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Message-ID: <20090305131538.GA32076@in.ibm.com>
Date: Thu, 5 Mar 2009 18:45:38 +0530
From: "K.Prasad" <prasad@...ux.vnet.ibm.com>
To: Ingo Molnar <mingo@...e.hu>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Alan Stern <stern@...land.harvard.edu>,
Roland McGrath <roland@...hat.com>
Subject: Re: [patch 11/11] ftrace plugin for kernel symbol tracing using HW
Breakpoint interfaces
On Thu, Mar 05, 2009 at 10:16:11AM +0100, Ingo Molnar wrote:
>
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
>
> > On Thu, Mar 05, 2009 at 10:13:33AM +0530, prasad@...ux.vnet.ibm.com wrote:
> > > This patch adds an ftrace plugin to detect and profile memory access over
> > > kernel variables. It uses HW Breakpoint interfaces to 'watch memory
> > > addresses.
> > >
> > > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> > > ---
> >
> >
> > Hi,
> >
> > Nice feature. And moreover the standardized hardware
> > breakpoints could be helpful for tracing.
>
> yeah. The feature is much more alive now.
>
> > Just some comments below.
>
> One other thing:
>
> +#ifdef CONFIG_FTRACE_SELFTEST
> +int trace_selftest_startup_ksym(struct tracer *trace, struct trace_array *tr)
> +{
> + /* TODO: Will be implemented later */
> + return 0;
> +}
> +#endif /* CONFIG_FTRACE_SELFTEST */
>
> This needs to be implemented before i can pick the code up into
> tip:tracing, as otherwise we will not notice it fast enough if
> some of this stuff breaks.
>
> Basically the ftrace plugin will be the main usage vector of
> this facility, so the self-test is a must-have.
>
> Looks very nice otherwise.
>
> Ingo
Thanks for the comments.
Test-cases for the hardware breakpoint interfaces can be the following:
- Basic sanity test to check if the API is intact
- Perform various types of memory accesses, like read, write (I/O and
others when implemented) on a dummy kernel variable and verify the
trigger of the exception handler.
While the above can be a part of trace_selftest_startup_ksym(),
rigorous testing would involve:
i) stressing the HW breakpoint infrastructure to confirm sane behaviour
when interoperated with other users of a)breakpoint register b)the
do_debug() exception. This will involve simultaneous use of kprobes,
hardware breakpoint interface and requests from user-space (say through
GDB).
ii) Verifying successful HB_NUM number of register_ requests.
iii) Verifying right priority resolution, and handling user-space
requests.
These, in my opinion, would better fit in a full-featured test-suite
such as LTP, as opposed to startup testing in ftrace.
I will implement trace_selftest_startup_ksym() to contain the first two
test-cases in the next iteration of this code.
Thanks,
K.Prasad
Thanks,
K.Prasad
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