Messages in this thread Patch in this message | | | Date | Wed, 22 Mar 2006 14:18:44 -0800 | From | Ravikiran G Thirumalai <> | Subject | Re: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage() |
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On Mon, Mar 20, 2006 at 09:04:56PM +0300, Oleg Nesterov wrote: > Hello Ravikiran,
Hi Oleg, sorry for the late response..
> > Ravikiran G Thirumalai wrote: > > > > 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). > > > > ... > > > > static void k_getrusage(struct task_struct *p, int who, struct rusage *r) > > @@ -1681,14 +1697,22 @@ static void k_getrusage(struct task_stru > > struct task_struct *t; > > unsigned long flags; > > cputime_t utime, stime; > > + int need_lock = 0; > > > > memset((char *) r, 0, sizeof *r); > > - > > - if (unlikely(!p->signal)) > > - return; > > - > > utime = stime = cputime_zero; > > > > + need_lock = (p != current || !thread_group_empty(p)); > > + if (need_lock) { > > + read_lock(&tasklist_lock); > > + if (unlikely(!p->signal)) { > > + read_unlock(&tasklist_lock); > > + return; > > + } > > + } else > > + /* See locking comments above */ > > + smp_rmb(); > > + > > I think now it is possible to improve this patch. > > Could you look at these patches? > > [PATCH] introduce lock_task_sighand() helper > http://marc.theaimsgroup.com/?l=linux-kernel&m=114028190927763 > > [PATCH 0/3] make threads traversal ->siglock safe > http://marc.theaimsgroup.com/?l=linux-kernel&m=114064825626496 > > I think we can forget about tasklist_lock in k_getrusage() completely > and just use lock_task_sighand(). > > What do you think?
Great!! Nice patches to avoid tasklist lock on thread traversal.
However, I was trying to comprehend the tasklist locking changes in 2.6.16-rc6mm2 and hit upon:
__exit_signal cleanup_sighand(tsk); kmem_cache_free(sighand) spin_unlock(sighand->lock)
It looked suspicious to me until I realised sighand cache now had SLAB_DESTROY_BY_RCU. Can we please add comments (at cleanup_sighand or __exit_signal) to make it a bit clearer for people like me :)
How is the following patch to avoid tasklist lock completely at getrusage? (Andrew, I can remake the patch against a reverted avoid-taking-global-tasklist_lock-for-single-threadedprocess-at-getrusage if you prefer it that way)
Thanks, Kiran
Change avoid-taking-global-tasklist_lock-for-single-threadedprocess-at-getrusage patch to not take the global tasklist lock at all. We don't need to take the tasklist lock for thread traversal of a process since Oleg's do-__unhash_process-under-siglock.patch and related work.
Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Index: linux-2.6.16-rc6mm2/kernel/sys.c =================================================================== --- linux-2.6.16-rc6mm2.orig/kernel/sys.c 2006-03-21 17:04:09.000000000 -0800 +++ linux-2.6.16-rc6mm2/kernel/sys.c 2006-03-22 12:46:03.000000000 -0800 @@ -1860,23 +1860,20 @@ out: * 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. + * Locking: + * We need to take the siglock for CHILDEREN, SELF and BOTH + * for the cases current multithreaded, non-current single threaded + * non-current multithreaded. Thread traversal is now safe with + * the siglock held. + * Strictly speaking, we donot need to take the siglock if we are current and + * single threaded, as 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 any lock, the + * signal-> fields could be read out of order while another thread was just + * exiting. So we should 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. But we don't do this yet to keep things simple. * */ @@ -1885,35 +1882,25 @@ static void k_getrusage(struct task_stru 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 (p != current || !thread_group_empty(p)) - need_lock = 1; - - if (need_lock) { - read_lock(&tasklist_lock); - if (unlikely(!p->signal)) { - read_unlock(&tasklist_lock); - return; - } - } else - /* See locking comments above */ - smp_rmb(); + rcu_read_lock(); + if (!lock_task_sighand(p, &flags)) { + rcu_read_unlock(); + return; + } switch (who) { case RUSAGE_BOTH: 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); if (who == RUSAGE_CHILDREN) break; @@ -1940,9 +1927,10 @@ static void k_getrusage(struct task_stru default: BUG(); } - - if (need_lock) - read_unlock(&tasklist_lock); + + unlock_task_sighand(p, &flags); + rcu_read_unlock(); + cputime_to_timeval(utime, &r->ru_utime); cputime_to_timeval(stime, &r->ru_stime); } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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