Peter,
quoted
On this second syscall, fetch_bp_busy_slots() sets slots.pinned to be 1,
despite there being no breakpoint on this CPU. This is because the call
the task_bp_pinned, checks all CPUs, rather than just the current CPU.
POWER7 only has one hardware breakpoint per CPU (ie. HBP_NUM=1), so we
return ENOSPC.
I think this comes from the ptrace legacy, we register a breakpoint on
all cpus because when we migrate a task it cannot fail to migrate the
breakpoint.
Its one of the things I hate most about the hwbp stuff as it relates to
perf.
Frederic knows more...
Maybe I should wait for Frederic to respond but I'm not sure I
understand what you're saying.
I can see how using ptrace hw breakpoints and perf hw breakpoints at the
same time could be a problem, but I'm not sure how this would stop it.
Are you saying that we need to keep at least 1 slot free at all times,
so that we can use it for ptrace?
Is "perf record -e mem:0x10000000 true" ever going to be able to work on
POWER7 with only one hw breakpoint resource per CPU?
Thanks,
Mikey