Messages in this thread | | | Date | Sun, 8 Jan 2006 11:58:48 -0800 | From | Ravikiran G Thirumalai <> | Subject | Re: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage() |
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On Sun, Jan 08, 2006 at 02:49:31PM +0300, Oleg Nesterov wrote: > Sorry for delay, > > Ravikiran G Thirumalai wrote: > > > > 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; > > Unneeded initialization
akpm changed the condition statement below with an if test. So it is needed now.
> > > 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(); > > This patch doesn't try to optimize ->sighand.siglock locking, > and I think this is right. But this also means we don't need > rmb() here. It was needed to protect against "another thread > just exited, cpu can read ->c* values before thread_group_empty() > without taking siglock" case, now it is not possible.
Don't we still need rmb for the RUSAGE_SELF case? we do not take the siglock for rusage self and the non c* signal fields are written to at __exit_signal...
What is wrong with optimizing by not taking the siglock in RUSAGE_BOTH and RUSAGE_CHILDREN? I would like to add that in too unless I am missing something and the optimization is incorrect.
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