Re: [Patch 0/1] HW-BKPT: Allow per-cpu kernel-space Hardware Breakpoint requests

K.Prasad
2009-08-19T16:11:34Z

On Mon, Aug 17, 2009 at 06:16:41PM +0530, K.Prasad wrote:
> Hi All,
> Please find a patch that enables kernel-space breakpoints to be
> requested for a subset of the available CPUs in the system. This allows
> per-CPU breakpoints and comes with the associated benefit of reduced
> overhead during (un)registration.
>
> This enhancement allows exploitation of hardware breakpoint registers by
> 'perf' which produces a CPU-wise information.
>
> Design changes
> --------------
> - Every breakpoint request 'consumes' the first available debug register
> (starting from HBP_NUM) in each CPU represented by 'cpumask' field in
> 'struct hw_breakpoint'.
>
> - 'hbp_kernel_pos' (that separates kernel-space breakpoints from the
> free/user-space breakpoints) now points to the maximum of debug
> registers consumed on any given CPU.
> -- 'hbp_kernel_pos' is decremented (one-at-a-time) to allow a new-slot
> for kernel-space requests iff all debug registers on the given CPU
> (from HBP_NUM - 1 to 'hbp_kernel_pos' are already consumed.
> -- 'hbp_kernel_pos' is incremented (one-at-a-time) to free a slot iff
> a removal request results in the release of a bkpt request that
> consumed maximum debug registers for kernel-space.
>
> - Every removal request results in compaction of breakpoint registers
> (on a per-cpu basis) to occupy the vacant debug register.
>
> The patch is based on commit b6c720b811aed0eeda89f277f13c1bd1bdf721fd of
> -tip tree and has been tested to work fine on an x86 machine for both
> cases (i.e. system-wide kernel breakpoints and bkpts for a subset of CPUs).
>
> Please let me know your comments on the same.
>
> Thanks,
> K.Prasad
>

Hi Frederic,
Do you find these patches, that provide the ability to restrict
kernel-space breakpoints to any given subset of CPUs, to bring the
requisite features for exploitation of hw-bkpt by 'perf tools'?

Also of interest would be the reduced overhead associated with
(un)register_kernel_hw_breakpoint() operations (no IPI in case of
single-CPU breakpoint request).

Thanks,
K.Prasad