Messages in this thread | | | Date | Tue, 27 Nov 2018 01:07:55 +0900 | From | Masami Hiramatsu <> | Subject | Re: [RFC][PATCH 11/14] function_graph: Convert ret_stack to a series of longs |
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On Fri, 23 Nov 2018 21:31:38 -0800 Joel Fernandes <joel@joelfernandes.org> wrote:
> On Wed, Nov 21, 2018 at 08:27:19PM -0500, Steven Rostedt wrote: > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org> > > > > In order to make it possible to have multiple callbacks registered with the > > function_graph tracer, the retstack needs to be converted from an array of > > ftrace_ret_stack structures to an array of longs. This will allow to store > > the list of callbacks on the stack for the return side of the functions. > > > > [ Note, this currently breaks architectures that access the ret_stack of a > > task to handle unwinding when 'return_to_handler' is on the stack ] > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> > > --- > > include/linux/sched.h | 2 +- > > kernel/trace/fgraph.c | 123 +++++++++++++++++++++++------------------- > > 2 files changed, 70 insertions(+), 55 deletions(-) > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index d6183a55e8eb..71a084a300da 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -1119,7 +1119,7 @@ struct task_struct { > > int curr_ret_depth; > > > > /* Stack of return addresses for return function tracing: */ > > - struct ftrace_ret_stack *ret_stack; > > + unsigned long *ret_stack; > > > > /* Timestamp for last schedule: */ > > unsigned long long ftrace_timestamp; > > diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c > > index 9b85638ecded..1389fe39f64c 100644 > > --- a/kernel/trace/fgraph.c > > +++ b/kernel/trace/fgraph.c > > @@ -23,6 +23,17 @@ > > #define ASSIGN_OPS_HASH(opsname, val) > > #endif > > > > +#define FGRAPH_RET_SIZE (sizeof(struct ftrace_ret_stack)) > > +#define FGRAPH_RET_INDEX (ALIGN(FGRAPH_RET_SIZE, sizeof(long)) / sizeof(long)) > > +#define SHADOW_STACK_SIZE (FTRACE_RETFUNC_DEPTH * FGRAPH_RET_SIZE) > > +#define SHADOW_STACK_INDEX \ > > + (ALIGN(SHADOW_STACK_SIZE, sizeof(long)) / sizeof(long)) > > +#define SHADOW_STACK_MAX_INDEX (SHADOW_STACK_INDEX - FGRAPH_RET_INDEX) > > + > > +#define RET_STACK(t, index) ((struct ftrace_ret_stack *)(&(t)->ret_stack[index])) > > +#define RET_STACK_INC(c) ({ c += FGRAPH_RET_INDEX; }) > > +#define RET_STACK_DEC(c) ({ c -= FGRAPH_RET_INDEX; }) > > + > [...] > > @@ -514,7 +531,7 @@ void ftrace_graph_init_task(struct task_struct *t) > > > > void ftrace_graph_exit_task(struct task_struct *t) > > { > > - struct ftrace_ret_stack *ret_stack = t->ret_stack; > > + unsigned long *ret_stack = t->ret_stack; > > > > t->ret_stack = NULL; > > /* NULL must become visible to IRQs before we free it: */ > > @@ -526,12 +543,10 @@ void ftrace_graph_exit_task(struct task_struct *t) > > /* Allocate a return stack for each task */ > > static int start_graph_tracing(void) > > { > > - struct ftrace_ret_stack **ret_stack_list; > > + unsigned long **ret_stack_list; > > int ret, cpu; > > > > - ret_stack_list = kmalloc_array(FTRACE_RETSTACK_ALLOC_SIZE, > > - sizeof(struct ftrace_ret_stack *), > > - GFP_KERNEL); > > + ret_stack_list = kmalloc(SHADOW_STACK_SIZE, GFP_KERNEL); > > > > I had dumped the fgraph size related macros to understand the patch better, I > got: > [ 0.909528] val of FGRAPH_RET_SIZE is 40 > [ 0.910250] val of FGRAPH_RET_INDEX is 5 > [ 0.910866] val of FGRAPH_ARRAY_SIZE is 16 > [ 0.911488] val of FGRAPH_ARRAY_MASK is 255 > [ 0.912134] val of FGRAPH_MAX_INDEX is 16 > [ 0.912751] val of FGRAPH_INDEX_SHIFT is 8 > [ 0.913382] val of FGRAPH_FRAME_SIZE is 168 > [ 0.914033] val of FGRAPH_FRAME_INDEX is 21 > FTRACE_RETFUNC_DEPTH is 50 > [ 0.914686] val of SHADOW_STACK_SIZE is 8400 > > I had a concern about memory overhead per-task. It seems the total memory > needed per task for the stack is 8400 bytes (with my configuration with > FUNCTION_PROFILE > turned off). > > Where as before it would be 32 * 40 = 1280 bytes. That looks like ~7 times > more than before.
Hmm, this seems too big... I thought the shadow-stack size should be smaller than 1 page (4kB). Steve, can we give a 4k page for shadow stack and define FTRACE_RETFUNC_DEPTH = 4096 / FGRAPH_RET_SIZE ?
> On my system with ~4000 threads, that becomes ~32MB which seems a bit > wasteful especially if there was only one or 2 function graph callbacks > registered and most of the callback array in the stack isn't used. > > Could we make the array size configurable at compile time and start it with a > small number like 4 or 6?
Or, we can introduce online setting :)
Thank you,
> > Also for patches 1 through 10: > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org> > > thanks, > > - Joel >
-- Masami Hiramatsu <mhiramat@kernel.org>
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