Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

[Jeremy Fitzhardinge]

Tim Chen wrote:
> Hi Herbet,
>
> The patch "Let WARN_ON/WARN_ON_ONCE return the condition"
> http://kernel.org/git/?
> p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=684f978347deb42d180373ac4c427f82ef963171
>
> introduced 40% more 2nd level cache miss to tbench workload
> being run in a loop back mode on a Core 2 machine.  I think the
> introduction of the local variables to WARN_ON and WARN_ON_ONCE
>
> typeof(x) __ret_warn_on = (x);
> typeof(condition) __ret_warn_once = (condition);
>
> results in the extra cache misses.  In our test workload profile, we see
> heavily used functions like do_softirq and local_bh_enable 
> takes a lot longer to execute.  
>
> The modification below helps fix the problem.  I made a slight
> modification to sched.c to get around a gcc bug.
>   

How does the generated code change?  Doesn't evaluating the condition 
multiple times have the potential to cause problems?

   J
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