Commit 2dd0ebcd authored by Ravikiran G Thirumalai's avatar Ravikiran G Thirumalai Committed by Linus Torvalds
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[PATCH] Avoid taking global tasklist_lock for single threadedprocess at getrusage()



Avoid 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).  Thanks to Oleg Nesterov for
review and suggestions.
Signed-off-by: default avatarNippun Goel <nippung@calsoftinc.com>
Signed-off-by: default avatarRavikiran Thirumalai <kiran@scalex86.org>
Signed-off-by: default avatarShai Fultheim <shai@scalex86.org>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 0c9e63fd
......@@ -1677,9 +1677,6 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim)
* 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
......@@ -1687,6 +1684,25 @@ asmlinkage long sys_setrlimit(unsigned int resource, struct rlimit __user *rlim)
* 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.
*
* tasklist_lock locking optimisation:
* If we are current and single threaded, we do not need to take the tasklist
* lock or the siglock. No one else can take our signal_struct away,
* no one else can reap the children to update signal->c* counters, and
* no one else can race with the signal-> fields.
* If we do not take the tasklist_lock, the signal-> fields could be read
* out of order while another thread was just exiting. So we place a
* read memory barrier when we avoid the lock. On the writer side,
* write memory barrier is implied in __exit_signal as __exit_signal releases
* the siglock spinlock after updating the signal-> fields.
*
* We don't really need the siglock when we access the non c* fields
* of the signal_struct (for RUSAGE_SELF) even in multithreaded
* case, since we take the tasklist lock for read and the non c* signal->
* fields are updated only in __exit_signal, which is called with
* tasklist_lock taken for write, hence these two threads cannot execute
* concurrently.
*
*/
static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
......@@ -1694,13 +1710,23 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
struct task_struct *t;
unsigned long flags;
cputime_t utime, stime;
int need_lock = 0;
memset((char *) r, 0, sizeof *r);
utime = stime = cputime_zero;
if (unlikely(!p->signal))
return;
if (p != current || !thread_group_empty(p))
need_lock = 1;
utime = stime = cputime_zero;
if (need_lock) {
read_lock(&tasklist_lock);
if (unlikely(!p->signal)) {
read_unlock(&tasklist_lock);
return;
}
} else
/* See locking comments above */
smp_rmb();
switch (who) {
case RUSAGE_BOTH:
......@@ -1740,6 +1766,8 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
BUG();
}
if (need_lock)
read_unlock(&tasklist_lock);
cputime_to_timeval(utime, &r->ru_utime);
cputime_to_timeval(stime, &r->ru_stime);
}
......@@ -1747,9 +1775,7 @@ static void k_getrusage(struct task_struct *p, int who, struct rusage *r)
int getrusage(struct task_struct *p, int who, struct rusage __user *ru)
{
struct rusage r;
read_lock(&tasklist_lock);
k_getrusage(p, who, &r);
read_unlock(&tasklist_lock);
return copy_to_user(ru, &r, sizeof(r)) ? -EFAULT : 0;
}
......
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