Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT

Jiri Olsa
2017-11-13T07:46:50Z

On Sun, Nov 12, 2017 at 11:09:23AM -0800, Milind Chabbi wrote:
> ,
>
> On Thu, Nov 9, 2017 at 10:59 AM, Milind Chabbi <chabbi.milind@gmail.com> wrote:
> > SNIP
> >
> > On Thu, Nov 9, 2017 at 5:12 AM, Jiri Olsa <jolsa@redhat.com> wrote:
> >>
> >>
> >> how about something like below (untested)
> >>
> >> looks like there's no irq caller for modify_user_hw_breakpoint,
> >> so we should be fine with locking nr_bp_mutex
> >>
> >> jirka
> >>
> >>
> >> ---
> >> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> >> index 3f8cb1e14588..f062b68399ea 100644
> >> --- a/kernel/events/hw_breakpoint.c
> >> +++ b/kernel/events/hw_breakpoint.c
> >> @@ -448,6 +448,8 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
> >> else
> >> perf_event_disable(bp);
> >>
> >> + release_bp_slot(bp);
> >> +
> >> bp->attr.bp_addr = attr->bp_addr;
> >> bp->attr.bp_type = attr->bp_type;
> >> bp->attr.bp_len = attr->bp_len;
> >> @@ -455,9 +457,9 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
> >> if (attr->disabled)
> >> goto end;
> >>
> >> - err = validate_hw_breakpoint(bp);
> >> + err = reserve_bp_slot(bp);
> >> if (!err)
> >> - perf_event_enable(bp);
> >> + err = validate_hw_breakpoint(bp);
> >>
> >> if (err) {
> >> bp->attr.bp_addr = old_addr;
> >> @@ -469,6 +471,7 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
> >> return err;
> >> }
> >>
> >> + perf_event_enable(bp);
> >> end:
> >> bp->attr.disabled = attr->disabled;
> >>
> >
> > We can do this accounting only if bp->attr.bp_type != attr->bp_type.
> >
> > -Milind
>
>
> Jirka,
>
> Neither of us seems to fully understand the convoluted logic used in
> breakpoint counting.

yea, I was hoping some of the guys would take over ;-)

the problem I have with the patch above is that we could
fail to reserve the slot at the end, which is not what
the caller might expect

>
> I tested the following sequence on an x86 machine, which has four
> debug registers (without your suggested patch for counting
> correction).
>
> fd1 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR1
> fd2 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR2
> fd3 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR3
> fd4 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR4
> ioctl(fd4, MODIFY, ...); // change fd4 to BP_TYPE= HW_BREAKPOINT_X @ ADDR5
> close(fd4);
> fd5 = perf_event_open(); //BP_TYPE=RW @ ADDR6
>
> We expected fd5 to fail because four BP_TYPE=TYPE_DATA are in use as
> per the accounting, but in reality, fd5 was successfully opened.

but you closed fd4 before openning fd5..?

>
> Is the accounting accidentally working on x86?
> Is there another architecture where TYPE_DATA and TYPE_INS are counted
> differently?

[jolsa@krava linux-perf]$ grep -r HAVE_MIXED_BREAKPOINTS_REGS arch/*
arch/Kconfig:config HAVE_MIXED_BREAKPOINTS_REGS
arch/sh/Kconfig: select HAVE_MIXED_BREAKPOINTS_REGS
arch/x86/Kconfig: select HAVE_MIXED_BREAKPOINTS_REGS

I'll try to check on it this week

jirka