Messages in this thread | | | Date | Wed, 29 Jul 2009 11:57:18 -0300 | From | Arnaldo Carvalho de Melo <> | Subject | Re: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardware breakpoints |
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Em Wed, Jul 29, 2009 at 11:22:52AM +0200, Peter Zijlstra escreveu: > On Wed, 2009-07-29 at 12:07 +0530, K.Prasad wrote: > > > > That still doesn't provide per-cpu breakpoints. > > > > > > > Yes, it doesn't provide a per-cpu only implementation. One can obtain > > the per-cpu data from the system-wide breakpoints by filtering it for a > > given CPU (agreed, it will associated overhead). > > > > A true per-cpu breakpoint implementation that co-exists with > > system-wide and per-task breakpoints will be difficult. It might require > > the re-introduction of some old features and a few new ones (like switching > > between kernel and user-space breakpoints at syscall time) that were > > rejected earlier by the community. > > I'm not clear on why you'd need to switch breakpoints on syscall entry. > You can simply leave the kernel address breakpoint around in userspace, > they're not able to poke at that address space anyway. > > > Also, the reason for a per-cpu only breakpoint (user and kernel-space) > > isn't very obvious. While kernel variables can be read/written > > throughout the system and user-space variables are per-task, the need > > for obtaining per-cpu information isn't clear. > > Well, suppose you're monitoring a per-cpu variable, or interested in the > effects of a workload confined to 1 cpu or node, there is no reason to > have this breakpoint on all cpus.
I was going to talk about exactly that, CPU isolation for one specific workload, I don't want that hits on tcp_v4_rcv on another CPU get counted, just the ones on that specific CPU.
The other CPUs can be given a break (pun intended :-P) by not having to process traps we're uninterested in.
- Arnaldo
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