Message-Id: <20240420175608.0e9664cbeee59c9aa2b5453d@kernel.org>
Date: Sat, 20 Apr 2024 17:56:08 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Florent Revest
 <revest@...omium.org>, linux-trace-kernel@...r.kernel.org, LKML
 <linux-kernel@...r.kernel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,
 bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>, Alexei
 Starovoitov <ast@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Arnaldo
 Carvalho de Melo <acme@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Alan Maguire <alan.maguire@...cle.com>, Mark Rutland
 <mark.rutland@....com>, Peter Zijlstra <peterz@...radead.org>, Thomas
 Gleixner <tglx@...utronix.de>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v9 07/36] function_graph: Allow multiple users to attach
 to function graph

On Fri, 19 Apr 2024 23:52:58 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Mon, 15 Apr 2024 21:50:20 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > @@ -27,23 +28,157 @@
> >  
> >  #define FGRAPH_RET_SIZE sizeof(struct ftrace_ret_stack)
> >  #define FGRAPH_RET_INDEX DIV_ROUND_UP(FGRAPH_RET_SIZE, sizeof(long))
> > +
> > +/*
> > + * On entry to a function (via function_graph_enter()), a new ftrace_ret_stack
> > + * is allocated on the task's ret_stack with indexes entry, then each
> > + * fgraph_ops on the fgraph_array[]'s entryfunc is called and if that returns
> > + * non-zero, the index into the fgraph_array[] for that fgraph_ops is recorded
> > + * on the indexes entry as a bit flag.
> > + * As the associated ftrace_ret_stack saved for those fgraph_ops needs to
> > + * be found, the index to it is also added to the ret_stack along with the
> > + * index of the fgraph_array[] to each fgraph_ops that needs their retfunc
> > + * called.
> > + *
> > + * The top of the ret_stack (when not empty) will always have a reference
> > + * to the last ftrace_ret_stack saved. All references to the
> > + * ftrace_ret_stack has the format of:
> > + *
> > + * bits:  0 -  9	offset in words from the previous ftrace_ret_stack
> > + *			(bitmap type should have FGRAPH_RET_INDEX always)
> > + * bits: 10 - 11	Type of storage
> > + *			  0 - reserved
> > + *			  1 - bitmap of fgraph_array index
> > + *
> > + * For bitmap of fgraph_array index
> > + *  bits: 12 - 27	The bitmap of fgraph_ops fgraph_array index
> 
> I really hate the terminology I came up with here, and would love to
> get better terminology for describing what is going on. I looked it
> over but I'm constantly getting confused. And I wrote this code!
> 
> Perhaps we should use:
> 
>  @frame : The data that represents a single function call. When a
>           function is traced, all the data used for all the callbacks
>           attached to it, is in a single frame. This would replace the
>           FGRAPH_RET_SIZE as FGRAPH_FRAME_SIZE.

Agreed.

> 
>  @offset : This is the word size position on the stack. It would
>            replace INDEX, as I think "index" is being used for more
>            than one thing. Perhaps it should be "offset" when dealing
>            with where it is on the shadow stack, and "pos" when dealing
>            with which callback ops is being referenced.

Indeed. @index is usually used from the index in an array. So we can use
@index for fgraph_array[]. But inside a @frame, @offset would be better.

> 
> 
> > + *
> > + * That is, at the end of function_graph_enter, if the first and forth
> > + * fgraph_ops on the fgraph_array[] (index 0 and 3) needs their retfunc called
> > + * on the return of the function being traced, this is what will be on the
> > + * task's shadow ret_stack: (the stack grows upward)
> > + *
> > + * |                                            | <- task->curr_ret_stack
> > + * +--------------------------------------------+
> > + * | bitmap_type(bitmap:(BIT(3)|BIT(0)),        |
> > + * |             offset:FGRAPH_RET_INDEX)       | <- the offset is from here
> > + * +--------------------------------------------+
> > + * | struct ftrace_ret_stack                    |
> > + * |   (stores the saved ret pointer)           | <- the offset points here
> > + * +--------------------------------------------+
> > + * |                 (X) | (N)                  | ( N words away from
> > + * |                                            |   previous ret_stack)
> > + *
> > + * If a backtrace is required, and the real return pointer needs to be
> > + * fetched, then it looks at the task's curr_ret_stack index, if it
> > + * is greater than zero (reserved, or right before poped), it would mask
> > + * the value by FGRAPH_RET_INDEX_MASK to get the offset index of the
> > + * ftrace_ret_stack structure stored on the shadow stack.
> > + */
> > +
> > +#define FGRAPH_RET_INDEX_SIZE	10
> 
> Replace SIZE with BITS.

Agreed.

> 
> > +#define FGRAPH_RET_INDEX_MASK	GENMASK(FGRAPH_RET_INDEX_SIZE - 1, 0)
> 
>   #define FGRAPH_FRAME_SIZE_BITS	10
>   #define FGRAPH_FRAME_SIZE_MASK	GENMASK(FGRAPH_FRAME_SIZE_BITS - 1, 0)
> 
> 
> > +
> > +#define FGRAPH_TYPE_SIZE	2
> > +#define FGRAPH_TYPE_MASK	GENMASK(FGRAPH_TYPE_SIZE - 1, 0)
> 
>   #define FGRAPH_TYPE_BITS	2
>   #define FGRAPH_TYPE_MASK	GENMASK(FGRAPH_TYPE_BITS - 1, 0)
> 
> 
> > +#define FGRAPH_TYPE_SHIFT	FGRAPH_RET_INDEX_SIZE
> > +
> > +enum {
> > +	FGRAPH_TYPE_RESERVED	= 0,
> > +	FGRAPH_TYPE_BITMAP	= 1,
> > +};
> > +
> > +#define FGRAPH_INDEX_SIZE	16
> 
> replace "INDEX" with "OPS" as it will be the indexes of ops in the
> array.
> 
>   #define FGRAPH_OPS_BITS	16
>   #define FGRAPH_OPS_MASK	GENMASK(FGRAPH_OPS_BITS - 1, 0)

OK, this looks good.

> 
> > +#define FGRAPH_INDEX_MASK	GENMASK(FGRAPH_INDEX_SIZE - 1, 0)
> > +#define FGRAPH_INDEX_SHIFT	(FGRAPH_TYPE_SHIFT + FGRAPH_TYPE_SIZE)
> > +
> > +/* Currently the max stack index can't be more than register callers */
> > +#define FGRAPH_MAX_INDEX	(FGRAPH_INDEX_SIZE + FGRAPH_RET_INDEX)
> 
> FGRAPH_MAX_INDEX isn't even used. Let's delete it.

OK.

> 
> > +
> > +#define FGRAPH_ARRAY_SIZE	FGRAPH_INDEX_SIZE
> 
>   #define FGRAPH_ARRAY_SIZE	FGRAPH_INDEX_BITS

OK.

> 
> > +
> >  #define SHADOW_STACK_SIZE (PAGE_SIZE)
> >  #define SHADOW_STACK_INDEX (SHADOW_STACK_SIZE / sizeof(long))
> >  /* Leave on a buffer at the end */
> > -#define SHADOW_STACK_MAX_INDEX (SHADOW_STACK_INDEX - FGRAPH_RET_INDEX)
> > +#define SHADOW_STACK_MAX_INDEX (SHADOW_STACK_INDEX - (FGRAPH_RET_INDEX + 1))
> 
> We probably should rename this is previous patches as well.
> 
> Unfortunately, it's getting close to the time for me to pick up my wife
> from the airport to start our vacation. But I think we should rename a
> lot of these variables to make things more consistent.

OK, Thanks for your review!

> 
> I'll try to look more at the previous patches as well to make my
> comments there, when I get some time. Maybe even later today.

Only if you have a time. I think I also refresh the code.

Thank you,

> 
> -- Steve
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>

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