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

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Message-ID: <20100726085324.GA32223@redhat.com>
Date:	Mon, 26 Jul 2010 10:53:24 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, andi.kleen@...el.com,
	stable@...nel.org
Subject: Re: [PATCH] Don't apply for write lock on tasklist_lock if parent
	doesn't ptrace other processes

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().

How this can make the 2% difference ? This looks like a noise to me,
or do you think I missed something?

> > 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.

Oleg.

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