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SubjectRe: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardware breakpoints
On Sun, 2009-07-26 at 05:27 +0530, K.Prasad wrote:

> I don't claim to have understood the requirements of perf-counters
> fully, but I sense the persistence of a few doubts about the register
> allocation mechanism of the hw-breakpoints API...thought it might be
> worthwhile to briefly explain.
> A few notes about the same:
> Kernel-space breakpoints: They are system-wide i.e. one kernel-space
> breakpoint request consumes one debug register on 'all' CPUs of the
> system.
> User-space breakpoints: Requests are stored in per-thread data
> structures. Written onto the debug register only when scheduled-into
> the CPU, and are removed when another task using the debug registers is
> scheduled onto the CPU.
> Debug register allocation mechanism: register request succeeds only when
> the availability of a debug register is guaranteed (for both user and
> kernel space). Allocation on a first-come-first-serve basis, no
> priorities for register requests and hence no pre-emption of requests.
> In short the available debug registers are divided into:
> # of debug registers = # of kernel-space requests + MAX(# of all
> user-space requests) + freely available debug registers.
> Thus on an x86 system with 4 debug registers, if there exists 1
> kernel-space request it consumes one debug register on all CPUs in the
> system. The remaining 3 debug registers can be consumed by user-space
> i.e. upto 3 user-space requests per-thread.
> There is no restriction on how many debug registers can be consumed by
> the kernel- or user-space requests and it is entirely dependent on the
> number of free registers available for use then.

Right, this is an utter miss-match for what perf counters would want.

Firstly, you seem to have this weird split of kernel/userspace
breakpoints. Perf counters looks at things in a per-cpu fashion, so the
all-cpus kernel breakpoint stuff is useless. Also, from perf counters'
POV its perfectly reasonable to have a per-task kernel breakpoint.

Secondly, perf counters wants to schedule the per task breakpoints
because we can optimize the context switch, saving lots of these MSR
writes under some common scenarios.

Thirdly, we can multiplex perf counters beyond their hardware maximum,
something you simply cannot do for a debug interface.

Like I said, please use the raw per-cpu breakpoint interface for perf
counters and connect that with the minimally required reservation you
need to make your other thing work.

You simply cannot put perf-counter breakpoints on top of whatever virt
layer you created going by what you say it is.

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