Messages in this thread | | | Date | Wed, 18 Mar 2009 08:55:58 +0530 | From | Bharata B Rao <> | Subject | Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used. |
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On Tue, Mar 17, 2009 at 03:04:46PM +0100, Peter Zijlstra wrote: > On Tue, 2009-03-17 at 19:29 +0530, Balbir Singh wrote: > > * Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-03-17 14:26:01]: > > > > > On Tue, 2009-03-17 at 18:42 +0530, Balbir Singh wrote: > > > > > > > I'd like to get the patches in -tip and see the results, I would > > > > recommend using percpu_counter_sum() while reading the data as an > > > > enhancement to this patch. If user space does not overwhelm with a lot > > > > of reads, sum would work out better. > > > > > > You trust userspace? I'd rather not. > > > > > > > Fair enough.. A badly written application monitor can frequently read > > this data and cause horrible performance issues. On the other hand > > large number of CPUs can make the lag even worse. Is it time yet for > > percpu_counter batch numbers? I've tested this patch and the results > > were not badly off the mark. > > I'd rather err on the side of caution here, you might get some crazy > folks depending on it and then expecting us to maintain it.
So if we want to be cautious, we could use percpu_counter_sum() as Balbir suggested. That would address both the issues with percpu_counter that I pointed out earlier:
- Readers are serialized with writers and we get consistent/correct values during reads. - Negates the effect of batching and reads would always get updated/current values.
Regards, Bharata.
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