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

[Oleg Nesterov]

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.

What if that thread has not exited yet? We take tasklist lock, but
this can't help, because this thread possibly updates it's ->xtime
right now on another cpu, and we have exactly same problem.

> > However, do you have any numbers or thoughts why this optimization
> > can make any _visible_ effect?
> 
> We know we don't need locks there, so I do not understand why we
> should keep them.  Locks are always serializing and expensive operations.  I
> believe on some arches disabling on-chip interrupts is also an expensive
> operation...some arches might use hypervisor calls to do that which I guess
> will have its own overhead...so why have it when we know we don't need it?

I think it is better not to complicate the code unless we can see
some difference in practice.

That said, I don't have a strong feeling that I am right (on both
issues), so please feel free to ignore me.

Oleg.
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