Re: [rfc][patch] Avoid taking global tasklist_lock for single threadedprocess at getrusage()

[Ravikiran G Thirumalai <kiran@scalex86.org>]

On Tue, Jan 17, 2006 at 10:59:02PM +0300, Oleg Nesterov wrote:
> Ravikiran G Thirumalai wrote:
> > 
> > Sorry for the delay..
> > 
> > On Tue, Jan 10, 2006 at 10:03:35PM +0300, Oleg Nesterov wrote:
> > >
> > > Sorry, I can't undestand. Could you please be more verbose ?
> > 
> > Last thread (RUSAGE_SELF)               Exiting thread
> >
> > [ ... ]
> >
> >         utime = cputime_add(utime, p->signal->utime); /* use cached load above */
> >         stime = cputime_add(stime, p->signal->stime); /* load from memory */
> 
> Thanks for your explanation, now I see what you mean.
> 
> But don't we already discussed this issue? I think that RUSAGE_SELF
> case always not 100% accurate, so it is Ok to ignore this race.

It is not 100% accurate as in we lose time accounting for one clock tick
for the task_struct->utime, stime counters.  But
task_struct->signal->utime,stime collect rusage times of an exiting thread,
so we would be introducing large inaccuracies if we don't use rmb here.
Take the case when an exiting thread has a large utime stime value, and
rusage reports utime before thread exit and stime after thread exit... the
result would look wierd.
So IMHO, while inaccuracies in task_struct->xxx time can be tolerated, it
might not be such a good idea to for task_struct->signal->xxx counters.

Thanks,
Kiran