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SubjectRe: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage()
On Wed, Dec 28, 2005 at 10:33:45AM -0800, Ravikiran G Thirumalai wrote:
> On Wed, Dec 28, 2005 at 03:38:39PM +0300, Oleg Nesterov wrote:
> > Christoph Lameter wrote:
> > >
> > > On Sat, 24 Dec 2005, Oleg Nesterov wrote:
> > >
> > > > I can't understand this. 'p' can do clone(CLONE_THREAD) immediately
> > > > after 'if (!thread_group_empty(p))' check.
> > >
> > > Only if p != current. As discussed later the lockless approach is
> > > intened to only be used if p == current.
> >
> > Unless I missed something this function (getrusage_both) is called
> > from wait_noreap_copyout,
>
> Hi Oleg,
> Yes, this patch needs to be reworked. I am on it. I'd also missed that
> p->signal was protected by the tasklist_lock. Thanks for pointing it out.
> I will put out the modified version soon.

Oleg, Here's the reworked patch.
Comments?

It would have been nice if we could avoid tasklist locking on the
getrusage_both case. Maybe we should explore using RCU here along with
other patches from Paul McKenney....
Thanks,
Kiran

Following patch avoids taking the global tasklist_lock when possible,
if a process is single threaded during getrusage(). Any avoidance of
tasklist_lock is good for NUMA boxes (and possibly for large SMPs).

Signed-off-by: Nippun Goel <nippung@calsoftinc.com>
Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Signed-off-by: Shai Fultheim <shai@scalex86.org>

Index: arch/mips/kernel/irixsig.c
===================================================================
--- arch/mips/kernel/irixsig.c.orig 2005-12-27 14:49:57.000000000 -0800
+++ arch/mips/kernel/irixsig.c 2005-12-27 14:52:47.000000000 -0800
@@ -540,7 +540,7 @@ out:
#define IRIX_P_PGID 2
#define IRIX_P_ALL 7

-extern int getrusage(struct task_struct *, int, struct rusage __user *);
+extern int getrusage_both(struct task_struct *, struct rusage __user *);

#define W_EXITED 1
#define W_TRAPPED 2
@@ -605,7 +605,7 @@ repeat:
remove_parent(p);
add_parent(p, p->parent);
write_unlock_irq(&tasklist_lock);
- retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
+ retval = ru ? getrusage_both(p, ru) : 0;
if (retval)
goto end_waitsys;

@@ -626,7 +626,7 @@ repeat:
current->signal->cutime += p->utime + p->signal->cutime;
current->signal->cstime += p->stime + p->signal->cstime;
if (ru != NULL)
- getrusage(p, RUSAGE_BOTH, ru);
+ getrusage_both(p, ru);
retval = __put_user(SIGCHLD, &info->sig);
retval |= __put_user(1, &info->code); /* CLD_EXITED */
retval |= __put_user(p->pid, &info->stuff.procinfo.pid);
Index: arch/mips/kernel/sysirix.c
===================================================================
--- arch/mips/kernel/sysirix.c.orig 2005-12-27 14:49:57.000000000 -0800
+++ arch/mips/kernel/sysirix.c 2005-12-27 14:52:47.000000000 -0800
@@ -234,7 +234,8 @@ asmlinkage int irix_prctl(unsigned optio
#undef DEBUG_PROCGRPS

extern unsigned long irix_mapelf(int fd, struct elf_phdr __user *user_phdrp, int cnt);
-extern int getrusage(struct task_struct *p, int who, struct rusage __user *ru);
+extern int getrusage_self(struct rusage __user *ru);
+extern int getrusage_children(struct rusage __user *ru);
extern char *prom_getenv(char *name);
extern long prom_setenv(char *name, char *value);

@@ -405,12 +406,12 @@ asmlinkage int irix_syssgi(struct pt_reg
switch((int) regs->regs[base + 5]) {
case 0:
/* rusage self */
- retval = getrusage(current, RUSAGE_SELF, ru);
+ retval = getrusage_self(ru);
goto out;

case -1:
/* rusage children */
- retval = getrusage(current, RUSAGE_CHILDREN, ru);
+ retval = getrusage_children(ru);
goto out;

default:
Index: kernel/exit.c
===================================================================
--- kernel/exit.c.orig 2005-12-27 14:49:57.000000000 -0800
+++ kernel/exit.c 2005-12-27 14:52:47.000000000 -0800
@@ -38,7 +38,7 @@
extern void sem_exit (void);
extern struct task_struct *child_reaper;

-int getrusage(struct task_struct *, int, struct rusage __user *);
+int getrusage_both(struct task_struct *, struct rusage __user *);

static void exit_mm(struct task_struct * tsk);

@@ -994,7 +994,7 @@ static int wait_noreap_copyout(task_t *p
struct siginfo __user *infop,
struct rusage __user *rusagep)
{
- int retval = rusagep ? getrusage(p, RUSAGE_BOTH, rusagep) : 0;
+ int retval = rusagep ? getrusage_both(p, rusagep) : 0;
put_task_struct(p);
if (!retval)
retval = put_user(SIGCHLD, &infop->si_signo);
@@ -1111,7 +1111,7 @@ static int wait_task_zombie(task_t *p, i
*/
read_unlock(&tasklist_lock);

- retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
+ retval = ru ? getrusage_both(p, ru) : 0;
status = (p->signal->flags & SIGNAL_GROUP_EXIT)
? p->signal->group_exit_code : p->exit_code;
if (!retval && stat_addr)
@@ -1260,7 +1260,7 @@ bail_ref:

write_unlock_irq(&tasklist_lock);

- retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
+ retval = ru ? getrusage_both(p, ru) : 0;
if (!retval && stat_addr)
retval = put_user((exit_code << 8) | 0x7f, stat_addr);
if (!retval && infop)
@@ -1321,7 +1321,7 @@ static int wait_task_continued(task_t *p
read_unlock(&tasklist_lock);

if (!infop) {
- retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
+ retval = ru ? getrusage_both(p, ru) : 0;
put_task_struct(p);
if (!retval && stat_addr)
retval = put_user(0xffff, stat_addr);
Index: kernel/sys.c
===================================================================
--- kernel/sys.c.orig 2005-12-27 14:49:57.000000000 -0800
+++ kernel/sys.c 2005-12-28 14:14:05.000000000 -0800
@@ -1657,6 +1657,10 @@ asmlinkage long sys_setrlimit(unsigned i
}

/*
+ * getrusage routines:
+ * getrusage_self() and getrusage_children() always use current.
+ * getrusage_both() need not be for the current task.
+ *
* It would make sense to put struct rusage in the task_struct,
* except that would make the task_struct be *really big*. After
* task_struct gets moved into malloc'ed memory, it would
@@ -1664,94 +1668,190 @@ asmlinkage long sys_setrlimit(unsigned i
* a lot simpler! (Which we're not doing right now because we're not
* measuring them yet).
*
- * This expects to be called with tasklist_lock read-locked or better,
- * and the siglock not locked. It may momentarily take the siglock.
- *
- * When sampling multiple threads for RUSAGE_SELF, under SMP we might have
- * races with threads incrementing their own counters. But since word
- * reads are atomic, we either get new values or old values and we don't
- * care which for the sums. We always take the siglock to protect reading
+ * In multi threaded scenario, we always take the siglock to protect reading
* the c* fields from p->signal from races with exit.c updating those
* fields when reaping, so a sample either gets all the additions of a
* given child after it's reaped, or none so this sample is before reaping.
+ *
+ * getrusage_children -- locking:
+ * In multithreaded scenario, we need to take tasklist_lock for read
+ * to avoid races against another thread doing exec and changing/freeing
+ * signal_struct of the current task. We need to take the sighand->siglock
+ * too as another thread could be reaping its children. However, both locks
+ * can be avoided if we are a single threaded process, since we are current;
+ * No one else can take our signal_struct away, and no one else can reap
+ * children to update our signal->c* counters.
+ *
*/
-
-static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
+int getrusage_children(struct rusage __user *ru)
{
- struct task_struct *t;
unsigned long flags;
+ int lockflag = 0;
cputime_t utime, stime;
+ struct task_struct *p = current;
+ struct rusage r;
+ memset((char *) &r, 0, sizeof (r));

- memset((char *) r, 0, sizeof *r);
+ if (!thread_group_empty(p)) {
+ read_lock(&tasklist_lock);
+ if (unlikely(!p->signal)) {
+ read_unlock(&tasklist_lock);
+ goto ret;
+ }
+ spin_lock_irqsave(&p->sighand->siglock, flags);
+ lockflag = 1;
+ }
+
+ utime = p->signal->cutime;
+ stime = p->signal->cstime;
+ r.ru_nvcsw = p->signal->cnvcsw;
+ r.ru_nivcsw = p->signal->cnivcsw;
+ r.ru_minflt = p->signal->cmin_flt;
+ r.ru_majflt = p->signal->cmaj_flt;
+ if (lockflag) {
+ spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ read_unlock(&tasklist_lock);
+ }
+ cputime_to_timeval(utime, &r.ru_utime);
+ cputime_to_timeval(stime, &r.ru_stime);

- if (unlikely(!p->signal))
- return;
+ret:
+ return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0;
+}

- switch (who) {
- case RUSAGE_CHILDREN:
- spin_lock_irqsave(&p->sighand->siglock, flags);
- utime = p->signal->cutime;
- stime = p->signal->cstime;
- r->ru_nvcsw = p->signal->cnvcsw;
- r->ru_nivcsw = p->signal->cnivcsw;
- r->ru_minflt = p->signal->cmin_flt;
- r->ru_majflt = p->signal->cmaj_flt;
- spin_unlock_irqrestore(&p->sighand->siglock, flags);
- cputime_to_timeval(utime, &r->ru_utime);
- cputime_to_timeval(stime, &r->ru_stime);
- break;
- case RUSAGE_SELF:
- spin_lock_irqsave(&p->sighand->siglock, flags);
- utime = stime = cputime_zero;
- goto sum_group;
- case RUSAGE_BOTH:
- spin_lock_irqsave(&p->sighand->siglock, flags);
- utime = p->signal->cutime;
- stime = p->signal->cstime;
- r->ru_nvcsw = p->signal->cnvcsw;
- r->ru_nivcsw = p->signal->cnivcsw;
- r->ru_minflt = p->signal->cmin_flt;
- r->ru_majflt = p->signal->cmaj_flt;
- sum_group:
- utime = cputime_add(utime, p->signal->utime);
- stime = cputime_add(stime, p->signal->stime);
- r->ru_nvcsw += p->signal->nvcsw;
- r->ru_nivcsw += p->signal->nivcsw;
- r->ru_minflt += p->signal->min_flt;
- r->ru_majflt += p->signal->maj_flt;
- t = p;
- do {
- utime = cputime_add(utime, t->utime);
- stime = cputime_add(stime, t->stime);
- r->ru_nvcsw += t->nvcsw;
- r->ru_nivcsw += t->nivcsw;
- r->ru_minflt += t->min_flt;
- r->ru_majflt += t->maj_flt;
- t = next_thread(t);
- } while (t != p);
- spin_unlock_irqrestore(&p->sighand->siglock, flags);
- cputime_to_timeval(utime, &r->ru_utime);
- cputime_to_timeval(stime, &r->ru_stime);
- break;
- default:
- BUG();
+/*
+ * getrusage_self:
+ * In multithreaded scenario, we need to take the tasklist_lock for read
+ * since we traverse the task TGID hash list. However, we do not need to
+ * take the siglock even for the multithreaded case, as the signal fields,
+ * which the siglock protects are only updated at __exit_signal with
+ * tasklist_lock taken for write, which cannot happen as we have the read_lock
+ * on tasklist_lock.
+ * If we are single threaded, since we are current, we don't need to take
+ * the tasklist_lock or the siglock as no one else can race with the
+ * signal fields.
+ *
+ * When sampling multiple threads for getrusage_self(), under SMP we might have
+ * races with threads incrementing their own counters. But since word
+ * reads are atomic, we either get new values or old values and we don't
+ * care which for the sums.
+ *
+ */
+int getrusage_self(struct rusage __user *ru)
+{
+ int lockflag = 0;
+ cputime_t utime, stime;
+ struct task_struct *t, *p = current;
+ struct rusage r;
+ memset((char *) &r, 0, sizeof (r));
+
+ if (!thread_group_empty(p)) {
+ read_lock(&tasklist_lock);
+ lockflag = 1;
}
+
+ if (unlikely(!p->signal)) {
+ if (lockflag)
+ read_unlock(&tasklist_lock);
+ goto ret;
+ }
+
+ utime = p->signal->utime;
+ stime = p->signal->stime;
+ r.ru_nvcsw = p->signal->nvcsw;
+ r.ru_nivcsw = p->signal->nivcsw;
+ r.ru_minflt = p->signal->min_flt;
+ r.ru_majflt = p->signal->maj_flt;
+ t = p;
+ do {
+ utime = cputime_add(utime, t->utime);
+ stime = cputime_add(stime, t->stime);
+ r.ru_nvcsw += t->nvcsw;
+ r.ru_nivcsw += t->nivcsw;
+ r.ru_minflt += t->min_flt;
+ r.ru_majflt += t->maj_flt;
+ t = next_thread(t);
+ } while (t != p);
+
+ if (lockflag)
+ read_unlock(&tasklist_lock);
+ cputime_to_timeval(utime, &r.ru_utime);
+ cputime_to_timeval(stime, &r.ru_stime);
+
+ret:
+ return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0;
}

-int getrusage(struct task_struct *p, int who, struct rusage __user *ru)
+/*
+ * getrusage_both:
+ * getrusage_both can be invoked for non current processes -- usually from
+ * wait_* routines, on the children. As even a single threaded process
+ * we are waiting on can spawn another thread and exec, we cannot
+ * avoid tasklist_lock for both single threaded and multithreaded cases.
+ * We also need to take the siglock to protect the c* fields of the
+ * signal_struct for both single threaded and multi threaded case.
+ *
+ */
+int getrusage_both(struct task_struct *p, struct rusage __user *ru)
{
+ unsigned long flags;
+ cputime_t utime, stime;
struct rusage r;
+ struct task_struct *t;
+ memset((char *) &r, 0, sizeof (r));
+
read_lock(&tasklist_lock);
- k_getrusage(p, who, &r);
+ if (unlikely(!p->signal)) {
+ read_unlock(&tasklist_lock);
+ goto ret;
+ }
+
+ spin_lock_irqsave(&p->sighand->siglock, flags);
+ utime = p->signal->cutime;
+ stime = p->signal->cstime;
+ r.ru_nvcsw = p->signal->cnvcsw;
+ r.ru_nivcsw = p->signal->cnivcsw;
+ r.ru_minflt = p->signal->cmin_flt;
+ r.ru_majflt = p->signal->cmaj_flt;
+ spin_unlock_irqrestore(&p->sighand->siglock, flags);
+
+ utime = cputime_add(utime, p->signal->utime);
+ stime = cputime_add(stime, p->signal->stime);
+ r.ru_nvcsw += p->signal->nvcsw;
+ r.ru_nivcsw += p->signal->nivcsw;
+ r.ru_minflt += p->signal->min_flt;
+ r.ru_majflt += p->signal->maj_flt;
+
+ t = p;
+ do {
+ utime = cputime_add(utime, t->utime);
+ stime = cputime_add(stime, t->stime);
+ r.ru_nvcsw += t->nvcsw;
+ r.ru_nivcsw += t->nivcsw;
+ r.ru_minflt += t->min_flt;
+ r.ru_majflt += t->maj_flt;
+ t = next_thread(t);
+ } while (t != p);
+
read_unlock(&tasklist_lock);
+ cputime_to_timeval(utime, &r.ru_utime);
+ cputime_to_timeval(stime, &r.ru_stime);
+
+ret:
return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0;
}

asmlinkage long sys_getrusage(int who, struct rusage __user *ru)
{
- if (who != RUSAGE_SELF && who != RUSAGE_CHILDREN)
- return -EINVAL;
- return getrusage(current, who, ru);
+ switch (who) {
+ case RUSAGE_SELF:
+ return getrusage_self(ru);
+ case RUSAGE_CHILDREN:
+ return getrusage_children(ru);
+ default:
+ break;
+ }
+ return -EINVAL;
}

asmlinkage long sys_umask(int mask)
-
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