Re: [PATCH] Don't apply for write lock on tasklist_lock if parent doesn't ptrace other processes

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Mon, 2010-07-26 at 10:53 +0200, Oleg Nesterov wrote:
> On 07/26, Zhang, Yanmin wrote:
> >
> > On Fri, 2010-07-23 at 19:34 +0200, Oleg Nesterov wrote:
> > > On 07/23, Zhang, Yanmin wrote:
> > > >
> > > > After applying my patch (although it's incorrect as there is a race with TRACEME),
> > > > perf shows write_lock_irq in forget_original_parent consumes less than 40% cpu time on
> > > > 8-socket machine.
> > >
> > > Any chance you can test the patch I sent? It should have the same effect,
> > > otherwise there is something interesting.
> > 1) with my patch, we got about 13% improvement;
> > 2) With your patch, we got about 11% improvement;
> >
> > Performance is very sensitive to spinlock contention on large machines.
> 
> Zhang, thank you very much.
> 
> But. In this case I do not trust these results or I missed something.
> I mean, they do not look 100% accurate.
> 
> With your patch:
> 
> 	forget_original_parent:
> 
> 		exit_ptrace:
> 			if (list_empty(ptraced))
> 				return;
> 
> 
> 		write_lock_irq(tasklist);
> 
> 		... do a lot more work ...
> 
> With my patch:
> 
> 	forget_original_parent:
> 
> 		write_lock_irq(tasklist);
> 	
> 		exit_ptrace:
> 			if (list_empty(ptraced))
> 				return;
> 
> 		... do a lot more work ...
> 
> The only difference is that we are doing the function call + list_empty()
> under tasklist, just a few instructions compared to "do a lot more work"
> in forget_original_parent().
If considering lock acquire/release on a big machine, plus cache-misses like
what Andi said, the result is reasonable. We did lots of testing on 8-socket
machine. Performance result is very sensitive to lock contentions and cache-misses.


> 
> How this can make the 2% difference ?
I reran the testing for a couple of times to make sure the result is stable.

>  This looks like a noise to me,
> or do you think I missed something?
No, you didn't miss anything. Any patch shouldn't introduce bugs, so your patch is
right and good.

> 
> > > Heh. We must optimize it. But it is not clear when ;)
> > Thanks. It's better to remove the big lock.
> 
> Yes. The only problem this is very much nontrival with the current code.
I agree that would be a big project.