Messages in this thread | | | Date | Thu, 21 May 2009 09:59:11 +0800 | From | Li Zefan <> | Subject | Re: [PATCH 0/3] tracepoints: delay argument evaluation |
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Jiaying Zhang wrote: > Is it possible to convert blktrace to use event tracer? Then in this case we
Yes, I'm doing this, see: http://marc.info/?l=linux-kernel&m=124228198011297&w=2
> can pass 'md' as the parameter to trace_block_bio_complete and dereference > md->queue during assignment. >
But the problem discussed here exists whether you use plain tracepoints or TRACE_EVENT.
Though we can add a new tracepoint named trace_md_bio_complete, this is not the way to solve it.
> Jiaying > > On Wed, May 20, 2009 at 8:42 AM, Jason Baron <jbaron@redhat.com> wrote: >> On Wed, May 20, 2009 at 09:33:48AM +0200, Ingo Molnar wrote: >>> hm, this is really a compiler bug in essence - the compiler should >>> delay the construction of arguments into unlikely branches - if the >>> arguments are only used there. >>> >>> We'd basically open-code a clear-cut: >>> >>> trace_block_bio_complete(md->queue, bio); >>> >>> into this form: >>> >>> trace(block_bio_complete, md->queue, bio); >>> >>> .. and this latter form could become moot (and a nuisance) if the >>> compiler is fixed. >>> >>> Have you tried very latest GCC, does it still have this optimization >>> problem? >>> >>> Note that the compiler getting this right would help a _lot_ of >>> other inline functions in the kernel as well. Arguments only used >>> within unlikely() branches are quite common. >>> >>> Ingo >> hi, >> >> I e-mailed the gcc list, where they suggested using a macro, as I've >> done. They also suggested filing an enhancement request for this, which >> I've done: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40207 It seems >> like they agree with the suggestion. >> >> It still might make sense to make this requirement explicit (by adding >> the extra macro), as the tracepoint off case should really be as optimized as >> possible. >> >> thanks, >> >> -Jason >>
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