Messages in this thread | | | Date | Sun, 16 Apr 2006 17:06:14 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 00/05] robust per_cpu allocation for modules |
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Steven Rostedt wrote: > On Sun, 16 Apr 2006, Nick Piggin wrote:
>>Why is your module using so much per-cpu memory, anyway? > > > Wasn't my module anyway. The problem appeared in the -rt patch set, when > tracing was turned on. Some module was affected, and grew it's per_cpu > size by quite a bit. In fact we had to increase PERCPU_ENOUGH_ROOM by up > to something like 300K.
Well that's easy then, just configure PERCPU_ENOUGH_ROOM to be larger when tracing is on in the -rt patchset? Or use alloc_percpu for the tracing data?
>>I don't think it would have been hard for the original author to make >>it robust... just not both fast and robust. PERCPU_ENOUGH_ROOM seems >>like an ugly hack at first glance, but I'm fairly sure it was a result >>of design choices. > > Yeah, and I discovered the reasons for those choices as I worked on this. > I've put a little more thought into this and still think there's a > solution to not slow things down. > > Since the per_cpu_offset section is still smaller than the > PERCPU_ENOUGH_ROOM and robust, I could still copy it into a per cpu memory > field, and even add the __per_cpu_offset to it. This would still save > quite a bit of space.
Well I don't think making it per-cpu would help much (presumably it is not going to be written to very frequently) -- I guess it would be a small advantage on NUMA. The main problem is the extra load in the fastpath.
You can't start the next load until the results of the first come back.
> So now I'm asking for advice on some ideas that can be a work around to > keep the robustness and speed. > > Is there a way (for archs that support it) to allocate memory in a per cpu > manner. So each CPU would have its own variable table in the memory that > is best of it. Then have a field (like the pda in x86_64) to point to > this section, and use the linker offsets to index and find the per_cpu > variables. > > So this solution still has one more redirection than the current solution > (per_cpu_offset__##var -> __per_cpu_offset -> actual_var where as the > current solution is __per_cpu_offset -> actual_var), but all the loads > would be done from memory that would only be specified for a particular > CPU. > > The generic case would still be the same as the patches I already sent, > but the archs that can support it, can have something like the above. > > Would something like that be acceptible?
I still don't understand what the justification is for slowing down this critical bit of infrastructure for something that is only a problem in the -rt patchset, and even then only a problem when tracing is enabled.
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