Messages in this thread | | | Date | Mon, 26 Jul 2010 10:53:24 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] Don't apply for write lock on tasklist_lock if parent doesn't ptrace other processes |
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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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