmod synchro-test.ko sm=2 ism=2
*
* See Documentation/synchro-test.txt for more information.
*/

#include <linux/module.h>
#include <linux/poll.h>
#include <linux/moduleparam.h>
#include <linux/sched.h>
#include <linux/stat.h>
#include <linux/init.h>
#include <asm/atomic.h>
#include <linux/personality.h>
#include <linux/smp_lock.h>
#include <linux/delay.h>
#include <linux/timer.h>
#include <linux/completion.h>
#include <linux/mutex.h>

#define MAX_THREADS 64

/*
* Turn on self-validation if we do a one-shot boot-time test:
*/
#ifndef MODULE
# define VALIDATE_OPERATORS
#endif

static int nummx;
static int numsm, seminit = 4;
static int numrd, numwr, numdg;
static int elapse = 5, load = 2, do_sched, interval = 2;
static int verbose = 0;

MODULE_AUTHOR("David Howells");
MODULE_DESCRIPTION("Synchronisation primitive test demo");
MODULE_LICENSE("GPL");

module_param_named(v, verbose, int, 0);
MODULE_PARM_DESC(verbose, "Verbosity");

module_param_named(mx, nummx, int, 0);
MODULE_PARM_DESC(nummx, "Number of mutex threads");

module_param_named(sm, numsm, int, 0);
MODULE_PARM_DESC(numsm, "Number of semaphore threads");

module_param_named(ism, seminit, int, 0);
MODULE_PARM_DESC(seminit, "Initial semaphore value");

module_param_named(rd, numrd, int, 0);
MODULE_PARM_DESC(numrd, "Number of reader threads");

module_param_named(wr, numwr, int, 0);
MODULE_PARM_DESC(numwr, "Number of writer threads");

module_param_named(dg, numdg, int, 0);
MODULE_PARM_DESC(numdg, "Number of downgrader threads");

module_param(elapse, int, 0);
MODULE_PARM_DESC(elapse, "Number of seconds to run for");

module_param(load, int, 0);
MODULE_PARM_DESC(load, "Length of load in uS");

module_param(interval, int, 0);
MODULE_PARM_DESC(interval, "Length of interval in uS before re-getting lock");

module_param(do_sched, int, 0);
MODULE_PARM_DESC(do_sched, "True if each thread should schedule regularly");

/* the semaphores under test */
static struct mutex ____cacheline_aligned mutex;
static struct semaphore ____cacheline_aligned sem;
static struct rw_semaphore ____cacheline_aligned rwsem;

static atomic_t ____cacheline_aligned do_stuff = ATOMIC_INIT(0);

#ifdef VALIDATE_OPERATORS
static atomic_t ____cacheline_aligned mutexes = ATOMIC_INIT(0);
static atomic_t ____cacheline_aligned semaphores = ATOMIC_INIT(0);
static atomic_t ____cacheline_aligned readers = ATOMIC_INIT(0);
static atomic_t ____cacheline_aligned writers = ATOMIC_INIT(0);
#endif

static unsigned int ____cacheline_aligned mutexes_taken [MAX_THREADS];
static unsigned int ____cacheline_aligned semaphores_taken [MAX_THREADS];
static unsigned int ____cacheline_aligned reads_taken [MAX_THREADS];
static unsigned int ____cacheline_aligned writes_taken [MAX_THREADS];
static unsigned int ____cacheline_aligned downgrades_taken [MAX_THREADS];

static struct completion ____cacheline_aligned mx_comp[MAX_THREADS];
static struct completion ____cacheline_aligned sm_comp[MAX_THREADS];
static struct completion ____cacheline_aligned rd_comp[MAX_THREADS];
static struct completion ____cacheline_aligned wr_comp[MAX_THREADS];
static struct completion ____cacheline_aligned dg_comp[MAX_THREADS];

static struct timer_list ____cacheline_aligned timer;

#define ACCOUNT(var, N) var##_taken[N]++;

#ifdef VALIDATE_OPERATORS
#define TRACK(var, dir) atomic_##dir(&(var))

#define CHECK(var, cond, val) \
do { \
int x = atomic_read(&(var)); \
if (unlikely(!(x cond (val)))) \
printk("check [%s %s %d, == %d] failed in %s\n", \
#var, #cond, (val), x, __func__); \
} while (0)

#else
#define TRACK(var, dir) do {} while(0)
#define CHECK(var, cond, val) do {} while(0)
#endif

static inline void do_mutex_lock(unsigned int N)
{
mutex_lock(&mutex);

ACCOUNT(mutexes, N);
TRACK(mutexes, inc);
CHECK(mutexes, ==, 1);
}

static inline void do_mutex_unlock(unsigned int N)
{
CHECK(mutexes, ==, 1);
TRACK(mutexes, dec);

mutex_unlock(&mutex);
}

static inline void do_down(unsigned int N)
{
CHECK(mutexes, <, seminit);

down(&sem);

ACCOUNT(semaphores, N);
TRACK(semaphores, inc);
}

static inline void do_up(unsigned int N)
{
CHECK(semaphores, >, 0);
TRACK(semaphores, dec);

up(&sem);
}

static inline void do_down_read(unsigned int N)
{
down_read(&rwsem);

ACCOUNT(reads, N);
TRACK(readers, inc);
CHECK(readers, >, 0);
CHECK(writers, ==, 0);
}

static inline void do_up_read(unsigned int N)
{
CHECK(readers, >, 0);
CHECK(writers, ==, 0);
TRACK(readers, dec);

up_read(&rwsem);
}

static inline void do_down_write(unsigned int N)
{
down_write(&rwsem);

ACCOUNT(writes, N);
TRACK(writers, inc);
CHECK(writers, ==, 1);
CHECK(readers, ==, 0);
}

static inline void do_up_write(unsigned int N)
{
CHECK(writers, ==, 1);
CHECK(readers, ==, 0);
TRACK(writers, dec);

up_write(&rwsem);
}

static inline void do_downgrade_write(unsigned int N)
{
CHECK(writers, ==, 1);
CHECK(readers, ==, 0);
TRACK(writers, dec);
TRACK(readers, inc);

downgrade_write(&rwsem);

ACCOUNT(downgrades, N);
}

static inline void sched(void)
{
if (do_sched)
schedule();
}

static int mutexer(void *arg)
{
unsigned int N = (unsigned long) arg;

daemonize("Mutex%u", N);
set_user_nice(current, 19);

while (atomic_read(&do_stuff)) {
do_mutex_lock(N);
if (load)
udelay(load);
do_mutex_unlock(N);
sched();
if (interval)
udelay(interval);
}

if (verbose >= 2)
printk("%s: done\n", current->comm);
complete_and_exit(&mx_comp[N], 0);
}

static int semaphorer(void *arg)
{
unsigned int N = (unsigned long) arg;

daemonize("Sem%u", N);
set_user_nice(current, 19);

while (atomic_read(&do_stuff)) {
do_down(N);
if (load)
udelay(load);
do_up(N);
sched();
if (interval)
udelay(interval);
}

if (verbose >= 2)
printk("%s: done\n", current->comm);
complete_and_exit(&sm_comp[N], 0);
}

static int reader(void *arg)
{
unsigned int N = (unsigned long) arg;

daemonize("Read%u", N);
set_user_nice(current, 19);

while (atomic_read(&do_stuff)) {
do_down_read(N);
#ifdef LOAD_TEST
if (load)
udelay(load);
#endif
do_up_read(N);
sched();
if (interval)
udelay(interval);
}

if (verbose >= 2)
printk("%s: done\n", current->comm);
complete_and_exit(&rd_comp[N], 0);
}

static int writer(void *arg)
{
unsigned int N = (unsigned long) arg;

daemonize("Write%u", N);
set_user_nice(current, 19);

while (atomic_read(&do_stuff)) {
do_down_write(N);
#ifdef LOAD_TEST
if (load)
udelay(load);
#endif
do_up_write(N);
sched();
if (interval)
udelay(interval);
}

if (verbose >= 2)
printk("%s: done\n", current->comm);
complete_and_exit(&wr_comp[N], 0);
}

static int downgrader(void *arg)
{
unsigned int N = (unsigned long) arg;

daemonize("Down%u", N);
set_user_nice(current, 19);

while (atomic_read(&do_stuff)) {
do_down_write(N);
#ifdef LOAD_TEST
if (load)
udelay(load);
#endif
do_downgrade_write(N);
#ifdef LOAD_TEST
if (load)
udelay(load);
#endif
do_up_read(N);
sched();
if (interval)
udelay(interval);
}

if (verbose >= 2)
printk("%s: done\n", current->comm);
complete_and_exit(&dg_comp[N], 0);
}

static void stop_test(unsigned long dummy)
{
atomic_set(&do_stuff, 0);
}

static unsigned int total(const char *what, unsigned int counts[], int num)
{
unsigned int tot = 0, max = 0, min = UINT_MAX, zeros = 0, cnt;
int loop;

for (loop = 0; loop < num; loop++) {
cnt = counts[loop];

if (cnt == 0) {
zeros++;
min = 0;
continue;
}

tot += cnt;
if (tot > max)
max = tot;
if (tot < min)
min = tot;
}

if (verbose && tot > 0) {
printk("%s:", what);

for (loop = 0; loop < num; loop++) {
cnt = counts[loop];

if (cnt == 0)
printk(" zzz");
else
printk(" %d%%", cnt * 100 / tot);
}

printk("\n");
}

return tot;
}

/*****************************************************************************/
/*
*
*/
static int __init do_tests(void)
{
unsigned long loop;
unsigned int mutex_total, sem_total, rd_total, wr_total, dg_total;

if (nummx < 0 || nummx > MAX_THREADS ||
numsm < 0 || numsm > MAX_THREADS ||
numrd < 0 || numrd > MAX_THREADS ||
numwr < 0 || numwr > MAX_THREADS ||
numdg < 0 || numdg > MAX_THREADS ||
seminit < 1 ||
elapse < 1 ||
load < 0 || load > 999 ||
interval < 0 || interval > 999
) {
printk("Parameter out of range\n");
return -ERANGE;
}

if ((nummx | numsm | numrd | numwr | numdg) == 0) {
int num = num_online_cpus();

if (num > MAX_THREADS)
num = MAX_THREADS;
nummx = numsm = numrd = numwr = numdg = num;

load = 1;
interval = 1;
do_sched = 1;
printk("No parameters - using defaults.\n");
}

if (verbose)
printk("\nStarting synchronisation primitive tests...\n");

mutex_init(&mutex);
sema_init(&sem, seminit);
init_rwsem(&rwsem);
atomic_set(&do_stuff, 1);

/* kick off all the children */
for (loop = 0; loop < MAX_THREADS; loop++) {
if (loop < nummx) {
init_completion(&mx_comp[loop]);
kernel_thread(mutexer, (void *) loop, 0);
}

if (loop < numsm) {
init_completion(&sm_comp[loop]);
kernel_thread(semaphorer, (void *) loop, 0);
}

if (loop < numrd) {
init_completion(&rd_comp[loop]);
kernel_thread(reader, (void *) loop, 0);
}

if (loop < numwr) {
init_completion(&wr_comp[loop]);
kernel_thread(writer, (void *) loop, 0);
}

if (loop < numdg) {
init_completion(&dg_comp[loop]);
kernel_thread(downgrader, (void *) loop, 0);
}
}

/* set a stop timer */
init_timer(&timer);
timer.function = stop_test;
timer.expires = jiffies + elapse * HZ;
add_timer(&timer);

/* now wait until it's all done */
for (loop = 0; loop < nummx; loop++)
wait_for_completion(&mx_comp[loop]);

for (loop = 0; loop < numsm; loop++)
wait_for_completion(&sm_comp[loop]);

for (loop = 0; loop < numrd; loop++)
wait_for_completion(&rd_comp[loop]);

for (loop = 0; loop < numwr; loop++)
wait_for_completion(&wr_comp[loop]);

for (loop = 0; loop < numdg; loop++)
wait_for_completion(&dg_comp[loop]);

atomic_set(&do_stuff, 0);
del_timer(&timer);

if (mutex_is_locked(&mutex))
printk(KERN_ERR "Mutex is still locked!\n");

/* count up */
mutex_total = total("MTX", mutexes_taken, nummx);
sem_total = total("SEM", semaphores_taken, numsm);
rd_total = total("RD ", reads_taken, numrd);
wr_total = total("WR ", writes_taken, numwr);
dg_total = total("DG ", downgrades_taken, numdg);

/* print the results */
if (verbose) {
printk("mutexes taken: %u\n", mutex_total);
printk("semaphores taken: %u\n", sem_total);
printk("reads taken: %u\n", rd_total);
printk("writes taken: %u\n", wr_total);
printk("downgrades taken: %u\n", dg_total);
}
else {
char buf[30];

sprintf(buf, "%d/%d", interval, load);

printk("%3d %3d %3d %3d %3d %c %5s %9u %9u %9u %9u %9u\n",
nummx, numsm, numrd, numwr, numdg,
do_sched ? 's' : '-',
buf,
mutex_total,
sem_total,
rd_total,
wr_total,
dg_total);
}

/* tell insmod to discard the module */
if (verbose)
printk("Tests complete\n");
return -ENOANO;

} /* end do_tests() */

module_init(do_tests);

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Next message: Christoph Lameter: "Re: [RESEND -v2][PATCH 3/3] mem-hotplug: fix potential race whilebuilding zonelist for new populated zone"
Previous message: Stephen Rothwell: "Re: [GIT PULL] x86/atomic changes for v2.6.35"
In reply to: David Howells: "Re: [PATCH 09/10] x86 rwsem: down_read_critical implementation"
Next in thread: Michel Lespinasse: "Re: [PATCH 10/10] Use down_read_critical() for /sys/<pid>/exe and /sys/<pid>/maps files"
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