Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH 10/10] Use down_read_critical() for /sys/<pid>/exe and /sys/<pid>/maps files | Date | Wed, 19 May 2010 16:21:32 +0100 |
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Michel Lespinasse <walken@google.com> 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@redhat.com) * * 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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